AlFoCS + F3D II: unexpectedly low gas-to-dust ratios in the Fornax galaxy cluster
Nikki Zabel, Timothy A. Davis, Matthew W. L. Smith, Marc Sarzi, Alessandro Loni, Paolo Serra, Maritza A. Lara-López, Phil Cigan, Maarten Baes, George J. Bendo, Ilse De Looze, Enrichetta Iodice, Dane Kleiner, Bärbel S. Koribalski, Reynier Peletier, Francesca Pinna, P. Tim de Zeeuw
MMNRAS , 1–22 (2021) Preprint 4 February 2021 Compiled using MNRAS L A TEX style file v3.0
AlFoCS + F3D II: unexpectedly low gas-to-dust ratios in the Fornax galaxycluster
Nikki Zabel, ★ Timothy A. Davis, Matthew W. L. Smith, Marc Sarzi, Alessandro Loni, , Paolo Serra, Maritza A. Lara-López, Phil Cigan, Maarten Baes, George J. Bendo, Ilse De Looze, , Enrichetta Iodice, Dane Kleiner, Bärbel S. Koribalski, , Reynier Peletier, Francesca Pinna, P. Tim de Zeeuw , Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands School of Physics and Astronomy, Cardiff University, Queen’s Building, The Parade, Cardiff, CF24 3AA, Wales, UK Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, UK INAF - Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047 Selargius (CA), Italy Dipartimento di Fisica, Università di Cagliari, Cittadella Universitaria, 09042, Monserrato, Italy George Mason University, 4400 University Dr, Fairfax, VA 22030-4444, USA Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, B-9000 Gent, Belgium UK ALMA Regional Centre Node, Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester,Oxford Road, Manchester M13 9PL, UK Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK INAF-Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131, Napoli, Italy Australia Telescope National Facility, CSIRO Astronomy and Space Science, P.O. Box 76, 1710, Epping, NSW, Australia Western Sydney University, Locked Bag 1797, 1797, Penrith South, NSW, Australia Max-Planck-Institut für Aststuhl 17, 69117, Heidelberg, Germany Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85741, Garching bei Muenchen, Germany
Accepted 2021 February 3. Received 2021 February 1; in original form 2020 November 18
ABSTRACT
We combine observations from ALMA, ATCA, MUSE, and
Herschel to study gas-to-dust ratios in 15 Fornax cluster galaxiesdetected in the FIR/sub-mm by
Herschel and observed by ALMA as part of the ALMA Fornax Cluster Survey (AlFoCS). Thesample spans a stellar mass range of 8 . ≤ log (M ★ / M (cid:12) ) ≤ .
16, and a variety of morphological types. We use gas-phasemetallicities derived from MUSE observations (from the Fornax3D survey) to study these ratios as a function of metallicity, andto study dust-to-metal ratios, in a sub-sample of nine galaxies. We find that gas-to-dust ratios in Fornax galaxies are systematicallylower than those in field galaxies at fixed stellar mass/metallicity. This implies that a relatively large fraction of the metals in theseFornax systems is locked up in dust, which is possibly due to altered chemical evolution as a result of the dense environment.The low ratios are not only driven by Hi deficiencies, but H -to-dust ratios are also significantly decreased. This is differentin the Virgo cluster, where low gas-to-dust ratios inside the virial radius are driven by low Hi-to-dust ratios, while H -to-dustratios are increased. Resolved observations of NGC1436 show a radial increase in H -to-dust ratio, and show that low ratios arepresent throughout the disc. We propose various explanations for the low H -to-dust ratios in the Fornax cluster, including themore efficient stripping of H compared to dust, more efficient enrichment of dust in the star formation process, and altered ISMphysics in the cluster environment. Key words: galaxies: clusters: individual: Fornax – galaxies: clusters: individual: Virgo – galaxies: evolution – galaxies: ISM– ISM: evolution
The environment in which a galaxy resides can significantly influ-ence the way it evolves. In particular, the more dense the environ-ment, the more early-type galaxies it harbours relative to late-type ★ E-mail: [email protected] galaxies (Oemler 1974; Dressler 1980). This suggests that dense en-vironments are capable of aiding the quenching of star formation ingalaxies. Various environmental mechanisms have been proposed tobe (co-)responsible for this, including ram pressure stripping (Gunn& Gott 1972), starvation (Larson et al. 1980), and galaxy-galaxyinteractions (Moore et al. 1996). While many of these processes pri-marily affect the more extended and easily stripped Hi gas, evidencehas started to accumulate that they can also directly affect the molec- © a r X i v : . [ a s t r o - ph . GA ] F e b N. Zabel et al. ular gas, despite its more tightly bound and centrally located nature(Vollmer et al. 2008; Fumagalli et al. 2009; Boselli et al. 2014; Zabelet al. 2019, hereafter Z19). Since molecular gas is the direct fuel forstar formation, this can, in turn, affect the star formation efficiencyin galaxies in these dense environments (e.g. Gullieuszik et al. 2020,Zabel et al. 2020, hereafter Z20).Besides gas, dust plays a vital role in star formation and galaxyevolution. If dust is environmentally stripped along with gas, thiscould have a significant impact on the evolution of galaxies in clus-ters, the most (galaxy )dense environments in the Universe. Dust actsas a catalyst for the formation of H (e.g. van Dishoeck et al. 1993;Scoville 2013; Wakelam et al. 2017 and references therein), and helpsshield it from the interstellar radiation field (e.g. van Dishoeck et al.1993; Ciolek & Mouschovias 1995). Therefore, dust and moleculargas are typically linked, and might be expected to be stripped simul-taneously. However, if the process of star formation is (indirectly)affected by dense environments, this may result in more subtle ef-fects on the chemical evolution of a galaxy, and the relative gas anddust contents.The gas-to-dust ratio provides a useful probe for studying the ef-fects of environmental stripping on galaxies’ gas and dust contents inmore detail. Especially if it is compared with metallicity, it can teachus a lot about the chemical evolution of galaxies. The gas-to-dust ra-tio (G/D) as a function of metallicity has been studied extensively infield galaxies (e.g. Issa et al. 1990; Lisenfeld & Ferrara 1998; Draineet al. 2007; Sandstrom et al. 2013; Rémy-Ruyer et al. 2014; De Viset al. 2017; De Looze et al. 2020). Rémy-Ruyer et al. (2014) com-piled and homogenised a range of recent samples. They found that(total) G/D correlates strongly with metallicity, following a broken(around 12 + log(O/H) = 7.96) power law, with large scatter ( ∼ ∼ ) G/D within clustergalaxies. Cortese et al. (2016) found that (total) gas-to-dust ratios incluster galaxies are decreased compared to those in isolated galaxiesat fixed mass and metallicity. They found that this decrease is drivenby the Hi-to-dust ratio, while the H -to-dust ratio, on the other hand,is increased. They attribute this difference to differential stripping ofthe ISM as a result of the different spatial distribution of Hi (mostextended), H (centrally concentrated) and dust (in between the twogas phases).Thus far, studies of gas-to-dust ratios in cluster galaxies havemostly focused on spiral galaxies in the Virgo cluster. To obtaina broader understanding of the role of dust stripping in the baryoncycle of cluster galaxies, in a wider variety of environments, we needto expand our studies to different clusters and galaxy types. The For-nax cluster is the more mature (though still active, e.g. Z19; Iodiceet al. 2019b; Raj et al. 2020; Loni et al. 2021), but smaller sibling of the Virgo cluster in the Southern sky. Although it has only ∼ ∼ × 𝑀 (cid:12) within twice the virial radius,2 × ∼ ∼ Herschel or in Hi, with the goal to study the effects ofenvironment on molecular gas in combination with other phases ofthe interstellar medium (ISM). It comprises 30 galaxies (of which15 were detected in CO) of various morphological types, both early-types and late-types, as well as (irregular) dwarf galaxies, spanning amass-range of 10 ∼ . − M (cid:12) . for reference, the optical Fornax Clus-ter Catalogue (FCC, Ferguson 1989) contains 340 galaxies, and withthe recent optical Fornax Deep Survey (FDS, Peletier et al. 2020),this number probably increases to several thousands of galaxies, withmany new photometric cluster candidates (564 dwarf galaxies alone)that have been identified in Venhola et al. (2018). In Z19 we pre-sented resolved molecular gas maps and H masses and deficiencies.In Z20, where we use both Atacama Large Millimiter/submillimeterArray (ALMA) and Multi Unit Spectroscopic Explorer (MUSE) ob-servations (see below), we studied the resolved star formation relationin the Fornax cluster. In this work, we study molecular gas-to-dustratios in relation to their metallicities.Recently, a 15 ×
15 degree blind survey, covering the Fornax clusterout to its virial radius, was performed with the Australia TelescopeCompact Array (ATCA, Loni et al. 2021). This survey resulted in thedetection of 16 Fornax galaxies in Hi. 15 of these have CO observa-tions or useful upper limits (i.e. upper limits that provide informativeconstraints on the H -to-dust ratio) from AlFoCS, allowing us tostudy their total gas-to-dust ratios.Fornax3D (F3D, Sarzi et al. 2018; Iodice et al. 2019b) targetedall galaxies from the Fornax Cluster Catalogue (Ferguson 1989)brighter than 𝑚 𝐵 =
15 within or close to the virial radius ( 𝑅 vir = 0.7Mpc; Drinkwater et al. 2001) with MUSE, mounted to the YepunUnit Telescope 4 at the Very Large Telescope (VLT). Nine of thesegalaxies were detected (or have a useful upper limit) with AlFoCSand ATCA.This paper is organised as follows. In §2 we describe our sampleand observations. In §3 we describe the methods used, in particularhow any masses and metallicities were estimated. In §4 we describethe DustPedia literature sample, to which we compare our results, andany underlying assumptions used to derive the quantities we utilise.In §5 we describe the main results. In §6 we look into the resolvedH -to-dust ratio in NGC1436, to highlight any radial or other spatialvariation, if present. The main results are discussed in §7. Finally,we summarise our findings in §8.Throughout this work, we assume a common distance of 19.95Mpc (Tonry et al. 2001) to galaxies in the Fornax cluster. MNRAS000
15 within or close to the virial radius ( 𝑅 vir = 0.7Mpc; Drinkwater et al. 2001) with MUSE, mounted to the YepunUnit Telescope 4 at the Very Large Telescope (VLT). Nine of thesegalaxies were detected (or have a useful upper limit) with AlFoCSand ATCA.This paper is organised as follows. In §2 we describe our sampleand observations. In §3 we describe the methods used, in particularhow any masses and metallicities were estimated. In §4 we describethe DustPedia literature sample, to which we compare our results, andany underlying assumptions used to derive the quantities we utilise.In §5 we describe the main results. In §6 we look into the resolvedH -to-dust ratio in NGC1436, to highlight any radial or other spatialvariation, if present. The main results are discussed in §7. Finally,we summarise our findings in §8.Throughout this work, we assume a common distance of 19.95Mpc (Tonry et al. 2001) to galaxies in the Fornax cluster. MNRAS000 , 1–22 (2021) lFoCS + F3D II: low gas-to-dust ratios Our Fornax sample consists of all AlFoCS galaxies that have far-infrared/sub-millimetre measurements from which dust masses canbe estimated reliably (i.e. are detected in ≥ Herschel bands, implyingat least one detection with the Spectral and Photometric ImagingREceiver (SPIRE, Griffin et al. 2010, see §2.3). It consists of 15galaxies, spanning a mass range of 8 . ≤ log(M ★ /M (cid:12) ) ≤ . measurements areupper limits, though these are only included if they provide a usefulconstraint on the H -to-dust ratio. Of these galaxies, 9 (of whichone having a CO upper limit) have MUSE observations from F3D,and can therefore be used to study H -to-dust ratios as a functionof metallicity. This sub-sample spans a stellar mass range of 8 . ≤ log(M ★ /M (cid:12) ) ≤ .
