Molecular gas masses of gamma-ray burst host galaxies
Michał J. Michałowski, A. Karska, J. R. Rizzo, M. Baes, A. J. Castro-Tirado, J. Hjorth, L. K. Hunt, P. Kamphuis, M. P. Koprowski, M. R. Krumholz, D. Malesani, A. Nicuesa Guelbenzu, J. Rasmussen, A. Rossi, P. Schady, J. Sollerman, P. van der Werf
aa r X i v : . [ a s t r o - ph . GA ] F e b Astronomy & Astrophysics manuscript no. ms c (cid:13)
ESO 2020February 25, 2020
Molecular gas masses of gamma-ray burst host galaxies
Micha l J. Micha lowski1 ,
2, A. Karska3, J. R. Rizzo4, M. Baes5, A. J. Castro-Tirado6, J. Hjorth7,L. K. Hunt8, P. Kamphuis9 ,
10, M. P. Koprowski11, M. R. Krumholz12, D. Malesani7, A. NicuesaGuelbenzu13, J. Rasmussen7 ,
14, A. Rossi15, P. Schady16, J. Sollerman17, and P. van der Werf18 Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, ul. S loneczna 36, 60-286Pozna´n, Poland, [email protected] SUPA ⋆ , Institute for Astronomy, University of Edinburgh, Royal Observatory, Blakford Hill, Edinburgh, EH9 3HJ,UK Centre for Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudzi¸adzka5, 87-100 Toru´n, Poland Centro de Astrobiolog´ıa (INTA-CSIC), Ctra. M-108, km. 4, E-28850 Torrej´on de Ardoz, Madrid, Spain Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281-S9, 9000, Gent, Belgium Instituto de Astrof´ısica de Andaluc´ıa (IAA-CSIC), Glorieta de la Astronom´ıa s/n, E-18008, Granada, Spain Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100Copenhagen Ø, Denmark INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy Astronomisches Institut der Ruhr-Universit¨at Bochum (AIRUB), Universit¨atsstrasse 150, 44801 Bochum, Germany National Centre for Radio Astrophysics, TIFR, Ganeshkhind, Pune 411007, India Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT, Australia Th¨uringer Landessternwarte Tautenburg, Sternwarte 5, D-07778 Tautenburg, Germany Technical University of Denmark, Department of Physics, Fysikvej, building 309, DK-2800 Kgs. Lyngby, Denmark INAF-OAS, via Piero Gobetti 93/3 - 40129 Bologna - Italy Max-Planck-Institut f¨ur Extraterrestrische Physik, Giessenbachstraße, D-85748 Garching bei M¨unchen, Germany The Oskar Klein Centre, Department of Astronomy, AlbaNova, Stockholm University, 106 91 Stockholm, Sweden Leiden Observatory, Leiden University, P.O. Box 9513, NL-2300 RA Leiden, The NetherlandsPreprint online version: February 25, 2020
ABSTRACT
Aims.
The objectives of this paper are to analyse molecular gas properties of the first substantial sample of GRB hostsand test whether they are deficient in molecular gas.
Methods.
We obtained CO(2-1) observations of seven GRB hosts with the APEX and IRAM 30m telescopes. Weanalysed these data together with all other hosts with previous CO observations.
Results.
We obtained detections for 3 GRB hosts (980425, 080207, and 111005A) and upper limits for the remaining4 (031203, 060505, 060814, and 100316D). In our entire sample of 12 CO-observed GRB hosts, 3 are clearly deficientin molecular gas, even taking into account their metallicity (980425, 060814, and 080517). Four others are close tothe best-fit line for other star-forming galaxies on the SFR- M H plot (051022, 060505, 080207, and 100316D). Onehost is clearly molecule rich (111005A). Finally, the data for 4 GRB hosts are not deep enough to judge whether theyare molecule deficient (000418, 030329, 031203, and 090423). The median value of the molecular gas depletion time, M H / SFR, of GRB hosts is ∼ . M H / SFR shows an only ∼ σ difference between GRB hostsand other galaxies. This difference can partly be explained by metallicity effects, since the significance decreases to ∼ σ for M H / SFR versus metallicity.
Conclusions.
We found that any molecular gas deficiency of GRB hosts has low statistical significance and that it can beattributed to their lower metallicities; and thus the sample of GRB hosts has molecular properties that are consistentwith those of other galaxies, and they can be treated as representative star-forming galaxies. However, the moleculargas deficiency can be strong for GRB hosts if they exhibit higher excitations and/or a lower CO-to-H conversion factorthan we assume, which would lead to lower molecular gas masses than we derive. Given the concentration of atomicgas recently found close to GRB and supernova sites, indicating recent gas inflow, our results about the weak moleculardeficiency imply that such an inflow does not enhance the SFRs significantly, or that atomic gas converts efficientlyinto the molecular phase, which fuels star formation. Key words. gamma ray bursts: general – ISM: lines and bands – ISM: molecules – galaxies: ISM – galaxies: star formation– radio lines: galaxies ⋆ Scottish Universities Physics Alliance
1. Introduction
Long gamma-ray bursts (GRBs) have long been confirmedto be the endpoints of lives of very massive stars (e.g.
