Low Surface Brightness Galaxies in z > 1 Galaxy Clusters: HST approaches the Progenitors of Local Ultra Diffuse Galaxies
Aisha Bachmann, Remco F.J. van der Burg, Jérémy Fensch, Gabriel Brammer, Adam Muzzin
AAstronomy & Astrophysics manuscript no. aanda © ESO 2021January 21, 2021 L etter to the E ditor Low Surface Brightness Galaxies in z > Galaxy Clusters:HST approaches the Progenitors of Local Ultra Diffuse Galaxies
Aisha Bachmann , (cid:63) , Remco F. J. van der Burg , Jérémy Fensch , , Gabriel Brammer , , Adam Muzzin Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute, Universitätsstr. 150, 44801 Bochum, Germanye-mail: [email protected] European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748, Garching, Germanye-mail: [email protected] Univ. Lyon, ENS de Lyon, Univ. Lyon 1, CNRS, Centre de Recherche Astrophysique de Lyon, UMR5574, 69007, Lyon, Francee-mail: [email protected] Cosmic Dawn Center (DAWN), Denmark Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, DK-2100 Copenhagen, Denmark Department of Physics and Astronomy, York University, 4700, Keele Street, Toronto, Ontario, ON MJ3 1P3, CanadaSubmitted 9 December 2020; accepted 15 January 2021
ABSTRACT
Ultra Di ff use Galaxies (UDGs), a type of large Low Surface Brightness (LSB) galaxies with particularly large e ff ective radii ( r e ff > . z < z > ffi cult to detect and studythere. This work uses the deepest Hubble Space Telescope (HST) imaging stacks of z > + z = z = ∼
3. A plausible explanation for the implied increase with time would be a significant size growth of these galaxies in the last ∼ Key words. galaxies: clusters: general – galaxies: dwarfs – galaxies: formation
1. Introduction
Dwarf galaxies are showing a vast range of properties in sizeand luminosity. Twenty particularly large (~10 kpc) dwarfs withlow surface brightness (LSB) were discovered through extensivephotometric studies of the Virgo cluster by Sandage & Binggeli(1984). An additional 27 examples were found in the Virgo clus-ter (Impey et al. 1988) as well as in the Fornax cluster (Ferguson& Sandage 1988). While these objects were found in high den-sity environments, similar low surface brightness objects werealso discovered in lower density environments such as the field(Dalcanton et al. 1997; Román et al. 2019).More recently, after discovering 47 similar LSB objects (r e ff ~3 − (cid:48)(cid:48) , or r e ff > µ ( g , = −
26 mag arcsec − )in the Coma cluster, the term "Ultra Di ff use Galaxies" (UDGs)was introduced for these objects (van Dokkum et al. 2015a). Dueto their projected density, and their spatial coincidence with theComa cluster, van Dokkum et al. (2015a) concluded these ob-jects to be a part of the Coma cluster, a statement that was con-firmed by a follow-up spectroscopic study (van Dokkum et al.2015b). (cid:63) As part of the 1st ESO Summer Research Programme
Even though it is still a challenge to obtain redshift measure-ments of sizable samples of UDGs, their overdensity in galaxyclusters allows us to study their properties in such over-denseenvironments. An approximately linear dependence between theabundance of UDGs in galaxy clusters, and the cluster mass wasfound (van der Burg et al. 2017; Janssens et al. 2017; Román& Trujillo 2017). UDGs within clusters appear to be found onthe red sequence (van Dokkum et al. 2015a; van der Burg et al.2016), while UDG-like galaxies found in the field appear to betypically bluer (Leisman et al. 2017; Prole et al. 2019).Important open questions surrounding the study of UDGs arerelated to their origin, for which di ff erent theories have been pro-posed in the literature. van Dokkum et al. (2015a) suggest that(some) UDGs may have formed in haloes with masses similar tothe Milky Way, but “failed” to form an L * galaxy. UDGs mayalso be the extremes in a continuous distribution in dwarf galaxyproperties, having acquired their expanded sizes due to internal(Amorisco & Loeb 2016; Di Cintio et al. 2017), or external (Ben-net et al. 2018) processes.Given the suggested low dark-matter content of some fieldUDGs (van Dokkum et al. 2018), they may also have formed astidal dwarf galaxies (Bennet et al. 2018; Fensch et al. 2019). Article number, page 1 of 9 a r X i v : . [ a s t r o - ph . GA ] J a n & A proofs: manuscript no. aanda
Since these formation processes happen over di ff erent timescales, observing the evolving properties of UDGs may help dis-tinguish between di ff erent scenarios. To this end, we search forLSB galaxies in the two galaxy clusters SPTCL-2106-5844 ( z = + z = Λ CDM cosmology with Ω m = Ω Λ = =
70 km s -1 Mpc -1 . At the redshift of our clusters, 1arcsec corresponds to ~8.2 - 8.4 kpc. For stellar masses we as-sume the Initial Mass Function (IMF) from Chabrier (2003). Allmagnitudes we quote are in the AB magnitude system.