0. Key parameters of the sample are listed inTable 1. Fornax Cluster Catalogue (FCC, Ferguson 1989) numbersare listed in column one. Corresponding common galaxy namesare listed in column 2. Column 3 lists stellar masses, from z0mgs(Leroy et al. 2019), or from Fuller et al. (2014) if not available there,indicated with a ‡ . Morphological types are given in column 4, fromSarzi et al. (2018) where available, otherwise from older referencesprovided in the Table caption. Column 5 lists the projected cluster-centric distance in kpc. Column 6 describes whether the moleculargas reservoir is regular (R) or disturbed (D), as classified in Z19.Column 7 lists X CO , estimated as described in §3.1. Molecular gas,atomic gas, and dust masses are listed in columns 8, 9, and 10,respectively. Finally, column 11 lists whether the galaxy is includedin F3D (Y) or not (N). ALMA data for our Fornax cluster targets were analysed in Z19,which describes the data and methods used in detail. We summarisesome important details here. ALMA Band 3 observations were car-ried out between the 7th and 12th of January 2016 under project ID2015.1.00497.S (PI: T. Davis), using the main (12m) array in theC36-1 configuration. The data were calibrated manually, clean-edinteractively, using a natural weighting scheme, and continuum sub-tracted using the Common Astronomy Software Applications pack-age (CASA, version 5.1.1, McMullin et al. 2007). The FWHM of therestoring beam is typically between ∼ (cid:48)(cid:48) and 3 (cid:48)(cid:48) (equivalent to ∼ − . Typical rms noise levels are ∼ CO(1-0)line emission using the masked moment method from Dame (2011).These maps were corrected for the primary beam response. Spectra,from which the H masses were estimated, were calculated by sum-ming over both spatial directions of the spectral cube, using a squarespatial field around the emission. At the distance of the Fornax clus-ter, the largest scales recoverable with the 12m array are significantlylarger than the expected sizes of the largest CO structures the galaxiesobserved. Therefore, we expect to have recovered the total CO(1-0)flux of each galaxy, and we expect the masses derived in Z19 to beaccurate. For one object, NGC1365, ALMA data was added fromthe archive (project ID: 2015.1.01135.S, PI: Fumi Egusa, see Z19 formore details). For a more detailed, resolved study of H -to-dust ratiosin NGC1436, described in §6, we use additional, deeper ALMA datafrom the archive to complement the observations from Z19 (projectID: 2017.1.00129.S, PI: Kana Morokuma, see §6 for more details). The far-infrared (FIR) maps used to estimate dust masses are from thePhotoconductor Array Camera and Spectrometer (PACS, Poglitschet al. 2010, 100 and 160 micron) and the Spectral and PhotometricImaging REceiver (SPIRE, Griffin et al. 2010, 250, 350, and 500micron), both mounted on the
Herschel
Space Observatory (Pilbrattet al. 2010). The SPIRE maps used here are identical to the ones usedby DustPedia (Davies et al. 2017, see §4), and the PACS maps werereprocessed using the same techniques as DustPedia. The FWHMof the beams of the
Herschel maps are ∼ . ∼ . ∼ . ∼ . ∼ . (cid:48)(cid:48) at 100, 160, 250, 350, and 500 micron, respectively,corresponding to ∼ , ∼ , ∼ , ∼ , and ∼ and thePACS Observer’s Manual ).Note that FIR measurements and derived dust masses for the For-nax cluster are already available in Fuller et al. (2014), on which theAlFoCS sample is based. However, since this work was published,several improvements have been made both to the Herschel data re-duction and the SED fitting methods. In order to obtain dust massesthat are as accurate as possible, we have opted to re-estimate dustmasses from improved FIR maps, using updated SED fitting tech-niques (see §3.3). Dust masses calculated here are slightly lower thanthose published in Fuller et al. (2014), with a median difference of0.28 dex and a 1 𝜎 spread in differences of 0.28 dex. i data Hi data are from ATCA, which was used to conduct a blind sur-vey of the Fornax cluster, covering an area of 15 deg out to 𝑅 vir .The observations and data reduction are presented and described indetail in Loni et al. (2021), and are summarised here. Observationswere carried out from December 2013 to January 2014 in the 750Bconfiguration (project code: C2894, PI: P. Serra). The data were re-duced manually using the MIRIAD software (Sault et al. 1995). Thedirty cube was obtained using the
INVERT task (using natural weight-ing), after which the tasks
MOSMEM and
RESTOR were used to cleanand restore the Hi emission. The synthesised beam has a FWHMof 95 (cid:48)(cid:48) × (cid:48)(cid:48) (corresponding to ∼ × . − . Typical noiselevels are 2.8 mJy beam − and go down to 2.0 mJy beam − in themost sensitive region. Hi sources were identified using the SoFiAsource-finding package (Serra et al. 2015). Whether detections areconsidered reliable is based on the algorithm from Serra et al. (2012),and by visual inspection where necessary. This has resulted in thedetection of Hi in 16 Fornax cluster galaxies, of which 15 have COdetections (or useful upper limits) and are thus included in this work.The spatial resolution of the ATCA data is 67 (cid:48)(cid:48) × (cid:48)(cid:48) ( ∼ × Optical spectra, used to measure the line ratios from which metal-licities are estimated, are from F3D Sarzi et al. (2018); Iodice et al.(2019b). A detailed description of the survey and data reduction http://herschel.esac.esa.int/Docs/SPIRE/spire_handbook.pdf Available from https://atoa.atnf.csiro.au/query.jsp
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Table 1.
Key properties of the galaxies in the sample.FCC Common name M ★ Type D Gas dist. X CO M H M Hi M dust in F3D?- - (log 𝑀 (cid:12) ) - (kpc) - 10 cm − (K km s − ) − (log 𝑀 (cid:12) ) (log 𝑀 (cid:12) ) (log 𝑀 (cid:12) ) (Y/N)(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)67 NGC1351A 9.56 ± a
694 R 2.2 ± ± ± + . − . N90 MGC-06-08-024 8.98 ‡ E4 pec 595 D 6.6 ± ± ± + . − . Y121 NGC1365 10.75 ± b
420 R 1.6 ± ± ± + . − . N167 NGC1380 8.75 ± ± ± ≤ + . − . Y179 NGC1386 10.92 ± ± ± ≤ + . − . Y184 NGC1387 9.51 ± ± ± ≤ + . − . Y235 NGC1427A 9.1 ± b
132 - 2.6 ± ≤ ± + . − . N263 PGC013571 8.75 ± ± ± ± + . − . Y282 FCC282 8.86 ± c
615 D 2.8 ± ± ≤ + . − . N285 NGC1437A 8.71 ± b
428 - 3.7 ± ≤ ± + . − . Y290 NGC1436 10.12 ± ± ± ± + . − . Y306 FCC306 8.68 ‡ S c
600 - 3.9 ± ≤ ± + . − . N308 NGC1437B 9.25 ± ± ± ± + . − . Y312 ESO358-G063 10.08 ± ± ± ± + . − . Y335 ESO359-G002 9.17 ± b
872 D 2.4 ± ± ≤ + . − . N Notes: : Fornax Cluster Catalogue (Ferguson 1989) number of the galaxy; : Common name of the galaxy; : Stellar mass from z0mgs (Leroy et al. 2019), orfrom Fuller et al. (2014) if not available there, indicated with a ‡ ; : Morphological type from Sarzi et al. (2018) if available, otherwise references are listed below; : Projected distance from brightest cluster galaxy NGC1399; : Whether the molecular gas in the galaxy is regular (R) or disturbed (D) as classified in Z19; :X CO , estimated as described in §3.1; : H mass from Z19; : Hi mass from Loni et al. (2021); : Dust mass calculated from the FIR fluxes published in Fulleret al. (2016); : Whether the galaxy was observed with MUSE as part of F3D (Y) or not (N). a (Lauberts & Valentijn 1989); b (de Vaucouleurs et al. 1991); c (Loveday 1996) can be found in Sarzi et al. (2018), and some important details aresummarised here.Integral-field spectroscopic observations were carried out withMUSE (Bacon et al. 2010, mounted to VLT Unit Telescope 4,“Yepun”) in Wide Field Mode, between July 2016 and December2017. A field of 1 × × − .Data reduction was performed using the MUSE pipeline (version1.6.2, Weilbacher et al. 2012, 2016) under the ESOREFLEX environ-ment (Freudling et al. 2013). In summary, the data reduction involvedbias and overscan subtraction, flat fielding, wavelength calibration,determination of the line spread function, illumination correctionwith twilight flats (to account for large-scale variation of the illumi-nation of the detectors) and similar with lamp flats (to correct foredge effects between the integral-field units). masses H masses of the Fornax galaxies are calculated as described in §4.3of Z19. Upper limits are one beam, 3 𝜎 upper limits, assuming alinewidth of 50 km s − , as described in §4.3.1 of Z19. In summary,we use the following equation: 𝑀 H = 𝑚 H 𝐷 𝑋 CO 𝜆 𝑘 B ∫ 𝑆 𝜈 d 𝑣, (1) where 𝑚 H is the mass of a hydrogen atom, 𝐷 the distance to thegalaxy (in this case assumed to be the distance to the Fornax cluster), 𝑋 CO is the CO-to-H mass conversion factor, 𝜆 the rest wavelength ofthe line observed, 𝑘 B the Boltzmann constant, and ∫ 𝑆 𝜈 d 𝑣 the totalflux of the line observed. For 𝑋 CO we use the metallicity-dependentmass conversion factor 𝛼 CO from Accurso et al. (2017, eqn. 25):log 𝛼 CO = . − . [ + log(O/H) ] + .