Hjorth et al. 2003; Stanek et al. 2003; Hjorth & Bloom2012). Most of the tracers of the star formation rate(SFR) of galaxies are connected with emission from mas-sive stars (e.g. Kennicutt 1998), so that GRBs were alsoused to measure the star formation history of the Universe(Y¨uksel et al. 2008; Kistler et al. 2009; Butler et al. 2010;Elliott et al. 2012; Robertson & Ellis 2012; Perley et al.2016a,b). This approach is valid if GRB hosts arerepresentative star-forming galaxies at a given redshift(Micha lowski et al. 2012; Hunt et al. 2014a; Schady et al.2014; Greiner et al. 2015; Kohn et al. 2015), or if biasesare known and can be corrected for (Perley et al. 2013,2015, 2016a,b; Boissier et al. 2013; Vergani et al. 2015;Schulze et al. 2015; Greiner et al. 2016). Gas is the fuel ofstar formation, so one of the important aspects of this issueis whether GRB hosts exhibit normal gas properties withrespect to other star-forming galaxies.The information about gas properties of GRB hostsis scarce. Micha lowski et al. (2015) and Arabsalmani et al.(2015) provided the only measurements so far of the atomicgas properties of five such galaxies. This led to a suggestionthat GRB hosts have experienced recent inflows of atomicgas. A resulting possibility of using GRBs to select galaxiesfor the study of gas accretion is important because the rateof the gas accretion onto galaxies is surprisingly constantsince z ∼
5, which is at odds with the significantly changingSFR volume density of the Universe (Spring & Micha lowski2017). Moreover, a fraction of star formation in GRB hostsmay be directly fuelled by atomic gas (Micha lowski et al.2015, 2016). The existence of this process is controversial,but it has been predicted theoretically (Glover & Clark2012; Krumholz 2012; Hu et al. 2016; Elmegreen 2018)and is supported by some observations (Bigiel et al. 2010;Fumagalli & Gavazzi 2008; Elmegreen et al. 2016).Clearly, most of the star formation in the Universeis fuelled by molecular gas (Fumagalli et al. 2009;Carilli & Walter 2013; Rafelski et al. 2016). There wereseveral unsuccessful searches of CO lines for GRB hosts(Kohno et al. 2005; Endo et al. 2007; Hatsukade et al.2007, 2011; Stanway et al. 2011) and only four detections sofar, for the hosts of GRB 980425 (Micha lowski et al. 2016),051022 (Hatsukade et al. 2014), 080517 (Stanway et al.2015b), and 080207 (Arabsalmani et al. 2018). These stud-ies resulted in mixed conclusions on whether GRB hostsare deficient in molecular gas with respect to the SFR- M H correlation of other star-forming galaxies.Hence, the objectives of this paper are i) to analysemolecular gas properties of the first substantial sample ofGRB hosts; and ii) to test whether these hosts are deficientin molecular gas. For this, we combined existing literaturedata with new observations using the APEX and IRAM30m telescopes.We use a cosmological model with H = 70 km s − Mpc − , Ω Λ = 0 .
7, and Ω m = 0 .
3. We also assume theChabrier (2003) initial mass function (IMF), to which allstar formation rates (SFRs) and stellar masses were con-verted (by dividing by 1.8) if given originally assuming theSalpeter (1955) IMF.
2. Target selection and data
We selected the host galaxies of all known GRBs at z < . i ob-servations from Micha lowski et al. 2015). These criteriawere fulfilled by GRB 980425 (the central pointing waspublished separately in Micha lowski et al. 2016), 031203,060505, 100316D, and 111005A. We performed CO(2-1) observations using the Swedish Heterodyne FacilityInstrument (SHeFI; Vassilev et al. 2008; Belitsky et al.2006) and the Swedish-ESO PI Instrument for APEX(SEPIA; Belitsky et al. 2018; only for the GRB 031203host) mounted at the Atacama Pathfinder Experiment(APEX; G¨usten et al. 2006) (project no. 096.D-0280,096.F-9302 and 097.F-9308, PI: M. Micha lowski). Table 1shows the observation log with total on-source integra-tion times. Two and three positions were observed for thehost of GRB 980425 and 111005A, respectively. The re-maining galaxies are smaller than the beam ( ∼ ′′ ). Allobservations were carried out in the on-off pattern andposition-switching mode. The fluxes were corrected usingthe main beam efficiency of 0.75. We reduced and analysedthe data using the Continuum and Line Analysis SingleDish Software ( Class ) package within the Grenoble Imageand Line Data Analysis Software ( Gildas ; Pety 2005).
We selected all GRB hosts in the northern hemisphere withinfrared or radio detections (Hunt et al. 2014a; Perley et al.2015; Micha lowski et al. 2015) and z > .