2. Data
We are making use of HST photometry taken as part of the SeeChange program (HST GO 13677, 14327; PI: Perlmutter), whichtargeted galaxy clusters in the range 1 . < z < .
75. The maingoal of their program was to find high- z supernovae (SN) type Ia , and to use these to constrain the expansion rate of the uni-verse. The survey strategy has therefore been to take data with a ∼ monthly cadence (e.g. Williams et al. 2020). Rather than usingthe individual exposures, we use the image stacks, which reach acombined exposure time from 3.99 to 5.13 hours in F140W percluster we examined. In this work we focus on the two lowest- z clusters from their sample.The first cluster we analyze is SPT-CL J2106-5844 (here-after SPTCL-2106) at redshift z = ff ect (SZ) signal, yielding a mass estimate of M = (1.27 ± × M (cid:12) (Foley et al. 2011). The second clus-ter is MOO J1014 + = = (5.6 ± × M sun and a strong SZ signature (Brodwin et al. 2015). RGBimages of the two clusters are shown in Figs. A.1 & A.2.To create the full-depth mosaics of both clusters we start byaligning each of the multiple SN monitoring “visits” first inter-nally to a catalog of sources detected in a single F140W visitand then globally to sources matched in the GAIA DR2 cata-log (Gaia Collaboration et al. 2018). We use the A stro D rizzle software package (Gonzaga & et al. 2012) to identify and maskcosmic rays and bad pixels in the aligned individual exposuresand perform source detection on the final combined F140W(WFC3 / IR) mosaic generated with 60 mas pixels. We further useF814W (WFC3 / UVIS) stacks to provide basic colour informa-tion (or limits on the inferred colour based on the F814W de-tection limit). The combination of these filters bridge the 4000Åbreak at the redshifts of our clusters, hence providing clues onstellar populations and a way to help assess the sample purity.
To estimate the level of contamination of the sample by fore-ground and background objects, we require a field survey withthe same filter bands and image depth. We therefore utilise the data stacks taken in the Hubble eXtreme Deep Field (XDF).These deep stacks are composed of data from 19 di ff erent HSTprograms covering the Hubble Ultra Deep Field from 2002 to2012. For details on the data reduction we refer to Illingworthet al. (2013).To ensure similar source detection limits as for the clusters,we add artificial noise so that the recovered fraction of simulatedsources were similar between the reference and cluster fields (seeSect. 3.3).
3. Analysis
Sources are detected by running
SExtractor (Bertin & Arnouts1996) on the F140W images of the clusters and the XDF. The pa-rameters used to ensure optimal detection of faint and extendedsources in the clusters can be found in Appendix B, and an iden-tical setup was used for the XDF images. We only considersources in the relatively central regions of the cluster stacks,where exposure time is nearly uniform (cf. Figs. A.1 & A.2). Ex-amples of detected objects in each of the two clusters are shownFig. 1.
For the detected sources, structural parameters such as magni-tude, radius, ellipticity and Sérsic index of the detected objectsare determined using GALFIT (Peng et al. 2002) on the F140Wimage after masking neighbouring objects that are detected by
SExtractor . GALFIT is also allowed to simultaneously fit aconstant value to the sky background to improve the overall fit.In order to measure reliable colours, the F814W fluxes of the de-tected objects are measured on the corresponding stack by forc-ing GALFIT to use the morphological parameters obtained fromthe F140W stack, and only fit the flux normalisation.To estimate the measurement uncertainty, the GALFIT mod-els are injected on a hundred di ff erent random locations in thecorresponding cluster, requiring that there were no prior de-tections, and measured exactly as before. In this way the 1- σ -uncertainties for size and magnitude in F140W, and magnitude inF814W, are obtained. We note that GALFIT measured a slightlylower flux (by about 0.2 mag) and smaller size (by about 0.3kpc) than the simulated inputs. In the following, we correct forthis small bias. To assess the completeness of our UDG progenitor selection, weperform all processing steps also on a range of image simula-tions. For this we inject objects with Sérsic profiles on randomlocations in the HST stacks. We choose a constant Sérsic- n pa-rameter of unity, which corresponds to typical light profiles ofUDGs measured in the local Universe (e.g. van Dokkum et al.2015a; Koda et al. 2015; van der Burg et al. 2016). Sizes aredrawn uniformly between 0 (cid:48)(cid:48) . (cid:48)(cid:48) .