062 log Δ (MS) , (2)where 12 + log(O/H) is the metallicity as estimated according to§3.4, and Δ (MS) the distance from the main sequence (from Elbazet al. 2007) as estimated in Z20. 𝛼 CO is then converted to 𝑋 CO bymultiplying it by 2 . × . Note that this prescription includes acorrection for helium, however for convenience we will use 𝑀 H torefer to the total molecular gas mass. The results are listed in Table1. i masses Hi masses are from Loni et al. (2021), who use the prescriptionfrom Meyer et al. (2017) to convert integrated fluxes to Hi masses.A common distance of 20 Mpc was assumed for all galaxies, anegligable difference with the 19.95 Mpc assumed in this work. Incases where there is no detectable Hi, upper limits are calculated.These are estimated as 3 × the local rms noise in the ATCA datacube, in one beam, and assuming the linewidths of the correspondingCO(1-0) lines from Z19. The resulting Hi masses are summarised inTable 1. Dust masses are estimated directly from the
Herschel maps describedin §2.3. To obtain FIR flux measurements for each galaxy, we performaperture photometry, using the Python package photutils (Bradleyet al. 2019). Reasonable aperture extents were determined by eye,
MNRAS000
MNRAS000 , 1–22 (2021) lFoCS + F3D II: low gas-to-dust ratios such that they encompass the dust emission from the entire galaxy(typical semimajor axis-sizes vary from 15 (cid:48)(cid:48) for small galaxies to400 (cid:48)(cid:48) for NGC1365, which is slightly smaller than the extent ofthe Hi disc in that galaxy, and closer to the extent of its stellardisc). To ensure all flux is included even in the lowest-resolutionimage, we define apertures in the longest wavelength image in whichthe galaxy is detected. In case there is no detection in each band,upper limits from the non-detected bands are used to constrain theSED at those wavelengths, which is reflected in the uncertainties.Background noise was estimated locally, by defining annuli aroundthe apertures chosen. The flux in these annuli is then subtracted fromthe measured source flux. Uncertainties were estimated by randomlyplacing 100 apertures of the same dimensions as the one used for thesource randomly in the Herschel map, and calculating the standarddeviation in these. This is done for each source in each of the 5wavelengths. The maps used are parallel-mode scan maps that coververy large areas (roughly 4.6 × ∼ . × . 𝜎 from the median.For small sources, the SPIRE beam is often larger than the apertureused to measure the flux: the full width at half maximum (FWHM) isup to ∼ (cid:48)(cid:48) , depending on the wavelength (see the SPIRE handbook ,while the radii of our apertures are as small as 15 (cid:48)(cid:48) for the smallestsources. To account for any missing flux as a result of this, whichmight result in an underestimation of dust masses (and therefore anoverestimation of gas-to-dust ratios), we apply aperture corrections.As the sources are not very extended at Herschel resolutions, weapply the recommended corrections for point-sources, provided aspart of the SPIRE calibration (Ott 2010; Bendo et al. 2013). Thesevalues depend on the aperture radius, which we estimate by takingthe arithmetic average of the semi-major and semi-minor axes of theelliptical apertures.Finally, dust masses are estimated from the resulting SEDs. Weperform modified blackbody fits, described by (a simplified versionof) equation 1 in Hildebrand (1983): 𝐹 𝜆 = 𝜅 𝜆 𝐵 𝜆 ( 𝑇 ) 𝐷 − 𝑀 dust , (3)where 𝑀 dust the dust mass in kg, 𝐷 the distance to the galaxy in Mpc, 𝐵 (T) Plank’s law of blackbody radiation, and 𝜅 𝜆 is described by 𝜅 𝜆 = 𝜅 ( 𝜆 / 𝜆 ) 𝛽 , (4)where 𝛽 is the dust opacity index, which we fix at 1.790 to match ourcomparison sample (DustPedia, see §4), and 𝜅 is the dust emissivity,which we fix at 0.192 m kg − at 𝜆 = 350 𝜇 m, again following thevalue used by DustPedia. Both constants adopted by DustPedia aredrawn from The Heterogeneous dust Evolution Model for InterstellarSolids (THEMIS) model (Jones et al. 2013; Köhler et al. 2014; Ysardet al. 2015). The fits were performed using our own SED fitter,which is based on PyMC3 (a Python package for Bayesian statisticalmodelling and Probabilistic Machine Learning focusing on advancedMarkov chain Monte Carlo (MCMC) and variational inference (VI)algorithms, Salvatier et al. 2016). It was written to include all beamand colour corrections as part of the fitting process, and account forcorrelated uncertainties between bands.For the (log) dust mass, we use a Gamma prior with a standarddeviation of 1 dex. To ensure that the code can handle a wide rangeof dust masses (from a small region of a galaxy to a ULIRG), themode is programmed to be the flux at the data point closest to the peak of the blackbody. For dust temperature, we also use a Gammadistribution, with a mode of 20K and a standard deviation of 6K. Wechoose the Gamma distribution as it does not allow for unphysicalnegative dust temperatures. It is also wider than a Gaussian distribu-tion, which more accurately reflects the wide range of dust mass andtemperatures.The assumed calibration uncertainties are 5% for PACS (Baloget al. 2014), and 1.7% for SPIRE (Bendo et al. 2013), as well as anadditional correlated uncertainty of 5% between PACS bands and4% between SPIRE bands. The correlated and uncorrelated uncer-tainties were conservatively added linearly for the SPIRE data asrecommended by the handbook , resulting in a total uncertainty of5.5%. For an overview of the general limitations of SED fitting tech-niques (in particular, dust emission in the FIR appearing to be a blendof emission from dust at different temperatures), we refer the readerto Bendo et al. (2015). Throughout this work, we use the oxygen abundance, 12 + log(O/H),as a proxy for the gas-phase metallicity. Metallicities are estimatedfrom F3D emission-line measurements presented in Iodice et al.(2019b), and using the strong-line calibration from Dopita et al.(2016, referred to as DOP16 throughout the rest of this work). Thiscalibration relies exclusively on the [NII] 𝜆 𝜆𝜆 𝛼 lines, as follows:12 + log(O/H) = . + log[NII]/[SII] + .
264 log[NII]/H 𝛼. (5)It is independent of the ionisation parameter, flux calibration, and ex-tinction correction, and valid for a wide range of oxygen abundances.The required emission-line fluxes were estimated by simultaneouslyfitting the spectral continuum and the nebular emission lines usingGas AND Absorption Line Fitting (gandalf, Sarzi et al. 2006),which makes use of Penalized Pixel-Fitting ( PPXF , Cappellari 2017)to derive stellar kinematic and corresponding absorption-line broad-ening. More details on this procedure can be found in Sarzi et al.(2018) and Iodice et al. (2019b). The resulting resolved metallic-ity measurements were averaged spatially to obtain global measure-ments.Of course, the choice of metallicity calibrator can significantlyimpact the resulting metallicity estimates. As can be seen in e.g.Sánchez et al. (2019), who compare various metallicity calibrationsas a function of stellar mass, and SFR, the DOP16 calibration showsa relatively steep gradient with stellar mass. It agrees reasonablywell with the calibration from PP04 at the low-mass end, with anoffset of ∼ ∼ MNRAS , 1–22 (2021)
N. Zabel et al.
Stellar masses are from the z = 0 Multiwavelength Galaxy Synthesis(z0MGS, Leroy et al. 2019) where available. These are based on datafrom the Wide-field Infrared Explorer (WISE, Wright et al. 2010)and the Galaxy Evolution Explorer (GALEX, Martin et al. 2005).Leroy et al. (2019) make use of data from the GALEX-SDSS-WISELegacy Catalog (GSWLC, Salim et al. 2016, 2018), which combinesWISE, GALEX, and Sloan Digital Sky Survey (SDSS) observations.In the GSWLC SED modeling with the Code Investigating GALaxyEmission (CIGALE, Burgarella et al. 2005; Noll et al. 2009; Boquienet al. 2019) was used to yield stellar mass estimates. In Leroy et al.(2019) these data are used to derive calibrations to estimate stellarmasses from their sample of WISE and GALEX data. The IMF fromKroupa & Weidner (2003) was assumed.If stellar masses are not available from z0MGS (this only applies totwo dwarf galaxies, FCC090 and FCC306), they are taken from Fulleret al. (2014). Although the stellar masses in Fuller et al. (2014) areestimated differently from those in z0MGS, even a significant errorin the stellar masses of these two dwarf galaxies would not affect ourresults. Stellar masses, along with their sources, are listed in Table 1.The two stellar masses that were taken from Fuller et al. (2014) areindicated with a ‡ . In order to compare the dust and gas in our cluster galaxies to thosein the field, we use data from the DustPedia project. DustPedia (Davies et al. 2017) covers all 875 extended galaxies within 3000km s − observed by the Herschel
Space Observatory. Here, we usea sub-sample of DustPedia, consisting of the 209 galaxies for whichM H measurements are available. This sample includes the Herschel
Virgo Cluster Survey (HeViCS, Davies et al. 2010) sample fromCorbelli et al. (2012). After eliminating Fornax galaxies, we splitup this sample in Virgo galaxies and field galaxies. Galaxies aredefined to be in the Virgo cluster if they are within twice the virialradius of the Virgo cluster, assumed to be 1.7 Mpc (Fukushige &Makino 2001). This corresponds to ∼ . o at the distance to theVirgo cluster, here assumed to be 16.5 Mpc (Mei et al. 2007). Dis-tances to individual DustPedia galaxies, adopted from the DustPediadatabase, are redshift-independent distance estimates from Hyper-LEDA where available, and redshift-independent estimates from theNASA/IPAC Extragalactic Database (NED) if not (more details onredshift-independent distance estimates by HyperLEDA and NEDcan be found in Makarov et al. 2014 and Steer et al. 2017, respec-tively). If both are unavailable, bulk flow-corrected redshift-derivedvalues provided by NED are used, assuming a Hubble constant of 𝐻 = .
24 km s − Mpc − (Riess et al. 2016). Finally, we createan additional sub-sample of Virgo galaxies located inside the clustervirial radius.H masses for DustPedia galaxies were compiled and homogenisedfrom a wide variety of sources by Casasola et al. (2020). We usetheir M H estimates that were derived using a fixed X CO , which werecalibrate to match the metallicity-dependent prescription used inthis work.We adopt dust masses estimated using a modified blackbodymodel, scaled to match the emissivity of 𝜅 = . m kg at 350 𝜇 m,used to estimate dust masses of the Fornax sample, and a 𝛽 -value of1.790 (Nersesian et al. 2019). http://dustpedia.astro.noa.gr/ To maximise consistency with our Fornax sample, rather thanadopting published metallicities, we apply the DOP16 calibration tothe (extinction corrected) line ratios from De Vis et al. (2019). Weaverage metallicities from all detected star forming regions for eachgalaxy, consistent with the spatially averaged metallicities used toestimate global metallicities in the Fornax sample. Details on theline flux measurements of the DustPedia sample can be found in DeVis et al. (2019) and on the DustPedia website.DustPedia Hi fluxes were compiled from the literature (Casasolaet al., in prep.) and converted to M Hi usingM Hi = . × 𝑓 Hi 𝐷 , (6)where 𝑓 Hi is the compiled Hi flux in Jy km s − and D the bestdistance measurement from Clark et al. (2018) in Mpc. More detailsand references can be found in De Vis et al. (2019).Stellar masses of the DustPedia sample were taken from z0MGS, asdescribed in §3.5. This means that the stellar masses of the DustPediasample are fully consistent with the vast majority of the Fornaxsample, with the exception of only the two objects discussed in §3.5.Thus, as explained above, the dust, H , Hi, and stellar masseswe use here are calculated identically for both our Fornax and theDustPedia comparison sample, and so can be directly compared. There are several other literature samples which have molecular gas,atomic gas, and dust masses available, as well as stellar massesand/or metallicities. These include the samples compiled by Rémy-Ruyer et al. (2014), such as the Dwarf Galaxy Survey (DGS, Maddenet al. 2013), and Key Insights on Nearby Galaxies: a Far-InfraredSurvey with
Herschel (KINGFISH, Kennicutt et al. 2011), the sub-sample of LITTLE THINGS from Cigan et al., in prep., and severalothers. Unfortunately, it has proven impossible to recalibrate thesesamples to rely on the same assumptions as the Fornax and DustPediasamples studied here. Therefore, they were not included in this work.Homogenising these datasets to follow the same assumptions usedhere (as far as is possible) suggests our conclusions above also holdin comparison to these samples.