5, so that theline is located at lower frequencies and easier to observe.This was fulfilled by GRB 060814 and 080207. We per-formed observations with the IRAM 30m telescope (projectno. 172-16, PI: M. Micha lowski) using the Eight MIxerReceiver (EMIR; Carter et al. 2012). We implementedwobbler-switching mode (with the offset to the referencepositions of 60 ′′ ), which provides stable and flat baselinesand optimises the total observing time. An intermediatefrequency (IF) covered the frequency of the CO(2-1) line.We used the Fourier Transform Spectrometers 200 (FTS-200) providing 195 kHz spectral resolution (correspondingto ∼ . − at the frequency of CO(2-1) of our targets)and 16 GHz bandwidth in each linear polarisation. The ob-servations were divided into 6 min scans, each consistingof 12 scans 30 s long. The pointing was verified every 1–2hr. The observing log is presented in Table 2 with totalon-source integration times. The observations were carriedout during good atmospheric conditions, and the opacity( τ ) was uniform across different runs. We reduced thedata using the Class package within
Gildas (Pety 2005).Each spectrum was calibrated, and corrected for baselineshape. The spectra were aligned in frequency and noise-weight averaged. Some well-known platforming, due to thefact that the instantaneous bandwidth of 4 GHz is sam-pled by three different FTS units, was corrected off-line bya dedicated procedure within
Class . In all cases, the COline is far away from the step of the platforming. -300 -200 -100 0 100 200 300Velocity relative to HI (km/s)-0.050.000.050.100.15 F l u x ( Jy ) F l u x ( Jy ) -300 -200 -100 0 100 200 300Velocity relative to HI (km/s)-0.2-0.10.00.1 F l u x ( Jy ) -300 -200 -100 0 100 200 300Velocity relative to HI (km/s)-0.06-0.04-0.020.000.020.040.06 F l u x ( Jy ) -300 -200 -100 0 100 200 300Velocity relative to the optical redshift (km/s)-0.004-0.0020.0000.0020.0040.006 F l u x ( Jy ) -1000 -500 0 500 1000Velocity relative to the optical redshift (km/s)-0.0010.0000.0010.0020.003 F l u x ( Jy ) -300 -200 -100 0 100 200 300Velocity relative to HI (km/s)-0.10-0.050.000.050.10 F l u x ( Jy ) -300 -200 -100 0 100 200 300Velocity relative to HI (km/s)-0.2-0.10.00.10.20.3 F l u x ( Jy ) F l u x ( Jy ) F l u x ( Jy ) Fig. 1.
For each GRB host (labelled in the top left corner of each panel), the first panel shows the optical image(Sollerman et al. 2005; Mazzali et al. 2006; Th¨one et al. 2008; Hjorth et al. 2012; Starling et al. 2011; Micha lowski et al.2018b) together with the green circles marking the positions of the pointings and the beam sizes of our CO(2-1) ob-servations. GRB positions are marked with red circles . North is up and east is to the left. The other panels showthe corresponding CO(2-1) spectra.
Vertical dotted lines show the velocity intervals within which the line fluxes weremeasured. ′ CO / K km s −1 pc )7891011121314 l og ( L I R / L o ) H2 / M o ) for α CO = 5.0 −3−2−101234 l og ( S F R / M o y r − ) GRB hostsMetal−poor Dwarfs (Hunt+14,15,17)Metal−poor Dwarfs (Cormier+14)Virgo Dwarfs (Grossi+16)HRSIRAS (Young+89)THINGS (Leroy+08)Local ULIRGs (Solomon+97)Local LIRGs (Sanders+91)Mass−selected (Bothwell+14)Mass−selected (Bertemes+18)Compilation (Krumholz+11)SMGs (Bothwell+13)BzK (Daddi+10)GRB 190114CCompanion
Fig. 2.
Infrared luminosity or the corresponding SFR as a function of CO luminosity, or the corresponding molecular gasmass with the CO-to-H conversion factor α CO = 5 M ⊙ (K km s − pc ) − . GRB hosts are marked with full red circlesor red arrows with crosses showing the errors. The symbols of other galaxies are indicated in the legend and describedin Sect. 2.4. The solid black line is a linear fit to the non-GRB galaxies excluding ULIRGs (Eq. 1), whereas the dashedblack line represents the fit including ULIRGs (Eq. 2). The ∼ . M H is notstatistically significant (see Sect. 3.1). In addition to the CO(2-1) measurements obtained here, weincluded all other CO measurements for GRB hosts fromthe literature. All molecular masses were converted into α CO = 5 M ⊙ (K km s − pc ) − and to the line luminosityratios in temperature units L ′ − /L ′ − = 0 . L ′ − /L ′ − =0 .
27, or L ′ − /L ′ − = 0 .
17 (the Milky Way values, see Table2 of Carilli & Walter 2013) if these masses were based onCO(2-1), CO(3-2), or CO(4-3) observations, respectively.These assumed line ratios are conservatively low, so thatthey lead to conservatively high M H . We are therefore ableto robustly test for any molecular deficiency of GRB hosts.We included the hosts of GRB 000418 (Hatsukade et al.2011), for which we converted the M H upperlimit from L ′ − /L ′ − = 1 into 0 . α CO = 0 . M ⊙ (K km s − pc ) − to 5; of GRB030329 (Kohno et al. 2005; Endo et al. 2007), forwhich we converted the M H upper limit from α CO = 40 M ⊙ (K km s − pc ) − into 5; of GRB 051022(Hatsukade et al. 2014), for which we converted the M H detection from L ′ − /L ′ − = 0 .
85 into 0 .
17 and from α CO = 4 . M ⊙ (K km s − pc ) − into 5; of GRB 080517(Stanway et al. 2015b), for which we converted the M H detection from α CO = 4 . M ⊙ (K km s − pc ) − into 5;and of GRB 090423 (Stanway et al. 2011), for which weconverted the M H detection from L ′ − /L ′ − = 1 into0 .
27 and from α CO = 0 . M ⊙ (K km s − pc ) − into 5.We did not use the CO(3-2) observations of GRB 980425of Hatsukade et al. (2007) because our deeper data resultedin a detection. Moreover, we excluded GRB 020819B be-cause the low-redshift galaxy with the existing CO mea-surement (Hatsukade et al. 2014) has been shown not tobe related to the GRB (Perley et al. 2017b). For theGRB 080207 host, the CO(3-2) line observations were re-cently reported by Arabsalmani et al. (2018). We did notuse these values in subsequent analysis, because our lowertransition likely traces a larger fraction of the total molec-ular gas content. We note, however, that the obtained gasmasses are consistent (see Sect. 3). Table 1.