0, and ellipticities f , de-fined as f = − b / a with b / a the axis ratio, uniformly between0.0 and 0.2. Each step was performed identically on the cluster-and on the reference field image.Fig. 2 shows the recovered fractions of the inserted objects,for the two clusters and the reference field. While the detectionlimits in the di ff erent panels looks similar overall, we note that https: // archive.stsci.edu / prepds / xdf / Article number, page 2 of 9isha Bachmann et al.: Low Surface Brightness Galaxies in distant Galaxy Clusters probed with HST
Fig. 1.
RGB (red = F140W, green = F105W, blue = F814W) images of members of the studied clusters.
Top:
Two spectroscopically-confirmedbright members of SPTCL-2106 (on the left) and three LSB galaxies that are likely members of SPTCL-2106 (based on a reference field compar-ison, on the right).
Bottom:
Three LSB galaxies that are likely members of the cluster MOO-1014 (based on a reference field comparison).
Fig. 2.
Left:
Recovery fractions for the simulated sources in SPTCL-2106.
Middle:
Same for MOO-1014.
Right:
Same for the XDF reference field,with noise level matched to the cluster fields. The colourbar shows the recovery fractions. While the detection limits are comparable between thepanels, the XDF shows overall a better recovery fraction for brighter targets than in the clusters due to a higher source crowding in the clusterfields. Also highlighted is the regime of objects with a surface brightness from 24.0 to 26.5 mag arcsec − in F140W and a radius from 1.5 to 7.0kpc. there is a substantial di ff erence between the clusters and the ref-erence field. Even for relatively bright sources, the recovery frac-tion is lower in the cluster than in the field. This is expected giventhe relatively high number of large and bright sources crowdingthe cluster stacks. These recovery fractions are used to determinethe limits of our analysis, and to correct the detected sources forincompleteness, both due to limiting depth and due to crowd-ing / obscuration. While the definition of UDGs is rather arbitrary, we initially filterthe sample by using a definition similar to that used in the LocalUniverse: – surface brightness in F140W ≥ − (i.e., notcorrected for surface brightness dimming), – e ff ective radius between 1.5 and 7.0 kpc, – Sérsic index ≤ – distance between SExtractor detection and GALFIT posi-tion of measurement < Article number, page 3 of 9 & A proofs: manuscript no. aanda in the reference field depends on the assumed angular diameterdistance, and thus the redshift of the cluster. We conservativelyonly consider sources that would have had a detection proba-bility of at least 50% in the reference field for both cluster andreference field objects and only count the objects above this limitto ensure a reasonable completeness correction. For the remain-ing sources we apply a completeness correction that is thus atmost a factor 2 based on the determined recovery fractions (seeSect. 3.3). We also apply a correction based on the di ff erent skyarea covered by the cluster and reference field. We are left witha statistical count of 99 ±
10 objects in SPTCL-2106 (67 ± ±
10 in MOO-1014 (70 ± ff erent covered sky areas,are taken into account. For more details on the subtraction ofthe reference field we refer to van der Burg et al. (2016). Afterthe subtraction we are left with a statistical count of 32 ±
13 inSPTCL-2106 and 20 ±
13 in MOO-1014.
4. Results and Discussion
To be able to compare our LSB candidates to likely progenitorsof UDGs studied in the local universe, we evolve local UDGsback to the redshift of our clusters following a simple stellarpopulation model. The model assumes a passively-evolving stel-lar population that was formed at z form = .