In order to examine the effect of the Fornax cluster environmenton the gas and dust in galaxies, we construct gas-to-dust ratios forour sample galaxies (listed in Table 2). We also tabulate the offsetbetween the gas-to-dust ratio of each Fornax galaxy and the me-dian gas-to-dust ratio of field galaxies at the same stellar mass, andmetallicity for the nine galaxies for which MUSE data from F3Dis available, (calculated using the DustPedia field galaxy sample).For comparison, we also include the DustPedia Virgo galaxies inour analysis. Total gas-to-dust ratios are shown in Figures 1 and 2,as a function of stellar mass, and metallicity, respectively. Similarly,Figures 3 and 4 show the H -to-dust ratio, and Figures 5 and 6the Hi-to-dust ratio. Fornax galaxies are plotted as diamond-shapedmarkers. Galaxies with regular molecular gas reservoirs are shownin black, and those with disturbed molecular gas reservoirs in red(as classified in Z19). CO upper limits (for which the molecular gasmorphology is unknown) are shown in purple. Virgo galaxies areshown in orange, with galaxies inside the virial radius highlightedwith larger markers. The DustPedia field sample is shown with grey MNRAS , 1–22 (2021) lFoCS + F3D II: low gas-to-dust ratios Table 2.
Estimated gas-to-dust ratios and residuals of galaxies in the sample.Object Total gas/dust res. (M ★ ) res. (Z) M H /dust res. ( 𝑀 ★ ) res. (Z) M Hi /dust res. ( 𝑀 ★ ) res. (Z)- - (dex) (dex) - (dex) (dex) - (dex) (dex)(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)FCC067 110 + − -0.75 - 2 + − -1.97 - 100 + − -0.67 -FCC090 460 + − -0.13 0.02 130 + − -0.26 -0.06 340 + − -0.18 -0.07FCC121 150 + − -0.62 - 25 + − -0.96 - 120 + − -0.62 -FCC167 60 + − -1 -0.65 36 + − -0.8 -0.05 ≤
25 -1.3 -0.55FCC179 50 + − -1.06 -0.74 48 + − -0.68 -0.14 ≤ ≤ -1.95 ≤ -1.41FCC184 70 + − -0.96 -0.62 60 + − -0.58 0.17 ≤ ≤ -1.88 ≤ -1.14FCC235 760 + − ≤ + − ≤ -1.39 - 750 + − + − -0.48 -0.41 90 + − -0.42 -0.37 120 + − -0.63 -0.61FCC282 140 + − -0.64 - 60 + − -0.57 - ≤
79 -0.81 -FCC285 200 + − -0.5 -0.57 ≤ + − ≤ -1.04 ≤ -1.52 170 + − -0.46 -0.53FCC290 50 + − -1.06 -0.72 29 + − -0.9 -0.32 24 + − -1.31 -0.76FCC306 8000 + − ≤ + − ≤ + − + − -0.79 -0.58 37 + − -0.79 -0.51 62 + − -0.91 -0.7FCC312 240 + − -0.42 -0.21 90 + − -0.43 -0.15 150 + − -0.53 -0.32FCC335 450 + − -0.14 - 140 + − -0.23 - ≤ ≤ -0.21 - Notes: : Object name according to the Fornax Cluster Catalogue; : Total gas-to-dust ratio ( (cid:0) M Hi + M H (cid:1) / D); : Total gas-to-dustresidual compared to the DustPedia median as a function of stellar mass (see Figure 1); : Total gas-to-dust residual comparedto the DustPedia median as a function of metallicity (see Figure 2); : H -to-dust ratio; : H -to-dust residual compared to theDustPedia median as a function of stellar mass (see Figure 3); : H -to-dust residual compared to the DustPedia median as afunction of metallicity (see Figure 4); : Hi-to-dust ratio; : Hi-to-dust residual compared to the DustPedia median as a functionof stellar mass (see Figure 5); : Hi-to-dust residual compared to the DustPedia median as a function of metallicity (see Figure6). markers. Upper limits are shown as downward-facing triangles forall samples. The solid, grey line represents the rolling median of theDustPedia field sample, which is calculated using bins with a fixednumber of 10 galaxies, overlapping by half a bin size. The bottompanels in these figures show the residuals of Fornax and Virgo galax-ies compared to this median, which is shown as a black solid line, ora black dashed line where extrapolated.In Figures 1 and 2 we can see that total gas-to-dust ratios in theFornax cluster are low compared to the field, both at fixed stellarmass, and metallicity. This also applies to galaxies in the Virgocluster, especially inside its virial radius. Figures 3 and 4 show thatmolecular gas-to-dust ratios are also decreased in the Fornax clustercompared to the field. However, galaxies in the Virgo cluster showthe opposite result: they are (marginally, but systematically) increasedcompared to the field. Finally, Figures 5 and 6 show that Hi-to-dustratios are decreased in both the Fornax and Virgo clusters, althoughthe difference between the Virgo cluster and the field is smaller ifgalaxies located inside 2 𝑅 vir are included. In the Fornax cluster thereis no significant difference in gas-to-dust ratio between galaxies withregular and disturbed CO reservoirs in any of the Figures.We quantify the results described above by applying Kolmogorov-Smirnov (KS) tests and Anderson-Darling (AD) tests to the residualsof the Fornax and Virgo data compared to DustPedia in all six Figures.Because these tests do not take into account uncertainties, we use aMonte Carlo approach to make sure these are implemented in thefinal result. We perturb each value by a random number drawn froma normal distribution with 𝜇 the measured value and 𝜎 the associateduncertainty. This is done 10 times, after which the 𝜇 and 𝜎 of theresulting distribution of the KS and AD statistics are adopted as thetest results. These are are summarised in Table 3. The KS statistic isbetween 0 and 1, and the closer to 1 it is, the less likely it is that bothsamples are drawn from the same distribution. The AD test returnsa statistic (referred to as the A2 statistic) as well as corresponding critical values at a discrete number of confidence intervals. The null-hypothesis that both samples are drawn from the same distributioncan be rejected with a certain probability if the difference betweenthe A2 statistic and the critical level is positive at the correspondingconfidence level. In Table 3 we report the difference between the A2statistic and the critical value at 0.1%.Both tests return similar results in all cases. In almost all cases, thenull-hypothesis that the Fornax and Virgo samples are drawn fromthe same distribution as the DustPedia field sample can be rejected.The exception is the Fornax Hi mass as a function of stellar mass,for which the spread in both statistics is too large to draw strongconclusions. In Figure 7 we show gas-to-dust ratios as a function of projectedcluster-centric distance for both clusters, which are represented by thesame colours as in previous Figures. These distances are normalisedby each cluster’s virial radius, indicated by a black dashed line.There is no clear relation between gas-to-dust ratio and cluster-centricradius for either of the clusters, although there is a possible increasein gas-to-dust ratio with cluster-centric radius in the Virgo cluster(a Kendall’s Tau test returns a tau statistic of 0.23 and a p-valueof 0.04). In the Virgo cluster, there is evidence that Hi deficienciesdecrease with cluster-centric radius (e.g. Haynes & Giovanelli 1986,Gavazzi et al. 2005), which could explain this possible trend. If Hideficiencies were the main driver of the decreased gas-to-dust ratiosin the Fornax cluster, we might expect to also see such a trend in thiscluster. However, no such trend is seen in Figure 7, although we onlyhave a small sample of Fornax cluster objects.
MNRAS , 1–22 (2021)
N. Zabel et al. M H I + H / M du s t DustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO ULDustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO UL10 M (M ) R e s i du a l s Figure 1.
Upper panel : Total gas-to-dust (Hi + H ) ratios in the Fornax cluster compared to those in the DustPedia (Davies et al. 2017) field (grey) and Virgo(orange) samples. Virgo galaxies inside 𝑅 vir are highlighted with larger markers. Fornax galaxies are indicated with diamond-shaped markers, in black forgalaxies with regular CO emission and in red for galaxies with disturbed CO emission as classified in Z19 (upper limits are shown in purple). NGC1436 (see §6)is indicated with a filled diamond-shaped marker. Upper limits are indicated with downward-facing triangles for all samples. The solid grey line indicates therolling median of the DustPedia field sample Lower panel : Residuals of gas-to-dust ratios in Fornax and Virgo cluster galaxies compared to the rolling medianof DustPedia field galaxies, shown as a solid line, or a dashed line where extrapolated. Note that, for the purpose of visibility, the bottom panel is set to showvalues between -1.5 and 1.5, which causes the strongly discrepant galaxy FCC067 (NGC1351A) to fall off the plot in some of the following figures. Accordingto KS tests, Fornax galaxies and Virgo galaxies inside 𝑅 vir have systematically lower gas-to-dust ratios than field galaxies at fixed stellar mass (see Table 3). M H I + H / M du s t DustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO ULDustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO UL7.8 8.0 8.2 8.4 8.6 8.8 9.0 9.2
12 + log(O/H) R e s i du a l s Figure 2.
Similar to Figure 1, showing total gas-to-dust ratios as a function of metallicity for a sub-sample of Fornax galaxies for which MUSE data is availablefrom F3D (see Table 1). Fornax cluster galaxies and Virgo galaxies inside 𝑅 vir have systematically lower gas-to-dust ratios than field samples.MNRAS000
Similar to Figure 1, showing total gas-to-dust ratios as a function of metallicity for a sub-sample of Fornax galaxies for which MUSE data is availablefrom F3D (see Table 1). Fornax cluster galaxies and Virgo galaxies inside 𝑅 vir have systematically lower gas-to-dust ratios than field samples.MNRAS000 , 1–22 (2021) lFoCS + F3D II: low gas-to-dust ratios M H / M du s t DustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO ULDustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO UL10 M (M ) R e s i du a l s Figure 3.
Similar to Figure 1, but showing molecular gas-to-dust ratios plotted as a function of stellar mass. Fornax cluster galaxies have systematically lowermolecular gas-to-dust ratios than field galaxies, whereas Virgo galaxies inside 𝑅 vir show in creased H -to-dust ratios. M H / M du s t DustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO ULDustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO UL7.8 8.0 8.2 8.4 8.6 8.8 9.0 9.2
12 + log(O/H) R e s i du a l s Figure 4.
Similar to Figure 2, but showing molecular gas-to-dust ratios as a function of metallicity. Molecular gas-to-dust ratios of Fornax galaxies aresystematically lower than those of field galaxies at fixed metallicity, while those inside the virial radius of the Virgo cluster are in creased. To better understand whether the decreased H -to-dust ratios in theFornax cluster are mostly the result of increased dust fractions, ordecreased H fractions, we show these as a function of stellar mass in Figure 8 (Figure 8a shows dust fractions and Figure 8b H fractions).Markers and symbols are the same as in previous figures. The dustcontent in most Fornax galaxies is normal to low compared to theDustPedia field sample, with the exception of a few dwarf galaxies,and NGC1365 at the high-mass end. The scatter in the H fractions in MNRAS , 1–22 (2021) N. Zabel et al. M H I / M du s t DustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO ULDustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO UL10 M (M ) R e s i du a l s Figure 5.
Similar to Figure 1, but showing Hi-to-dust ratios as a function of stellar mass. Fornax galaxies and Virgo galaxies inside the virial radius havesystematically lower Hi-to-dust ratios than galaxies in the field at fixed stellar mass. A possible decrease is also seen in Virgo galaxies between 1 and 2 𝑅 vir . M H I / M du s t DustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO ULDustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO UL7.8 8.0 8.2 8.4 8.6 8.8 9.0 9.2
12 + log(O/H) R e s i du a l s Figure 6.