Log of APEX observations.
GRB Obs. Date time/hr pwv/mm980425 Center Total 4.042015 Aug 29 0.70 1.64–1.702015 Sep 12 0.30 0.75–0.852015 Sep 16 0.70 1.43–1.572015 Oct 31 1.17 1.22–1.962015 Nov 01 1.17 0.66–0.85980425 WR Total 6.572015 Nov 02 2.17 0.75–3.482016 Apr 03 0.10 2.02–2.152016 Apr 04 4.30 3.33–5.23031203 2015 Sep 10 0.80 0.83–0.91060505 Total 7.002015 Aug 28 1.20 1.50-1.672015 Aug 29 1.40 1.38–1.622015 Sep 02 1.40 1.55–1.862015 Sep 03 1.00 3.36–3.612015 Sep 04 1.00 2.50–2.732015 Sep 06 1.00 2.45–3.40100316D Total 6.582015 Aug 28 2.11 1.50–1.622015 Sep 02 1.67 1.32–1.932015 Sep 06 2.80 2.45–4.80111005A Center Total 1.652015 Sep 01 0.75 1.00–1.212015 Sep 12 0.20 0.72–0.842015 Sep 15 0.70 0.64–0.82111005A NW Total 3.202015 Sep 17 0.50 1.52–1.612016 Apr 02 1.00 2.15–2.472016 Apr 03 0.60 1.96–2.312016 Jun 10 1.60 2.98–3.34111005A SE Total 2.202015 Sep 17 0.50 1.55–1.652016 Jun 10 0.60 3.12–3.322016 Jun 11 1.60 2.49–2.83
Table 2.
Log of IRAM 30m observations.
GRB Obs. Date time/hr τ
225 GHz
For all GRB hosts in our CO sample we used the lit-erature values for their redshifts, SFRs and metallicities,as listed in Table A.1 For the host of GRB 060814, wecalculated the metallicity based on the R method ofKobulnicky & Kewley (2004) based on the [O ii ], [O iii ],and H β emission lines, using the fluxes reported inKr¨uhler et al. (2015). We obtained 12 + log(O / H) ∼ . ± .
35. Additionally, we included values measured for the hostof SN 2009bb, the relativistic supernova (SN) type Ic(Micha lowski et al. 2018a) and plot them in Figs. 2 and4. SNe of this type may have similar engines as GRBs, butno γ -rays were detected. Therefore we did not use it for thestatistical analysis quoted for GRB hosts, and it does notappear in Figs. 3 and 5. In order to place the GRB hosts in the context ofgeneral galaxy populations, we compared their proper-ties with those of the following galaxy samples, cho-sen based on the availability of the gas mass estimates:the optical-flux-limited spirals and irregulars with IRASdata (Young et al. 1989), local luminous infrared galaxies(LIRGs; Sanders et al. 1991), local ultra-luminous infraredgalaxies (ULIRGs; Solomon et al. 1997), the
Herschel
Reference Survey (HRS; Boselli et al. 2010; Cortese et al.2012, 2014; Boselli et al. 2014; Ciesla et al. 2014), H i -dominated, low-mass galaxies and large spiral galaxies(Leroy et al. 2008), 0 . < z < .
03 mass-selected galax-ies with 8 . < log( M ∗ /M ⊙ ) <
10 (Bothwell et al. 2014),0 . < z < . M ∗ /M ⊙ ) >
10 and infrared detections (Bertemes et al. 2018), metal-poor dwarfs (Hunt et al. 2014b, 2015, 2017; Leroy et al.2007), metal-poor dwarfs from the
Herschel
Dwarf GalaxySurvey (Madden et al. 2013; Cormier et al. 2014), Virgo-cluster dwarfs (Grossi et al. 2016), z ∼ . . < z < . α CO =5 M ⊙ (K km s − pc ) − and to the Milky Way line ra-tios if they were based on higher CO transitions. Namely,Bothwell et al. (2014), Daddi et al. (2010), and Leroy et al.(2008) assumed L ′ − /L ′ − = 1, 0 .
16, and 0 . L ′ − /L ′ − = 0 . α CO is appropriate for 0 . α , infrared,and radio), they are broadly consistent (Salim et al. 2007;Wijesinghe et al. 2011; Davies et al. 2016; Wang et al.2016), even in dwarf galaxies, except at very low SFR < . M ⊙ yr − (Huang et al. 2012; Lee et al. 2009), not dis-cussed here.
3. Results
The positions of our APEX and IRAM 30m pointings andthe obtained CO(2-1) spectra are shown in Fig. 1. The spec-tra were binned to a velocity resolution of 20 km s − , exceptfor the GRB 080207 host, for which 50 km s − channels wereadopted. The derived parameters are shown in Table 3. Thefluxes were integrated within the velocity ranges shown inFig. 1 as vertical dotted lines. They were chosen to encom-pass the full extent of the lines for the detected targets, Table 3.
APEX and IRAM 30m CO(2-1) line fluxes and luminosities.
GRB F int S/N F int log L log L ′ log M H , CO (Jy km s − ) (10 − W m − ) ( L ⊙ ) (K km s − pc ) ( M ⊙ )(1) (2) (3) (4) (5) (6) (7)980425 5 . ± .
27 4 . . ± .
98 3 . +0 . − . . +0 . − . . +0 . − . . ± .
28 1 . . ± .
98 2 . +0 . − . . +0 . − . . +0 . − . . ± .
35 2 . . ± .
58 5 . +0 . − . . +0 . − . . +0 . − . . ± .