5, which is in linewith intermediately old ages of UDGs measured locally (Ferré-Mateu et al. 2018; Ruiz-Lara et al. 2018; Fensch et al. 2019). Itis based on stellar population synthesis models from Bruzual &Charlot (2003), a star formation history SFR ∝ e − t /τ with a shorte-folding time of τ =
10 Myr, a Chabrier (2003) initial massfunction and no dust extinction. We use the magnitudes, physi-cal sizes and filters of van Dokkum et al. (2015a) as anchor point,and estimate how those UDGs would appear at the redshift of ourcluster as observed through the WFC3 / F140W filter. This esti-mate accounts, by construction, for surface brightness dimming.The 47 UDG candidates are shown in Fig. 3. Additionally weevolved the dwarf and giant galaxies found by Mobasher et al.(2001) in the Coma cluster back to the redshift of our clusters inthe same way and plotted them as well in Fig. 3.Figure 3 shows that the LSB samples of our clusters lie inthe area between the samples by van Dokkum et al. (2015a) andMobasher et al. (2001), making them fainter than the progeni-tors of normal dwarf and giant galaxies in the Coma cluster, andalmost as faint as the expected progenitors of the UDGs stud-ied by van Dokkum et al. (2015a). We can see a small overlapbetween our samples and the compared objects from both otherstudies. As a reference, we also plot the curves of constant sur-face brightness, µ ( r , r e ff , circ ) = . , . − , which isa common selection boundary for UDGs in the local universe,also evolved to the redshifts of our clusters. This indicates thatonly half of the objects we are able to detect in both clusterscould classify as progenitors of the brightest UDGs known inthe local universe, based on this evolution model. We note that, while projecting the local Coma galaxies backto higher redshift, we have only evolved their fluxes and ig-nored any potential size evolution.However, numerical simula-tions suggest that a typical UDG may see its radius increasewith age from around 2.5 - 3 kpc at z = z = ff ected by it is plotted in Fig. 4. Figure 5 shows the colours and magnitudes of the sample ofLSB galaxies. The residual weight shows the number of galax-ies we expect in the observed cluster area per detected objectafter accounting for the subtraction of the reference field. Alsoshown in Fig. 5 are the positions in the colour-magnitude di-agram of galaxies which were spectroscopically confirmed asmembers of SPTCL-2106 (by the GOGREEN collaboration,Balogh et al. 2021). The colours were measured in the samefilter bands and following an identical method as for the LSBcandidates. Ignoring some of the data points that may be partof a bluer / star-forming cluster population, we note that the bulkof the LSB galaxies lie on an extended red-sequence, shown asa pink-dashed line, as defined by the brighter cluster galaxies.We note that the slope of this estimate red-sequence, -0.12, isconsistent with z ∼ ff erence between the two clusters. The LSB galaxies ofSPTCL-2106 and MOO-1014 show colours that are consistentwith the red sequence, suggesting that these galaxies are likelyquenched and have thus already stopped forming stars. To put the measured abundance of LSB galaxies in our clustersinto context, we compare it to the abundance of UDGs in localclusters, within projected R < R . For this, we have to makeassumptions regarding the underlying magnitude and size distri-bution of dwarf galaxies, and how these evolve with redshift. Weassume a flat magnitude distribution for di ff erent size bins (con-sistent with what is observed in the Coma cluster, by Danieli &van Dokkum 2019), and the same size distribution as measuredfor UDGs in local clusters (for radii between 1.5 and 7.0 kpcvan der Burg et al. 2016). Based on Fig. 2 we can also assumethat our samples in the range with surface brightness from 24.0 to26.5 mag in F140W and a radius from 1.5 to 7.0 kpc are mostlycomplete for the parameter range studied of both clusters.The available HST imaging does not allow us to probe radiiout to R , but only to radii corresponding to ∼ . · R forSPTCL-2106 and ∼ . · R for MOO-1014. To correct forthe missing area, we assume that LSBs approximately trace theoverall matter distribution in the cluster, which is described byan NFW (Navarro et al. 1997) profile with concentration c = ff y et al. 2008). Integrating this profile along the line-of-sight indicates that we probe a fraction of ∼ . ± . ∼ . ± . Article number, page 4 of 9isha Bachmann et al.: Low Surface Brightness Galaxies in distant Galaxy Clusters probed with HST
Fig. 3.
A selection by size and apparent magnitude, in F140W, of our LSB galaxies. The panels show the di ff erent clusters. Red:
LSB galaxiesdetected following our selection criteria and with a weight higher than 0.5.
Blue and Grey: the samples by van Dokkum et al. (2015a) and Mobasheret al. (2001), both shifted to our observed redshift by accounting for an E + K correction (see Sect. 4). We plot curves of constant surface brightness, µ (r e ff , circ ) = − evolved from the Coma cluster redshift to the high- z clusters. Average errorbars for our sample are shown. Fig. 4.