Similar to Figure 2, but showing Hi-to-dust ratios as a function of metallicity. These are systematically lower in Fornax cluster galaxies and Virgogalaxies inside 𝑅 vir compared to field galaxies at fixed metallicity.MNRAS000
Similar to Figure 2, but showing Hi-to-dust ratios as a function of metallicity. These are systematically lower in Fornax cluster galaxies and Virgogalaxies inside 𝑅 vir compared to field galaxies at fixed metallicity.MNRAS000 , 1–22 (2021) lFoCS + F3D II: low gas-to-dust ratios Table 3.
Results of a Monte Carlo analysis of KS and AD tests applied to the residuals (data of the respective clusters comparedto the rolling median of the DustPedia field sample) in Figures 1 through 6.Fornax Virgo (inside 𝑅 vir ) Virgo (inside 2 𝑅 vir )Figure x-axis y-axis D statistic A2 - crit. D statistic A2 - crit. D statistic A2 - crit.(1) (2) (3) (4) (5) (6) (7) (8) (9)1 M ★ M Hi + H ± ± ± ± ± ±
63 M ★ M H ± ± ± ± ± ±
95 M ★ M Hi ± ±
12 0.99 ± ± ± ±
122 12 + log(O/H) M Hi + H ± ± ± ± ± ±
44 12 + log(O/H) M H ± ± ± ± ± ±
46 12 + log(O/H) M Hi ± ± ± ± ± ± Notes: : reference to the Figure showing the data the KS test is applied to; : x-axis of that Figure; : y-axis of that Figure;
4, 6, 8 :D-statistic resulting from the KS test applied to the Fornax, Virgo inside 𝑅 vir , and Virgo inside 2 𝑅 vir data, respectively, includinguncertainties from a Monte Carlo analysis;
5, 7, 9 : difference between the A2 statistic and critical value at the 0.1% confidencelevel resulting from an Anderson-Darling test applied to the Fornax, Virgo inside 𝑅 vir , and Virgo inside 2 𝑅 vir data respectively; Cluster-centric distance / R vir10 M H + H I / M du s t VirgoFornax, regular CO Fornax, disturbed COFornax, CO UL
Figure 7.
Total gas-to-dust ratios as a function of relative (projected) distance to the cluster centre are shown with colours and markers similar to those in Figure3. The virial radius is indicated with a black dashed line. No significant gradient in gas-to-dust fractions is observed in any of the clusters, although there ispossibly a slight increase in gas-to-dust radio with cluster-centric radius in the Virgo cluster (a Kendall’s Tau test returns a tau statistic of 0.23 and a p-value of0.04).
Figure 8b is similar to that in Figure 8a, however there is a more pro-nounced systematic offset towards lower H fractions in the Fornaxcluster compared to the field. This suggests the difference seen in theFornax cluster is driven by the lower H content of these galaxies. InVirgo dust fractions are normal to slightly decreased inside the virialradius, whereas H fractions are increased, especially in low-stellarmass galaxies. Outside 𝑅 vir no difference with the DustPedia fieldsample is seen. A different way of visualising what fraction of metals are locked upin dust relative to gas, is to show dust-to-metal ratios: the ratio of thedust mass and the total metal-mass in the galaxy. Since dust consistsexclusively of metals, the ratio of the total metal mass and the dustmass is a measure of how much of the metal content in the ISM is locked up in dust grains. To estimate these, we use the prescriptionby De Vis et al. (2019): 𝑀 𝑍 ≡ 𝑓 𝑍 × 𝑀 𝑔 + 𝑀 𝑑 , (7)where 𝑀 𝑔 is the total gas mass, 𝑀 𝑑 the dust mass, and 𝑓 𝑍 is thefraction of metals by mass calculated using 𝑓 𝑍 = . × O/H . (8)The factor 27.36 comes from the assumption that the Solar metallicityis 12 + log(O/H) = 8.69, and the Solar metal mass fraction Z = 0.0134(Asplund et al. 2009). Using this method we should keep in mindthat, in reality, oxygen abundance does not scale directly to the totalmetal mass, as the oxygen-to-total metal ratio can vary depending onstar formation history and stellar metallicity. Similarly, the fractionof oxygen locked up in dust grains does not scale directly with theamount of metals locked up into them, as this depends on the dust MNRAS , 1–22 (2021) N. Zabel et al. composition. Keeping in mind these caveats, the result is shown inFigure 9; in Figure 9a as a function of stellar mass, and in Figure 9bas a function of metallicity. Markers and symbols are the same as inprevious figures. In both panels, dust-to-metal ratios in the Fornaxcluster are significantly increased compared to those in the field. Thisimplies that a relatively large fraction of the metals in Fornax galaxiesare locked up in dust. A similar, though less significant, difference isseen in Virgo galaxies inside 𝑅 vir . There is no significant differencebetween Virgo galaxies outside 𝑅 vir and the DustPedia field sample. Any (radial) trend in the gas-to-dust ratio could give us a hint asto what process might be responsible for the low gas-to-dust (total,H , and Hi) ratios observed (e.g. the outside-in removal of dust). Toinvestigate whether there might be any radial H -to-dust gradientspresent in these Fornax galaxies with low gas-to-dust ratios, andwhether the H -to-dust ratio differs between star forming and morepassive regions, we study the spatially resolved H -to-dust ratio inNGC1436. While it would be preferred to study resolved total gas-to-dust ratios, this is not feasible, as the ATCA beam is >
20 times largerthan the ALMA beam (and a similar size to the entire molecular discof this object). This will be done in a future paper (Loni et al., inprep.), after completion of the MeerKAT Fornax Survey (Serra et al.2016).NGC1436 is an almost face-on, flocculent spiral (Figure 10, seealso Z19 and Z20), making it an ideal candidate to study resolvedproperties of cluster galaxies in more detail, and currently the onlygalaxy in our sample for which this is possible. NGC1436 is highlyHi-deficient, and has a high H -to-Hi ratio (Loni et al. 2021). Ithas been suggested that this galaxy is undergoing a morphologicaltransition into a lenticular, based on the absence of a clear spiralstructure in its outer regions (Raj et al. 2019). From Figures 1 through5, where NGC1436 is highlighted with a filled diamond symbol, wecan see that it has a decreased gas-to-dust ratio, and that this is partlydriven by a decreased H -to-dust ratio, in addition to a decreasedHi-to-dust ratio.To ensure we have as complete as possible information on theCO emission, we combine our AlFoCS data with deep observationsfrom the ALMA archive (project ID: 2017.1.00129.S, PI: Kana Mo-rokuma). As part of this program NGC1436 was observed on 30November 2017 using the Morita array (see Morokuma-Matsui et al.2019 for a description of the data for NGC1316 from this survey).Its primary beam size is ∼ (cid:48)(cid:48) at ∼
115 GHz, and the total areacovered is ∼
182 square arcseconds. The spectral window coveringthe CO(1-0) was centred on 114.756 GHz, with a bandwidth of1.875 GHz, covering 3840 channels. The spectral resolution is 5.06km s − .We reduced this data using the casa pipeline (version 5.4.0-68,McMullin et al. 2007) before combining it with our own data usingthe task concat. We then imaged the resulting data set by cleaningit interactively, using the task tclean (Högbom 1974). A Briggsweighting scheme was used (Briggs 1995) with a robust parameterof 0.5. The pixel size in the final datacube is 0 . (cid:48)(cid:48)
5, and the velocityresolution is 10 km s − . The synthesised beam size is ∼ × (cid:48)(cid:48) .The sensitivity reached is ∼ 𝜇 m resolution (FWHM ≈ (cid:48)(cid:48) ), the highest resolution of the range of Herschel images usedto estimate its dust properties.The resulting images are shown in Figures 10 and 11. In the upper-left panel of Figure 10 the extent of the CO (red) and dust (orange)emission is shown (both are clipped to the 3 𝜎 level), overplotted on anoptical ugb -image from the FDS. In the upper-right panel H -to-dustratios within the galaxy are shown. The bottom-left and bottom-rightpanels show the H and dust surface densities, respectively. Figure11 shows the radial profile of the H -to-dust ratio, correspondingto the top-right panel in Figure 10. Each marker corresponds to theaverage H -to-dust ratio in an elliptical annulus (the sum of the ratioin each pixel divided by the total area of the annulus) at each radius,where the radius corresponds to the semi-major axis of the annulus.There are small-scale variations in the H -to-dust ratio. Peaks inthe H -to-dust ratio appear to mainly correlate with peaks in the H surface density. There is an increase in H -to-dust ratio with radius,after which it drops off sharply at 𝑅 ≈ . -to-dust ratios. It could also mean that dust is moreeasily removed from the outer parts of the disc than molecular gas,causing the gradient observed.The average H -to-dust ratio does not exceed ∼
20 at any radius.This suggests that molecular gas, and likely also dust, can be affectedby environment even in the inner parts of galaxies. A comparisonwith the star formation dominated H 𝛼 map from MUSE (from F3D,see §2.5) shows no clear correlation between star forming regionsand variations in the H -to-dust ratio. In §5.1 we have shown that gas-to-dust ratios in Fornax galaxies aresuppressed compared to a field comparison sample at fixed stellarmass and metallicity. Decreased total gas-to-dust ratios might beexpected in clusters as a result of stripping and truncation of Hi discs,which typically have scale lengths much larger than H . Indeed, this isobserved both in Fornax cluster galaxies and Virgo galaxies (Figures1 and 2). The low gas-to-dust ratios in the Fornax cluster are partlydriven by decreased Hi-to-dust ratios compared to field galaxies atfixed mass (Figures 5 and 6). However, these low Hi-to-dust ratios arenot the full story. H -to-dust ratios are also significantly decreasedin the Fornax cluster (Figures 3 and 4). In Figure 8 we can see that,while dust fractions are decreased in the Fornax cluster compared tofield galaxies at fixed stellar mass (panel a), molecular gas fractionsare even more strongly decreased (panel b). Broadly, there are threeways in which we could end up with the H -to-dust ratios observed:(i) H is destroyed/removed more efficiently than dust,(ii) both dust and H are destroyed, but the dust reservoir is re-plenished more efficiently than the H reservoir,(iii) the physics of the ISM in these cluster galaxies is unusualsuch that “standard” observational probes fail to return accurate H -to-dust ratios. MNRAS000
20 at any radius.This suggests that molecular gas, and likely also dust, can be affectedby environment even in the inner parts of galaxies. A comparisonwith the star formation dominated H 𝛼 map from MUSE (from F3D,see §2.5) shows no clear correlation between star forming regionsand variations in the H -to-dust ratio. In §5.1 we have shown that gas-to-dust ratios in Fornax galaxies aresuppressed compared to a field comparison sample at fixed stellarmass and metallicity. Decreased total gas-to-dust ratios might beexpected in clusters as a result of stripping and truncation of Hi discs,which typically have scale lengths much larger than H . Indeed, this isobserved both in Fornax cluster galaxies and Virgo galaxies (Figures1 and 2). The low gas-to-dust ratios in the Fornax cluster are partlydriven by decreased Hi-to-dust ratios compared to field galaxies atfixed mass (Figures 5 and 6). However, these low Hi-to-dust ratios arenot the full story. H -to-dust ratios are also significantly decreasedin the Fornax cluster (Figures 3 and 4). In Figure 8 we can see that,while dust fractions are decreased in the Fornax cluster compared tofield galaxies at fixed stellar mass (panel a), molecular gas fractionsare even more strongly decreased (panel b). Broadly, there are threeways in which we could end up with the H -to-dust ratios observed:(i) H is destroyed/removed more efficiently than dust,(ii) both dust and H are destroyed, but the dust reservoir is re-plenished more efficiently than the H reservoir,(iii) the physics of the ISM in these cluster galaxies is unusualsuch that “standard” observational probes fail to return accurate H -to-dust ratios. MNRAS000 , 1–22 (2021) lFoCS + F3D II: low gas-to-dust ratios M (M ) M du s t / M DustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO ULDustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO UL (a) M (M ) M H / M DustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO ULDustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO UL (b)
Figure 8.