64 0 . . ± . < . < . < . − . ± . − . − . ± . < . < . < . . ± .
11 3 . . ± .
08 6 . +0 . − . . +0 . − . . +0 . − . − . ± . − . − . ± . < . < . < . . ± .
94 9 . . ± .
26 4 . +0 . − . . +0 . − . . +0 . − . . ± .
13 4 . . ± .
63 3 . +0 . − . . +0 . − . . +0 . − . . ± .
52 3 . . ± .
17 3 . +0 . − . . +0 . − . . +0 . − . Notes. (1) GRB (2) Integrated flux within the velocity interval shown by the dotted lines in Fig. 1. (3) Signal-to-noise ratioof the line within this velocity interval. (4) Corresponding integrated flux in W m − . (5) Line luminosity. (6) Line luminosityin temperature units based on Equation 3 in Solomon et al. (1997). (7) Molecular gas mass estimated assuming L ′ CO(1 − =2 × L ′ CO(2 − (see Sects. 2.3 and 3) and the Galactic CO-to-H conversion factor α CO = 5 M ⊙ / K km s − pc . H2 for α CO = 5.0 / SFR / yr)0.00.20.40.60.81.0 N ( > M H / S F R ) −3.0 −2.5 −2.0 −1.5 −1.0log[(L ′ CO / K km s −1 pc ) / (L IR / L o )] GRB hostsOther galaxies
Fig. 3.
Cumulative distribution of molecular gas depletiontime (or the inverse of the star formation efficiency), i.e. theratio of the CO luminosity to the infrared luminosity or thecorresponding molecular gas mass with the CO-to-H con-version factor α CO = 5 M ⊙ (K km s − pc ) − to the starformation rate (SFR). The distribution of GRB hosts isshown as the dashed red line , whereas that of other galax-ies is shown as the solid black line . We treated the upperlimits as actual values, so the histogram for GRB hosts isan upper limit. GRB hosts are systematically shifted to theleft on this diagram (lower M H given their SFRs), but thisis not statistically significant (see Sect. 3.1).and the most significant positive feature within the veloc-ity range from −
300 to 300 km s − relative to the opticalredshift for the non-detected targets in order to obtain themost conservative upper limits. For these non-detected tar-gets we integrated the spectra in the region of a width of200 km s − , likely to be the velocity width of such galaxies,and of 50 km s − for the WR region, as it is unlikely thatthis pointing traces gas at a wider range of velocities (seeFig. 3 of Christensen et al. 2008). The CO(2-1) line lumi-nosities were calculated using Equation 3 in Solomon et al. (1997) and converted into the CO(1-0) luminosities assum-ing L ′ − = 2 × L ′ − . The Galactic CO-to-H conversion fac-tor α CO = 5 M ⊙ / K km s − pc was used to calculate molec-ular gas masses ( M H = α CO L ′ − ). M H The infrared luminosity (or SFR) as a function of CO lineluminosity (or M H ) for GRB hosts and other galaxies isshown in Fig. 2. The best linear fit in log-log space to allnon-GRB galaxies with SFRs lower than those of ULIRGs(SFR < M ⊙ yr − ) is (the solid line in Fig. 2)log(SFR /M ⊙ yr − ) = 0 . × log( M H /M ⊙ ) − . . (1)The scatter around this relation is ∼ .
42 dex. WhenULIRGs are included, this equation changes to (the dashedline in Fig. 2)log(SFR /M ⊙ yr − ) = 1 . × log( M H /M ⊙ ) − . . (2)As reported in Micha lowski et al. (2016), we found alow molecular gas content in the GRB 980425 host givenits SFR. Similarly, the hosts of GRB 100316D and 060814are deficient in M H given their SFRs. Our M H upperlimit for the GRB 031203 host is ∼ . ∼ H M H upper limit for theGRB 060505 host is not sufficiently strong to test for anymolecular gas deficiency, but it is close to the best-fit line forother star-forming galaxies, which means that this galaxy isnot richer in molecular gas than the average of other galax-ies. We found that the GRB 080207 host is very close to thebest-fit line for other galaxies on the SFR- M H diagram,consistent with the results of Arabsalmani et al. (2018)based on the CO(3-2) line. The host of GRB 111005Ais molecule rich with log( M H / SFR / yr) ∼ .
34, that is, ∼ .
24 dex above the best-fit relation for other galaxiesat the relevant SFR. Consistently with Micha lowski et al.(2018a), we show that the host of SN 2009bb has a molecu-lar gas mass that is a few times lower than its SFR suggests.