Same es Fig. 3 but for both clusters combined and with the sam-ple by van Dokkum et al. (2015a) with the size evolution taken out.
Cyan: the sample by van Dokkum et al. (2015a), where the suggestedsize evolution since z ∼ Based on the assumed magnitude and size distribution , and af-ter applying the needed correction for missed area, we wouldestimate a total number of 80 ±
38 UDGs in SPTCL-2106 and36 ±
25 UDGs in MOO-1014. Studies of local clusters suggestan abundance of ∼ z ∼
1. A possible explanation for this impliedevolution is that we are assuming the UDG progenitors to be ofthe same size as local Universe UDGs. If their progenitors wereactually smaller at z = We considered uncertainties in the assumed size distribution (as mea-sured in van der Burg et al. 2016) and magnitude distribution (as mea-sured in Danieli & van Dokkum 2019), finding that these a ff ect our es-timated number of high-z UDGs by at most 14%, hence not impactingour conclusions. Martin et al. 2019; Wright et al. 2021) they would not fall intoour selection criteria and thus be missed in the current analysis.Assuming the size distribution of UDGs in the local universe,as described by the power law shown in Fig. 7 of van der Burget al. (2016), we find that a size growth by a factor ∼ ∼ z ∼
1. Hydrodynamical simulationsby Martin et al. (2019) would predict a slightly larger size growthby a factor ∼ ffi cient toexplain the observed underabundance of UDGs.
5. Summary and outlook
This paper studies the abundance of LSB galaxies in two z > – Within the parameter space we defined, we find a statisticaloverdensity of 32 ±
13 LSB galaxies in SPTCL-2106 and 20 ±
13 in MOO-1014. – We find the colours of those LSB galaxies in SPTCL-2106and MOO-1014 to be consistent with an extension of the redsequence, as defined by spectroscopically identified brightercluster members. This suggests that the LSB galaxies in bothclusters are already evolving passively. – Based on a simple stellar population evolution model, wecompare our detected LSB galaxies with the expected pro-genitors of local UDGs in the Coma cluster. This suggeststhat the faintest sources we can detect approximate the ex-pected progenitors of local UDGs. – Based on an extrapolation, motivated by local scaling rela-tions, we estimate an overall abundance of 80 ±
38 UDGsin SPTCL-2106 and 36 ±
25 UDGs in MOO-1014. We notethat this is about three times lower than the abundance ofUDGs in local galaxy clusters having similar masses.
Article number, page 5 of 9 & A proofs: manuscript no. aanda
Fig. 5.
Colour-Magnitude diagrams for the sample of LSB candidates in the clusters after correcting for fore- and background interlopers andincompleteness. The colourbar gives the residual weight of the data points (as detailed in Sect. 3.4). The left panel shows the cluster SPTCL-2106 and the right panel the cluster MOO-1014. Additionally shown in the left panel are the positions in the colour-magnitude diagram ofspectroscopically-confirmed members of SPTCL-2106, and an extrapolation of the red sequence as defined by the confirmed members of SPTCL-2106, ignoring some of the data points that may be part of a bluer / starforming cluster population to guide the reader’s eye. This line has a slope of-0.12. – One way to interpret the implied evolution is by assuming asubstantial size growth of dwarf galaxies since z ∼
1, whichwould then increase the numbers of those that classify asUDGs. As we discuss, this is qualitatively consistent withresults from hydrodynamical simulations.We stress that this study uses the deepest data available forgalaxy clusters at z > Acknowledgements.
We thank the anonymous referee for their useful commentsthat substantially clarified the paper. AB acknowledges a 6-week ESO summerstudentship during which a substantial part of this research was done.
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Appendix A: RGB images of the clustersAppendix B:
SExtractor parameters
Table B.1.
SExtractor parameters used. All other parameters wereleft to their defaults.
Parameter ValueDETECT_MINAREA 7DETECT_THRESH 1.1ANALYSIS_THRESH 1.1BACK_TYPE MANUALBACK_VALUE 0FILTER_TYPE GAUSSIANFILTER default
Appendix C: Additional Figures
Article number, page 7 of 9 & A proofs: manuscript no. aanda
Fig. A.1.
RGB (red = F140W, green = F105W, blue = F814W) image of the cluster SPTCL-2106.Article number, page 8 of 9isha Bachmann et al.: Low Surface Brightness Galaxies in distant Galaxy Clusters probed with HST
Fig. A.2.
RGB (red = F140W, green = F105W, blue = F814W) image of the cluster MOO-1014.