Dust- and molecular gas-to-stellar mass fractions (panel a and panel b, respectively) in the Fornax cluster compared to the field and the Virgo cluster.Samples and symbols are the same as in previous figures. While both are low, molecular gas-to-stellar mass fractions in the Fornax cluster are especially lowcompared to the field and the Virgo cluster. The dust content in the Virgo cluster is normal to slightly decreased, whereas molecular gas-to-stellar mass fractionsare normal to slightly increased, in particular at low stellar mass. is destroyed/removed more efficiently than dust Molecular gas could be destroyed/removed more efficiently than dustby the cluster environment if our sample galaxies had strong radialH -to-dust gradients, and their gas discs were truncated from theoutside in (i.e. by ram pressure stripping). If this is the case, we areseeing the “relic” of a gas/dust reservoir that was larger before thegalaxies fell into the cluster. This could alter the total H -to-dustmass ratio we would measure. For example, Bekki (2014) shows thatram pressure stripping can lead to more centrally concentrated star formation. Studies of nearby galaxies and galaxies in the Virgo clusterare inconclusive as to whether such radial molecular gas-to-dustratios are observed. Several studies suggest that observed moleculargas-to-dust gradients are driven by a metallicity gradient (i.e. a radialchange in X CO , Bendo et al. 2010; Magrini et al. 2011; Pappalardoet al. 2012). Therefore, it is unclear whether an actual (non-X CO -driven) gradient in gas-to-dust ratio is also present. Cortese et al.(2010) find that the dust discs of Hi-deficient galaxies are truncatedas well as the Hi discs. Corbelli et al. (2012) find that gas-to-dustratios decrease as Hi-deficiency increases, but only up to a certain MNRAS , 1–22 (2021) N. Zabel et al. M (M ) M du s t / M Z DustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO ULDustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO UL (a)
12 + log(O/H) M du s t / M Z DustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO ULDustPedia (field)DustPedia (Virgo)DustPedia (Virgo, < R vir ) Fornax, regular COFornax, disturbed COFornax, CO UL (b)
Figure 9.
Dust-to-metal ratios as a function of stellar mass (panel a) and metallicity (panel b). Samples and symbols are the same as in previous figures.Dust-to-metal ratios in Fornax cluster galaxies are significantly higher than those in field galaxies of similar mass and metallicity, which means that a relativelylarge fraction of the metals in these galaxies are locked up in dust grains. Virgo galaxies inside 𝑅 vir also show a slight, but less significant, increase in dust-to-metalratio at fixed stellar mass. This difference is less clear at fixed metallicity. deficiency threshold, after which they remain constant. Hi-deficiencyis defined as the difference between the observed Hi mass and thatexpected in an isolated galaxy. This is because in highly disturbedgalaxies both gas and dust are stripped from the inner parts of thegalaxies. However, they also find stronger dust than H deficiencies inthese galaxies. This suggests that, although Hi-deficient galaxies havelower gas-to-dust ratios, and both H and dust can be stripped fromtheir inner parts, H -to-dust ratios in these galaxies remain constant,or even increase slightly. This is also observed by Cortese et al.(2016) and in Virgo galaxies in this work, and is discussed further in §7.2. Since truncation of the gas/dust disc is concluded to result in an in crease in H -to-dust ratios in these two studies, which is also seenhere in the Virgo cluster, it is unlikely to be the explanation for theH -to-dust ratios observed in Fornax. Observations of the moleculargas and dust in FCC167 (NGC1380, included in this sample) show anested ISM, in which the dust extends further out than the moleculargas (Viaene et al. 2019). This could indeed mean that the moleculargas in this galaxy was stripped more severely than the dust. However,the molecular gas disc in this galaxy is extremely truncated (e.g. Z19,Viaene et al. 2019), and it is unclear how representative this is for the MNRAS000
Dust-to-metal ratios as a function of stellar mass (panel a) and metallicity (panel b). Samples and symbols are the same as in previous figures.Dust-to-metal ratios in Fornax cluster galaxies are significantly higher than those in field galaxies of similar mass and metallicity, which means that a relativelylarge fraction of the metals in these galaxies are locked up in dust grains. Virgo galaxies inside 𝑅 vir also show a slight, but less significant, increase in dust-to-metalratio at fixed stellar mass. This difference is less clear at fixed metallicity. deficiency threshold, after which they remain constant. Hi-deficiencyis defined as the difference between the observed Hi mass and thatexpected in an isolated galaxy. This is because in highly disturbedgalaxies both gas and dust are stripped from the inner parts of thegalaxies. However, they also find stronger dust than H deficiencies inthese galaxies. This suggests that, although Hi-deficient galaxies havelower gas-to-dust ratios, and both H and dust can be stripped fromtheir inner parts, H -to-dust ratios in these galaxies remain constant,or even increase slightly. This is also observed by Cortese et al.(2016) and in Virgo galaxies in this work, and is discussed further in §7.2. Since truncation of the gas/dust disc is concluded to result in an in crease in H -to-dust ratios in these two studies, which is also seenhere in the Virgo cluster, it is unlikely to be the explanation for theH -to-dust ratios observed in Fornax. Observations of the moleculargas and dust in FCC167 (NGC1380, included in this sample) show anested ISM, in which the dust extends further out than the moleculargas (Viaene et al. 2019). This could indeed mean that the moleculargas in this galaxy was stripped more severely than the dust. However,the molecular gas disc in this galaxy is extremely truncated (e.g. Z19,Viaene et al. 2019), and it is unclear how representative this is for the MNRAS000 , 1–22 (2021) lFoCS + F3D II: low gas-to-dust ratios h m s s s s -35°50'00"30"51'00"30"52'00" RA (J2000) D ec ( J ) H (M pc ) h m s s s s RA (J2000) dust (M pc ) M H / M dust -35°50'00"30"51'00"30"52'00" D ec ( J ) H dust Figure 10.
Resolved H and dust properties in the flocculent spiral NGC1436. Upper left panel: optical ugb -image with the extent of the H (red) and dust(orange) emission overplotted. Upper right panel: resolved H -to-dust ratios. Lower-left panel: H surface density at the resolution of the PACS 100 𝜇 memission. Bottom-right panel : dust surface density at the PACS 100 𝜇 m resolution from ppmap. The beam of the CO observations is shown in the bottom-leftcorner of the bottom-left panel. There is no clear spatial trend in the H -to-dust ratios in this galaxy. rest of the sample. The observed radial increase in H -to-dust ratioin NGC1436 (see Figure 11) suggests that the stripping of the outerparts of the disc could indeed result in lower integrated H -to-dustratios.Figure 12 shows a histogram of the ratio of R CO and effectiveradius R 𝑒 (as estimated from the FDS, Peletier et al. 2020; Venholaet al. 2018; Iodice et al. 2019a; Raj et al. 2019) in the Fornax clus-ter (crimson) compared to this ratio in a field sample of gas-richearly-type galaxies (ETGs) from ATLAS (Davis et al. 2013) andnearby spiral galaxies from the Berkeley-Illinois-Maryland Associ-ation Survey of Nearby Galaxies (BIMA SONG, Regan et al. 2001)in lilac. Although the numbers are small, there is no evidence thatFornax cluster galaxies have smaller R CO /Re than the field sample (aKS test is not able to reject the null-hypothesis that the Fornax and ATLAS /BIMA SONG samples are drawn from the same distribu-tion, D=0.29, p=0.5 ∼ . 𝜎 ).In order to more generally test whether truncation could play asignificant role in creating low H -to-dust ratios, we create a toymodel consisting of an exponential gas/dust disc, with a moleculargas-to-dust ratio that increases linearly outward. We then explorethe parameter space describing the shape of the exponential discand the molecular gas-to-dust gradient resulting in the observed H deficiencies (see table 3 in Z19) and molecular gas-to-dust ratios,using a Markov chain Monte Carlo approach. We fix H deficienciesand molecular gas-to-dust ratios at the more conservative observedvalues of 1 and 200, respectively, before truncation, and -1 and 100,respectively, after truncation (see Table 2 and Table 3 in Z19). Trun-cation is simulated by removing a percentage of the disc from theoutside in. This percentage is a free parameter. Although, inevitably, MNRAS , 1–22 (2021) N. Zabel et al.
Radius [kpc] M H / M D Figure 11.