The second pointing for the GRB 980425 host, to-wards the Wolf-Rayet (WR) region (for its proper-ties, see Hammer et al. 2006; Le Floc’h et al. 2006, 2012;Christensen et al. 2008; Micha lowski et al. 2009, 2014,2016; Kr¨uhler et al. 2017) resulted in an upper limit closeto the best-fit line. While we cannot establish any molec-ular deficiency for this region, it is therefore definitely notmolecule rich, in contrast with its high abundance of atomicgas (Arabsalmani et al. 2015). Both the central and NW re-gions of the GRB 111005A host are molecule rich, but theSE region is at least slightly molecule deficient, given itsCO upper limit.Because of our choice to adopt the Milky Way COline ratios instead of those of M82 (see Sect. 2.3), we ob-tained a molecular gas mass that is approximately five timeshigher for the GRB 051022 host, and hence its moleculargas deficiency is not as dramatic as presented originallyin Hatsukade et al. (2014), but still apparent (Fig. 2). Ourcorrection for the GRB 080517 is small with respect to thevalues used in Stanway et al. (2015b), so we recover its re-ported molecular gas deficiency.The revised, lower value of the infrared luminosity ofthe host of GRB 000418 (compare Micha lowski et al. 2008and Perley et al. 2017b) means that the CO observations(Hatsukade et al. 2011) do not provide useful constraintson its location on the SFR- M H diagram (see Fig. 2).Similarly, the upper limits on L IR available for GRB 030329(Endo et al. 2007) and 090423 (Stanway et al. 2011) do notconstrain the positions of these galaxies relative to the best-fit SFR- M H relation. Hence we did not use these threehosts with upper limits for both SFRs and M H in the sta-tistical analysis.The median value of the molecular gas depletion timefor non-GRB galaxies is log( M H / SFR / yr) = 9 . +0 . − . ,whereas for GRB hosts it is 8 . +0 . − . (the errors were ob-tained by randomly perturbing 500 times the measured val-ues within their errors and assessing the 68% confidence in-terval of the obtained medians), where we treated the upperlimits as actual values. The value for GRB hosts thereforeis an upper limit. Hence, GRB hosts have molecular gasmasses ∼ . M H /SFR ra-tio (molecular gas depletion time) is shown in Fig. 3.For these statistics we excluded hosts with weak up-per limits (031203) and those with upper limits for both M H and SFRs (000418, 030329, and 090423). Using theKolmogorov–Smirnov (K-S) test, we found that we canrule out the null hypothesis that the M H / SFR values ofthe GRB hosts were drawn from the same distribution asthose of other star-forming galaxies at a significance level p = 0 .
05, corresponding to a difference with a low statisti-cal significance of ∼ . σ . In order to assess the influenceof the measurement errors on this result, we repeated theK-S test using the GRB values perturbed by their errorsand found that the significance remains similar. M H /SFR vs. metallicity The CO-to-H conversion factor is metallicity depen-dent (e.g. Bolatto et al. 2013), therefore we explored the M H /SFR ratio as a function of metallicity (Fig. 4). Using the galaxies with metallicity measurement, the linear fit toall non-GRB galaxies is (the solid line in Fig. 4)log( M H / SFR / yr) = 2 . × [12 + log(O / H)] − . . (3)The scatter around this relation is ∼ .
35 dex.The molecular deficiency of the GRB 980425 is con-firmed, even taking into account its sub-solar metallicity,that is to say, it has a shorter molecular gas depletion timethan expected for its SFR and metallicity. This is at oddswith the discussion in Arabsalmani et al. (2018) that thisgalaxy has normal molecular gas properties. However, theycompared M H with stellar mass, not SFR, as we do here,and also used the dwarf sample of Grossi et al. (2016) as acomparison, but these galaxies exhibit much lower metallic-ities than the GRB 980425 host (see Fig. 4). Similarly, themolecular gas deficiency of the hosts of GRB 080517 and060814 is confirmed after taking into account their metal-licities.The hosts of GRB 051022, 080207, and 100316D havedepletion times consistent with the expected values giventheir metallicities (the GRB 100316D host represents an up-per limit, therefore we do not know whether it is close tothe best-fit relation). Only the GRB 111005A host is clearlymolecule rich for its metallicity. The limits for the hosts ofGRB 031203 and 060505 are not constraining because theyare significantly above the best fit line.Our upper limit for the WR region of the GRB 980425host is ∼ . − . ± .
07 yr − , where we treated the upperlimits as actual values. This value is therefore an upperlimit.The cumulative distributions of residuals around thebest-fit line (Eq. 3) is shown in Fig. 5. For these statis-tics we excluded hosts with weak upper limits (031203 and060505) and those with upper limits for both M H andSFRs (000418, 030329, and 090423). Using the K-S test,we we found that we can reject the null hypothesis that theresiduals around the best-fit line for GRB hosts were drawnfrom the same distribution as those for other star-forminggalaxies only at a significance level p = 0 .
33, correspondingto a ∼ σ difference. Using the H i data from Micha lowski et al. (2015), we canconstrain the molecular gas fraction ( M H / ( M H + M HI ))to be ∼
7% for the GRB 980425 host, <
15% for theGRB 060505 host, and ∼
13% for the GRB 111005A host.This is within the scatter of but on the lower side com-pared to other star-forming galaxies (a few to a few tensof percent ; Young et al. 1989; Devereux & Young 1990;Leroy et al. 2008; Saintonge et al. 2011; Cortese et al.2014; Boselli et al. 2014) and SN hosts (Galbany et al.2017; Micha lowski et al. 2018a). l og ( M H f o r α C O = . / S F R / y r) −5−4−3−2−1 l og [ ( L ′ C O / K k m s − p c ) / ( L I R / L o ) ] GRB hostsMetal−poor Dwarfs (Hunt+14,15,17)Metal−poor Dwarfs (Cormier+14)Virgo Dwarfs (Grossi+16)HRSMass−selected (Bothwell+14)Compilation (Krumholz+11)Mass−selected (Bertemes+18)
GRB 190114C
Fig. 4.
Molecular gas depletion time (or the inverse of the star formation efficiency), i.e. the ratio of the CO lu-minosity to the infrared luminosity or the corresponding molecular gas mass with the CO-to-H conversion factor α CO = 5 M ⊙ (K km s − pc ) − to the SFR as a function of metallicity. GRB hosts are marked with full red circlesor red arrows with vertical bars showing the errors. The symbols of other galaxies are indicated in the legend and de-scribed in Sect. 2.4. The solid black line is our fit to the non-GRB galaxies (eq. 3), whereas the dashed black line is therelation found by Hunt et al. (2015). GRB hosts are consistent with other galaxies (see Sect. 3.2).