Radial profile of the H -to-dust ratio in NGC1436. It increases withradius, and drops off sharply at 𝑅 ≈ . -to-dust ratios. It could also imply that dust is more easily removed fromthe outer parts of the disc than molecular gas. there is a high degree of degeneracy in the parameters describing theexponential disc, it is clear that we would need very steep H -to-dustgradients and severe truncation ( >
80% of the gas/dust disc) for this toexplain the range of H -to-dust mass ratios and H -deficiencies ob-served. Therefore, based on this toy model, truncation in combinationwith a H -to-dust gradient is unlikely to be the sole explanation ofthe decreased H -to-dust ratios observed. In reality, the spatial extentof the dust may exceed that of the molecular gas, as, for example, ob-served the nearby spiral galaxy NGC2403 (Bendo et al. 2010). Thisimplies that it would be removed by ram pressure stripping beforethe molecular gas, as suggested by Cortese et al. (2016). This impliesthat it would be even more difficult to remove significant amounts ofmolecular gas compared to dust in this way. reservoir If dust is created at a higher rate than H in the star formationcycle, this could result in decreased gas-to-dust ratios. Both gas anddust are observed in supernova remnants (see Matsuura 2017 for areview). Recent, in-depth studies of supernova remnants have shownthat large amounts of dust can be produced in supernovae (Dunneet al. 2003; De Looze et al. 2017, 2019; Cigan et al. 2019; Priestleyet al. 2019). Gas, on the other hand, is produced in relatively smallquantities, resulting in gas-to-dust ratios in supernova remnants thatare much smaller (5 - 10 times) than typical ISM values (Owen& Barlow 2015; Matsuura et al. 2017; Arias et al. 2018; Priestleyet al. 2019). Therefore, ongoing star formation in combination withstarvation could be (co-)responsible for the decrease in H -to-dustratios observed, possibly sped up by the active removal of gas anddust. Several galaxies in the Fornax cluster have slightly increasedstar formation efficiencies. Furthermore, starbursts can already haveoccurred in the pre-processing phase, as galaxies start to interact withthe intracluster medium, or in infalling groups (Pinna et al. 2019a,b),although this has mainly been observed in early-type galaxies. Whilethis does not mean their SFRs or sSFRs are also increased (in fact, thevast majority of them lie below the star formation main sequence, seeZ20, so we do not expect them to produce more dust than “average” galaxies), it is the H -to-dust ratio we are interested in. This can beincreased as a result of this preferential dust production.An inspection of gas-to-dust ratios in the Fornax cluster as a func-tion of depletion time shows that they decrease with increasing de-pletion time: galaxies currently undergoing starbursts, often showingdisturbed molecular gas reservoirs, still have relatively high gas-to-dust ratios. This could mean two things: either this is not a goodexplanation, or it takes a while for the dust to accumulate, and theeffect is only noticeable after the star burst phase is over. The latterexplanation is supported by the fact that old stellar populations arealso a significant source of interstellar dust (while their gas feed-back is insufficient to keep star formation going, Matsuura et al.2009; Boyer et al. 2012; Höfner & Olofsson 2018). Thus, as gas anddust are stripped and depleted, but star formation has not yet beenquenched (the galaxy is experiencing “starvation”, Larson et al. 1980)the dust produced by older stars and supernovae accumulates withinthe galaxy. Even without ongoing star formation, the dust mass canstill continue to increase through interstellar grain growth (assumingsome residual cool gas is present in the galaxy to shield the dust fromsputtering in the hot intracluster medium, Hirashita 2012; Mattssonet al. 2014; Zhukovska 2014; Galliano et al., submitted). This sug-gests that the gas-to-dust ratio could even continue to decrease afterthe star formation has been quenched.Many galaxies in the Fornax cluster are deficient in Hi, or evencompletely devoid of it (at the survey sensitivity limit of ∼ × 𝑀 (cid:12) , Loni et al. 2021). If the main formation of H is throughthe condensation of Hi, this could mean that the production of H is slower than in non-Hi deficient galaxies. However, there also is asignificant fraction of Hi-deficient galaxies in the Virgo cluster (Yoonet al. 2017), so if this was the main explanation, we might expect tosee similarly low molecular gas-to-dust ratios in the Virgo cluster.Moreover, most of these galaxies have high rather than low H -to-Hiratios, which does not support this theory (Loni et al. 2021). It is possible that the radiation field in the cluster destroys CO (but notH ), leading us to underestimate H -to-dust ratios. In the Galaxy, theradiation field only affects the outermost layers of molecular clouds(e.g. Maloney & Black 1988). However, if the radiation field is moreintense, it can affect the entire cloud. In fact, besides metallicity,the strength of the FUV radiation field is the most important factordetermining X CO e.g. Bisbas et al. 2015 and references therein). Itis possible that X-rays from the hot intracluster medium (e.g. Joneset al. 1997) have a similar effect. However, it would be difficult todestroy CO while leaving dust grains intact. Moreover, in that casewe might expect to see a similar or stronger decrease in H -to-dustratios in the more massive Virgo cluster, which is not observed.An alternative factor that could be affecting our measurements isthe dust composition. Large silicate grains are not as easily destroyedby the radiation field, which means that H might be destroyed whileleaving a large fraction of the dust unaffected. Indeed, in their detailedanalysis of the dust content of FCC167, Viaene et al. (2019) find thatalmost no small grains are present in the dust reservoir of this galaxy.Only larger, self-shielding grains survive the effects of the clusterenvironment. Another possibility is that larger grains are broken upby the radiation field, resulting in a relatively high fraction of smalldust grains. If we then assume a standard dust composition with alower fraction of small grains to estimate the dust mass, we could beoverestimating the dust mass. It seems, however, more likely that sucha radiation field would continue to destroy bonds until no significant MNRAS000
80% of the gas/dust disc) for this toexplain the range of H -to-dust mass ratios and H -deficiencies ob-served. Therefore, based on this toy model, truncation in combinationwith a H -to-dust gradient is unlikely to be the sole explanation ofthe decreased H -to-dust ratios observed. In reality, the spatial extentof the dust may exceed that of the molecular gas, as, for example, ob-served the nearby spiral galaxy NGC2403 (Bendo et al. 2010). Thisimplies that it would be removed by ram pressure stripping beforethe molecular gas, as suggested by Cortese et al. (2016). This impliesthat it would be even more difficult to remove significant amounts ofmolecular gas compared to dust in this way. reservoir If dust is created at a higher rate than H in the star formationcycle, this could result in decreased gas-to-dust ratios. Both gas anddust are observed in supernova remnants (see Matsuura 2017 for areview). Recent, in-depth studies of supernova remnants have shownthat large amounts of dust can be produced in supernovae (Dunneet al. 2003; De Looze et al. 2017, 2019; Cigan et al. 2019; Priestleyet al. 2019). Gas, on the other hand, is produced in relatively smallquantities, resulting in gas-to-dust ratios in supernova remnants thatare much smaller (5 - 10 times) than typical ISM values (Owen& Barlow 2015; Matsuura et al. 2017; Arias et al. 2018; Priestleyet al. 2019). Therefore, ongoing star formation in combination withstarvation could be (co-)responsible for the decrease in H -to-dustratios observed, possibly sped up by the active removal of gas anddust. Several galaxies in the Fornax cluster have slightly increasedstar formation efficiencies. Furthermore, starbursts can already haveoccurred in the pre-processing phase, as galaxies start to interact withthe intracluster medium, or in infalling groups (Pinna et al. 2019a,b),although this has mainly been observed in early-type galaxies. Whilethis does not mean their SFRs or sSFRs are also increased (in fact, thevast majority of them lie below the star formation main sequence, seeZ20, so we do not expect them to produce more dust than “average” galaxies), it is the H -to-dust ratio we are interested in. This can beincreased as a result of this preferential dust production.An inspection of gas-to-dust ratios in the Fornax cluster as a func-tion of depletion time shows that they decrease with increasing de-pletion time: galaxies currently undergoing starbursts, often showingdisturbed molecular gas reservoirs, still have relatively high gas-to-dust ratios. This could mean two things: either this is not a goodexplanation, or it takes a while for the dust to accumulate, and theeffect is only noticeable after the star burst phase is over. The latterexplanation is supported by the fact that old stellar populations arealso a significant source of interstellar dust (while their gas feed-back is insufficient to keep star formation going, Matsuura et al.2009; Boyer et al. 2012; Höfner & Olofsson 2018). Thus, as gas anddust are stripped and depleted, but star formation has not yet beenquenched (the galaxy is experiencing “starvation”, Larson et al. 1980)the dust produced by older stars and supernovae accumulates withinthe galaxy. Even without ongoing star formation, the dust mass canstill continue to increase through interstellar grain growth (assumingsome residual cool gas is present in the galaxy to shield the dust fromsputtering in the hot intracluster medium, Hirashita 2012; Mattssonet al. 2014; Zhukovska 2014; Galliano et al., submitted). This sug-gests that the gas-to-dust ratio could even continue to decrease afterthe star formation has been quenched.Many galaxies in the Fornax cluster are deficient in Hi, or evencompletely devoid of it (at the survey sensitivity limit of ∼ × 𝑀 (cid:12) , Loni et al. 2021). If the main formation of H is throughthe condensation of Hi, this could mean that the production of H is slower than in non-Hi deficient galaxies. However, there also is asignificant fraction of Hi-deficient galaxies in the Virgo cluster (Yoonet al. 2017), so if this was the main explanation, we might expect tosee similarly low molecular gas-to-dust ratios in the Virgo cluster.Moreover, most of these galaxies have high rather than low H -to-Hiratios, which does not support this theory (Loni et al. 2021). It is possible that the radiation field in the cluster destroys CO (but notH ), leading us to underestimate H -to-dust ratios. In the Galaxy, theradiation field only affects the outermost layers of molecular clouds(e.g. Maloney & Black 1988). However, if the radiation field is moreintense, it can affect the entire cloud. In fact, besides metallicity,the strength of the FUV radiation field is the most important factordetermining X CO e.g. Bisbas et al. 2015 and references therein). Itis possible that X-rays from the hot intracluster medium (e.g. Joneset al. 1997) have a similar effect. However, it would be difficult todestroy CO while leaving dust grains intact. Moreover, in that casewe might expect to see a similar or stronger decrease in H -to-dustratios in the more massive Virgo cluster, which is not observed.An alternative factor that could be affecting our measurements isthe dust composition. Large silicate grains are not as easily destroyedby the radiation field, which means that H might be destroyed whileleaving a large fraction of the dust unaffected. Indeed, in their detailedanalysis of the dust content of FCC167, Viaene et al. (2019) find thatalmost no small grains are present in the dust reservoir of this galaxy.Only larger, self-shielding grains survive the effects of the clusterenvironment. Another possibility is that larger grains are broken upby the radiation field, resulting in a relatively high fraction of smalldust grains. If we then assume a standard dust composition with alower fraction of small grains to estimate the dust mass, we could beoverestimating the dust mass. It seems, however, more likely that sucha radiation field would continue to destroy bonds until no significant MNRAS000 , 1–22 (2021) lFoCS + F3D II: low gas-to-dust ratios R CO / R e024681012 F r e qu e n c y ATLAS + BIMA SONGFornax Figure 12.