4. Discussion
We obtained mixed results from analysing CO data for12 GRB hosts from our survey and from the literature.Three GRB hosts are clearly deficient in molecular gas, eventaking into account their metallicity (980425, 060814, and080517). Four others are close to the best fit-line for otherstar-forming galaxies in the SFR- M H plot (051022, 060505,080207, and 100316D). One host is clearly molecule-rich(111005A). Finally, for 4 GRB hosts the data are notdeep enough to judge whether they are molecule deficient(000418, 030329, 031203, and 090423).These results suggest that GRB hosts may be prefer-entially found in galaxies with lower molecular gas contentthan other star-forming galaxies, as there are more exam-ples of GRB hosts in the M H -poor part of the M H -SFRdiagram, and the median molecular depletion timescale( M H /SFR) of GRB hosts is ∼ . ∼ σ level when analysing M H /SFR (Figs. 2 and 3). Moreover, the statistical significance of this tentative difference de-creases further to the ∼ σ level when taking the metal-licity into account (Figs. 4 and 5). Hence, our sample isstatistically consistent with other star-forming galaxies.Recent high-resolution observations of GRB and SNhosts showed concentrations of atomic gas close to theGRB and SN positions (Micha lowski et al. 2015, 2018a;Arabsalmani et al. 2015), strongly supporting the hypoth-esis of recent inflow of gas at these sites. The sample ofGRB/SN hosts can then be used to study recent gas in-flow. Our result of a very weak molecular deficiency (if any)implies that either the SFRs of GRB/SN hosts are not sig-nificantly enhanced by such inflow, or that atomic gas isefficiently converted into the molecular phase, so that SFRand M H increase hand in hand.However, if molecular deficiency is confirmed with alarger sample of GRB hosts, then this will be consistentwith a scenario in which their SFRs are enhanced by a re-cent inflow of atomic gas that did not have time to convertinto the molecular phase. Moreover, a low molecular gas −1.0 −0.5 0.0 0.5 1.0residual log(M H2 for α CO = 5.0 / SFR / yr) vs. 12+log(O/H)0.00.20.40.60.81.0 N ( > r e s i dua l ) GRB hostsOther galaxies
Fig. 5.
Cumulative distribution of the residuals with re-spect to the solid line in Fig. 4 (Eq. 3), showing the rela-tion between metallicity and molecular gas depletion time(or the inverse of the star formation efficiency), i.e. the ra-tio of the CO luminosity to the infrared luminosity or thecorresponding molecular gas mass with the CO-to-H con-version factor α CO = 5 M ⊙ (K km s − pc ) − to the SFR.The distribution of GRB hosts is shown as the dashed redline , whereas that of other galaxies is shown as the solidblack line . We treated the upper limits as actual values,so the histogram for GRB hosts is an upper limit. GRBhosts are systematically shifted to the left on this diagram(lower M H given their SFRs and metallicity), but this isnot statistically significant (see Sect. 3.2).content would be consistent with star formation fuelled di-rectly by atomic gas (Micha lowski et al. 2015).Two other issues need to be pointed out. First, most ofour M H estimates are based on the CO(2-1) line or highertransitions. In order to calculate molecular gas masses, weconverted these line luminosities into those of the CO(1-0)line assuming a conservatively low Milky Way L ′ − / L ′ − ratio, giving conservatively high M H . If however the gas inGRB hosts is even less excited than the Milky Way, thenthe real 2-1/1-0 ratio ratio is even lower, and our assump-tion would result in too low M H . This is unlikely, how-ever, because GRB hosts are usually found to have a highSFR given their stellar masses (Castro Cer´on et al. 2006,2010; Savaglio et al. 2009; Th¨one et al. 2009), which likelyleads to high excitations (see Micha lowski et al. 2016) andhigh L ′ − / L ′ − ratios in turn. If this is the case gener-ally, then our M H are overestimated, and the differencebetween GRB hosts and other galaxies is stronger than sug-gested by our analysis. In particular, if we were to adoptthe SMG or M82 2-1/1-0 ratios, then the molecular gasmasses of GRB hosts would be 1 . . α CO . We did take into ac-count the variation of α CO with metallicity (Fig. 4), but itis possible that other properties (e.g. gas density or turbu- lence) lead to high α CO and result in weak CO emission. Onthe other hand, if the correct α CO for GRB hosts is closer tothe low value measured for starbursts (Bolatto et al. 2013),then the real molecular masses of GRB hosts are approxi-mately five times lower than we measure and the moleculardeficiency is statistically significant. This aspect is muchmore difficult to investigate (also for non-GRB galaxies),because there is no robust way of measuring α CO , espe-cially in non-standard environments.We also stress that it is important to investigate themolecular gas properties with high-resolution observations.If a molecular deficiency is found locally close to the GRBpositions, then this will be consistent with star formationfuelled directly by atomic gas. In such a scenario, we arenot able to capture this effect using the existing CO datawith low spatial resolution, as the hosts on average are notsignificantly molecule poor.This analysis can be improved by investigating a largersample of GRB hosts, and possibly with deeper observa-tions that allowing probing well below the average molec-ular gas depletion time of other star-forming galaxies.Moreover, the caveat of our sample is that it is heteroge-nous, including low- z hosts and highly star-forming hostsat higher redshifts (Hunt et al. 2011, 2014a; Svensson et al.2012; Perley et al. 2015). This demonstrates the need of ob-taining CO data for a larger sample of homogeneously se-lected GRB hosts. This is likely possible only with ALMA,because we have targeted nearby and bright hosts with COemission that is potentially easier to detect. ALMA will beable to detect fainter targets and thus will enable studiesof a larger and unbiased sample.