Histogram of R CO /Re in the Fornax cluster (crimson) comparedto a field sample consisting of gas-rich early-type galaxies from ATLAS (Davis et al. 2013) and spiral galaxies from BIMA SONG (Regan et al.2001), shown in lilac. Fornax cluster galaxies do not have more significantlytruncated CO than the field galaxies (a KS test is not able to reject the null-hypothesis that the Fornax and ATLAS /BIMA SONG samples are drawnfrom the same distribution, D=0.29, p=0.5 ∼ . 𝜎 ). amount of dust is left. Moreover, this contradicts the observations ofFCC167 described above.Furthermore, the carbon-to-silicate ratio of the dust grains canplay a role. Due to environmental conditions and typical grain sizes,silicate grains can have longer lifetimes ( ∼ < in sensitive observations obtained as part of ATLAS ).Suggested explanations for this observation include the majority ofthe gas being warm or hot rather than cold, the presence of a relativelylarge fraction of CO-dark gas, or a recent merger with a metal-poordwarf galaxy. There could be a larger fraction of warm/hot gas presentin Fornax galaxies if the gas at larger scale-heights is affected by thesurrounding hot intracluster medium. However, the molecular gas isdense and centrally located, so it would be difficult to explain thedecreased H -to-dust ratios in this way. Furthermore, there is noobvious reason why Fornax galaxies would contain more CO-darkgas than field galaxies (at fixed stellar mass), as their metallicitiesare average or even high compared to the DustPedia field sample(see §5.1). Moreover, the latter explanation is unlikely to apply tothe entire Fornax sample. Therefore, it seems more likely that theobserved low gas-to-dust ratios are a result of environmental effects. In this work, the main difference between the Fornax and Virgo clus-ters is that the low total gas-to-dust ratios in Fornax are partly drivenby decreased H -to-dust ratios, while these are in creased inside thevirial radius of the Virgo cluster. If Virgo galaxies within twice itsvirial radius are included, this result disappears (see Figures 1 - 6 andTable 3). Thus, in Virgo, decreased total-gas-to-dust ratios are purelydriven by the low Hi-to-dust ratios. This is in agreement with Corteseet al. (2016), who find that the decrease in total gas-to-dust ratios inHi-poor galaxies is purely driven by the decrease in Hi-to-dust ratios,while H -to-dust ratios are increased compared to field galaxies atfixed mass and metallicity. They attribute these anomalous ratios tothe different spatial distribution of the two gas phases and the dust,resulting in the differential stripping of the ISM. As described above,observations of the ISM in FCC167 indeed show a nested ISM, inwhich the the molecular gas disc is more compact than the dust ringpresent in this galaxy (Viaene et al. 2019). However, since this galaxyis significantly deficient in molecular gas, it is unclear whether thisnested ISM was already present before the galaxy entered the cluster,or if this is the result of the more efficient stripping of molecular gascompared to dust by the cluster environment.The two main differences between the Fornax and Virgo clusterare that Virgo is more dynamically active, while Fornax is moredynamically evolved, and the Virgo cluster is much more massivethan the Fornax cluster (see §1). The former difference could be anexplanation for option ii: dust is accumulated in the star formationprocess while H is slowly depleted. It could be that there is aninitial boost in H content, after which it stops being replenished,and this effect becomes more visible over time. However, since thegalaxies studied here all still have a detectable ISM, which is notexpected after several pericentric passages, it is unlikely that theFornax galaxies have spent significantly more time in the clusterthan the Virgo galaxies, or they would have no gas left.It may be possible that ram pressure stripping results in the in-creased H -to-dust ratios observed in the Virgo cluster and in thegalaxies studied by Cortese et al. (2016) if dust is stripped beforemolecular gas. This is supported by an ongoing study of the Virgocluster (Zabel et al., in prep.), where we observe normal to high H masses in the Virgo galaxies most affected by (past and ongoing)ram pressure stripping. It is possible that ram pressure compressesatomic gas, aiding its condensation into H , and thus contributingto the increase in H -to-dust ratios. This has indeed been observedin jellyfish galaxies (Moretti et al. 2020). Since the Fornax cluster isless massive than Virgo, ram pressure stripping might play less of arole there, or not be strong/violent enough to quickly remove Hi orcompress it into H . It is possible that starvation is more importantin Fornax, slowly depriving its galaxies of H , while the dust con-tinues to accumulate. However, this is mostly speculation, and thewildly different results between both clusters are difficult to explain.Expansion of this sort of analysis to other clusters is clearly neededto determine the true cause of this effect. It is often assumed that the dust-to-metal ratio is constant with time,due to the dust formation timescale being similar to the dust de-struction timescale. A constant dust-to-metal ratio implies that thegas-to-dust ratio depends on metallicity as G/D ∝ Z − , referred toas the “reference trend” by Rémy-Ruyer et al. (2014). This trend hasbeen shown to hold for galaxies with metallicities close to solar, butto break down at the low-metallicity regime where dwarf galaxies are MNRAS , 1–22 (2021) N. Zabel et al. found, which have higher observed gas-to-dust ratios than predictedby this trend (Rémy-Ruyer et al. 2014 and references therein). How-ever, a recent review by Péroux & Howk (2020) has shown that therelation between metallicity and dust-to-metal ratio indeed holds inthe low-metallicity/high redshift regime, albeit with more scatter dueto the more complex chemistry here. From Figures 2 and 9 we can seethat gas-to-dust ratios in the galaxies in our sample are consistentlylower than those in the DustPedia sample, suggesting that they do notfollow such a reference trend. This implies that their chemical evo-lution is different from regular field galaxies, which likely has to dowith the environment they reside in. Alternatively, the physics of theISM, such as the radiation field or dust composition, in these galaxiescould be different such that they contain unusually high fractions of“CO-free” molecular gas, as discussed above.Figures 2 through 9 imply that the fraction of metals locked upin dust (versus gas) is high in the Fornax cluster. If this is the resultof stripping of high-metallicity gas (and dust), these are enrichingthe intracluster medium. Some Fornax galaxies are quite metal-rich,while they still have significant amounts of gas left. This is in broadagreement with what is found by Hughes et al. (2013), who findthat gas-poor galaxies are relatively metal-rich. This could be dueto the more metal-poor outer regions of the galaxy being strippedfirst. However, they do not find any significant difference betweenmetallicities in the Virgo cluster and in the field. These metal- andgas-rich galaxies could indicate that we are observing a special timein the lifespan of the galaxies (i.e. we expect many of them to beon their first infall, which is also suggested by Loni et al. 2021), orwitnessing a special time for the cluster itself.
We have studied gas-to-dust ratios in a sample of 15 Fornax galaxiesfrom the ALMA Fornax Cluster Survey (AlFoCS) as a function ofstellar mass. In addition, in a sub-sample of 9 galaxies that werealso observed with VLT/MUSE, as part of the Fornax3D project,the gas-to-dust ratio was also studied as a function of metallicity.We have separated H and Hi to separately study H -to-dust andHi-to-dust ratios. Each gas-to-dust ratio (from Hi, H , and Hi + H )was compared to a field sample and galaxies in the Virgo cluster(separated into galaxies inside 𝑅 vir and inside 2 𝑅 vir ), both fromDustPedia. Dust, H , Hi, and stellar masses were calculated usingthe same assumptions and methods as DustPedia where possible,to maximise homogeneity between both samples. We also studieddust-to-metal ratios as a function of stellar mass and metallicity, andH -to-dust ratios as a function of distance from the cluster centre. Wemade use of ppmap to study resolved H -to-dust ratios in NGC1436,an almost face-on, flocculent spiral galaxy, at the PACS 100 𝜇 mresolution. Our main conclusions are as follows: • Gas-to-dust ratios in the Fornax cluster are systematicallydecreased compared to the field. Kolmogorov-Smirnov (KS) andAnderson-Darling (AD) tests are able to reject the hypothesis thatthe Fornax sample and DustPedia field sample are drawn from thesame distribution at (cid:29) 𝜎 . According to the same tests, this differ-ence is not only driven by Hi deficiencies, but H -to-dust ratios arealso decreased. We propose a number of explanations for this:- H is destroyed/removed from the galaxies more efficientlythan dust. This is possible if there is a radial H -to-dust gradientin combination with a truncated H /dust disc. However, Fornaxgalaxies do not show any evidence of having significantly trun-cated H discs. Moreover, past studies of Hi-deficient galaxies and galaxies in the Virgo cluster suggest that dust is stripped beforemolecular gas. Finally, from inspection of a toy model, one wouldneed quite steep radial gradients, in combination with severe trun-cation of the gas disc in these objects. Thus, it seems unlikelythat this is the sole explanation for the low ratios observed. How-ever, the radial increase in H -to-dust ratio observed in NGC1436suggests that this effect can contribute.- Both H and dust are destroyed, but the dust reservoir is re-plenished more efficiently than the H reservoir. This is possible ifrelatively large amounts of dust are created by the deaths of mas-sive stars, and/or H is not replenished as efficiently as a result ofstarvation, stripping of Hi, and/or the inefficient condensation ofHi into H . Recent observations of gas-to-dust ratios in supernovaremnants, as well as the known production of dust by old stellarpopulations, and interstellar grain growth, suggest that this couldbe a possible explanation. The slightly increased star formationefficiencies observed in Z19 possibly speed up this process. How-ever, the H -to-Hi ratios in Fornax galaxies are high comparedto those in field galaxies at fixed stellar mass, suggesting that theconversion of Hi into H does take place efficiently (Loni et al.2021).- The physics of the ISM is altered in such a way that “standard”assumptions we make in order to estimate H and dust masses areno longer valid. For example, the strong radiation field in thecluster could disintegrate CO, while leaving H intact, leading usto underestimate H masses. Alternatively, the dust compositioncould be altered by the radiation field, possibly resulting in ahigher fraction of small grains, which leads us to overestimate thedust mass. We consider these explanations less likely, however wecannot rule them out. • Gas-to-dust ratios as a function of metallicity are also decreased,while dust-to-metal ratios are increased. This suggests that a rela-tively large fraction of metals is locked up in dust in these Fornaxgalaxies. • Total gas-to-dust ratios in the Virgo cluster are decreased signif-icantly, especially inside its virial radius. However, unlike in Fornax,this decrease is purely driven by a decrease in Hi-to-dust ratios, whileH -to-dust ratios are in creased. The difference in dynamical state andmass between both clusters are suggested as possible explanationsfor this, however the differences between the two clusters remainpuzzling. • Gas-to-dust ratios do not show any obvious variation with (pro-jected) cluster-centric distance, although this is difficult to measurebecause of projection effects and small-number statistics. In the Virgocluster there is possibly a weak correlation between cluster-centricdistance and gas-to-dust ratio (a Kendall’s Tau test returns a tau statis-tic of 0.23 with a p-value of 0.04). This likely reflects the differencebetween galaxies inside and outside 𝑅 vir , but is diluted by projectioneffects. • Resolved H -to-dust ratios in NGC1436 show an increase withratio, ending in a sharp drop at 𝑅 ≈ . ∼
40, suggesting that, aside from small-scale variations, low H -to-dust ratios are low throughout the entiregas/dust disc. There is no clear correlation with H 𝛼 emission.In summary, total gas-to-dust, Hi-to-dust and H -to-dust ratios inthe Fornax cluster are significantly decreased compared to those infield galaxies at fixed stellar mass and metallicity. Their significantlyincreased dust-to-metal ratios suggest that a relatively large fractionof metals in these galaxies is locked up in dust. There is a varietyof environmental mechanisms that could explain this, and it ispossible that two or all three of the suggested explanations play MNRAS , 1–22 (2021) lFoCS + F3D II: low gas-to-dust ratios a role. We see an opposite effect inside the virial radius of theVirgo cluster, where decreased total gas-to-dust ratios are purelydriven by decreased Hi-to-dust ratios, while H -to-dust ratios are in creased. These results disappear when galaxies inside 2 𝑅 vir are included. There are several differences between both clustersthat could possibly explain this, however, it remains a puzzling result.Further study is clearly required to determine what is drivingthe low gas-to-dust mass ratios in cluster galaxies. To eliminateany X CO related effects on the gas-to-dust ratio, one could attemptto estimate X CO from the IR emission, simultaneously with dustmasses. Such an approach is, for example, taken by Leroy et al.(2011) and Sandstrom et al. (2013). Their models, however, rely onresolved measurements, which are not available for our sample. Ifsuitable data could be obtained it would interesting to investigatethis further in a future work.Furthermore, it is crucial to expand this analysis to study othergalaxy clusters and groups. An ongoing, similar study of H -to-dustratios in the Coma cluster (Zabel et al., in prep.) will possibly helpdisentangle which environmental effects and cluster properties havea significant effect on these ratios. ACKNOWLEDGEMENTS
We would like to thank the anonymous referee for taking the time torevise and provide constructive feedback on our manuscript.The authors would also like to thank the (other) dust experts at thePHYSX department at Cardiff University for helpful suggestions,and pointing us to some useful literature.NZ acknowledges support from the European Research Council(ERC) in the form of Consolidator Grant CosmicDust (ERC-2014-CoG-647939).TAD acknowledges support from the Science and TechnologyFacilities Council through grant ST/S00033X/1.R.F.P. and E.I. acknowledge financial support from the EuropeanUnion’s Horizon 2020 research and innovation program under theMarie Skłodowska-Curie grant agreement No. 721463 to the SUN-DIAL ITN network.This project has received funding from the European ResearchCouncil (ERC) under the European Union’s Horizon 2020 researchand innovation programme (grant agreement no. 679627; projectname FORNAX).This work made use of the H mass data of DustPedia late-typegalaxies (Casasola et al. 2020; Davies et al. 2017).DustPedia is a collaborative focused research project supportedby the European Union under the Seventh Framework Programme(2007-2013) call (proposal no. 606847). The participating institu-tions are: Cardiff University, UK; National Observatory of Athens,Greece; Ghent University, Belgium; Université Paris Sud, France;National Institute for Astrophysics, Italy and CEA, France.This paper makes use of the followingALMA data: ADS/JAO.ALMA a community-developed corePython package for Astronomy (Astropy Collaboration et al. 2013,2018).This research made use of APLpy, an open-source plotting packagefor Python (Robitaille & Bressert 2012). The data underlying this paper are available in theALMA archive: ADS/JAO.ALMA
Herschel maps from the
Herschel
Fornax Cluster Survey areavailable from . REFERENCES
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