5. Conclusions
We observed the CO(2-1) line for 7 GRB hosts, obtainingdetections for 3 GRB hosts (980425, 080207, and 111005A)and upper limits for the remaining 4 (031203, 060505,060814, and 100316D). In our entire sample of 12 CO-observed GRB hosts, including objects from the literature,3 are clearly deficient in molecular gas, even taking into ac-count their metallicity (980425, 060814, and 080517). Fourothers are close to the best-fit line for other star-forminggalaxies in the SFR- M H plot (051022, 060505, 080207,and 100316D). One host is clearly molecule rich (111005A).Finally, for 4 GRB hosts, the data are not deep enough tojudge whether they are molecule deficient (000418, 030329,031203, and 090423). The median value of the moleculargas depletion time, M H / SFR, of GRB hosts is ∼ . M H / SFR shows only ∼ σ difference be-tween GRB hosts and other galaxies. This difference canpartially be explained by metallicity effects, since the sig-nificance decreases to ∼ σ for M H / SFR versus metallicity.We found that any molecular gas deficiency of GRBhosts has low statistical significance and that it can beattributed to their lower metallicities; and thus the sam-ple of GRB hosts has consistent molecular properties toother galaxies and can be treated as representative of star-forming galaxies. However, the molecular gas deficiency canbe strong for GRB hosts if they exhibit higher excitationsand/or a lower CO-to-H conversion factor than we assume,which would lead to lower molecular gas masses than we de-rive. Given the concentration of atomic gas recently found close to GRB and SN sites, indicating recent gas inflow,our results about the weak molecular deficiency imply thatsuch inflow does not enhance the SFRs significantly, or thatatomic gas converts efficiently into the molecular phase,which fuels star formation. Only if the analysis of a largerGRB host sample reveals molecular deficiency (especiallyclose to the GRB position) would this support the hypoth-esis of star formation fuelled directly by atomic gas. Acknowledgements.
We thank Joanna Baradziej and our referee forhelp with improving this paper, Per Bergman, Carlos De Breuck, PalleMøller and Katharina Immer for help with the APEX observations,and Claudia Marka for help with IRAM30m observations.M.J.M. acknowledges the support of the National Science Centre,Poland through the POLONEZ grant 2015/19/P/ST9/04010; andthe UK Science and Technology Facilities Council; this projecthas received funding from the European Union’s Horizon 2020 re-search and innovation programme under the Marie Sk lodowska-Curie grant agreement No. 665778. A.K. acknowledges support fromthe Polish National Science Center grants 2014/15/B/ST9/02111and2016/21/D/ST9/01098. J.R.R. acknowledges the support fromproject ESP2015-65597-C4-1-R (MINECO/FEDER). A.J.C.T. ac-knowledges support from the Spanish Ministry Project AYA2015-71718-R. J.H. was supported by a VILLUM FONDEN Investigatorgrant (project number 16599). L.K.H. acknowledges funding from theINAF PRIN-SKA program 1.05.01.88.04. M.R.K. acknowledges sup-port from the Australian government through the Australian ResearchCouncil’s Discovery Projects funding scheme (project DP160100695).Based on observations collected at the European Organisationfor Astronomical Research in the Southern Hemisphere underESO programmes 096.D-0280(A), 096.F-9302(A), and 097.F-9308(A).This publication is based on data acquired with the AtacamaPathfinder Experiment (APEX). APEX is a collaboration betweenthe Max-Planck-Institut fur Radioastronomie, the European SouthernObservatory, and the Onsala Space Observatory. This work is basedon observations carried out under project number 172-16 withthe IRAM 30m telescope. IRAM is supported by INSU/CNRS(France), MPG (Germany) and IGN (Spain). This research hasmade use of the GHostS database ( ),which is partly funded by Spitzer/NASA grant RSA AgreementNo. 1287913; the NASA/IPAC Extragalactic Database (NED) whichis operated by the Jet Propulsion Laboratory, California Instituteof Technology, under contract with the National Aeronautics andSpace Administration; SAOImage DS9, developed by SmithsonianAstrophysical Observatory (Joye & Mandel 2003); and the NASA’sAstrophysics Data System Bibliographic Services.
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Appendix A: Additional table
Table A.1.
Properties of our sample of GRB hosts.
GRB z opt Ref SFR Ref 12 + log(O / H) Ref( M ⊙ yr − )980425 0.0085 1 0 .
26 17 8.60 27980425 WR 0.0085 1 0 .
02 17 8.16 28000418 1.1181 2 <
77 18 8.43 29030329 0.1685 3,4 <
17 19 8.13 30031203 0.1050 5 2 . . .
69 22 8.30 31060814 1.9229 8 256 23 8.38 32080207 2.0858 8 170 21 8.74 32080517 0.089 9 7 . <
39 24 - -100316D 0.0591 12,13 1 .
73 22 8.30 33111005A 0.01326 14,15 0 .
42 14 8.50 25111005A CENT 0.01326 14,15 0 .
26 25 8.56 25111005A NW 0.01326 14,15 0 .
06 25 8.49 25111005A SE 0.01326 14,15 0 .
09 25 8.43 25SN 2009bb 0.009877 16 5 .
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