A search for faint low surface brightness galaxies in the relaxed cluster Abell 496
Melville P. Ulmer, Christophe Adami, Florence Durret, Olivier Ilbert, Loic Guennou
aa r X i v : . [ a s t r o - ph . C O ] J a n Astronomy&Astrophysicsmanuscript no. Ulmer2011 c (cid:13)
ESO 2018November 20, 2018
A search for faint low surface brightness galaxies in the relaxedcluster Abell 496 ⋆ M. P. Ulmer , , C. Adami , F. Durret , , O. Ilbert , and L. Guennou LAM, Pˆole de l’Etoile Site de Chˆateau-Gombert, 38 rue Fr´ed´eric Joliot-Curie, 13388 Marseille Cedex 13, France Department of Physics and Astronomy, Northwestern University, 2131 Sheridan Road, Evanston IL 60208-2900, USA UPMC Universit´e Paris 06, UMR 7095, Institut d’Astrophysique de Paris, F-75014, Paris, France CNRS, UMR 7095, Institut d’Astrophysique de Paris, F-75014, Paris, FranceAccepted . Received ; Draft printed: November 20, 2018
ABSTRACT
Context.
Cluster faint low surface brightness galaxies (fLSBs) are di ffi cult to observe. Consequently, their origin, physical properties andnumber density are not well known. After a first search for fLSBs in the highly substructured Coma cluster, we present here a search for fLSBsin the nearly relaxed Abell 496 cluster. Aims.
Abell 496 appears to be a much more relaxed cluster than Coma, but still embedded in a large scale filament of galaxies. Our aim is tocompare the properties of fLSBs in these two very di ff erent clusters, to search for environmental e ff ects. Methods.
Based on deep CFHT / Megacam images in the u ∗ , g ′ , r ′ and i ′ bands, we selected galaxies with r ′ >
21 and µ r ′ >
24 mag arcsec − .We estimated photometric redshifts for all these galaxies and kept the 142 fLSBs with photo − z < . Results.
In a g ′ − i ′ versus i ′ color-magnitude diagram, we find that a large part of these fLSBs follow the red sequence (RS) of brighter galaxies.The fLSBs within ± σ of the RS show a homogeneous spatial distribution, while those above the RS appear to be concentrated along the largescale filament of galaxies. Conclusions.
These properties are interpreted as agreeing with the idea that RS fLSBs are formed in groups prior to cluster assembly. Theformation of red fLSBs could be related to infalling galaxies.
Key words.
Galaxies: clusters: individual (Abell 496), Galaxies: luminosity function
1. Introduction
Faint low surface brightness galaxies (fLSBs hereafter) remaina poorly known class of galaxies, though they are interest-ing objects for several reasons, as already discussed in detailby Adami et al. (2009a). We define fLSBs as galaxies with acentral surface brightness fainter than µ r ′ =
24 mag arcsec − and a total magnitude r ′ >
21, to be consistent with Adamiet al. 2006, hereafter ASU06. Briefly: fLSBs could accountfor part of the missing low luminosity structures predictedby CDM models of hierarchical structure formation (White &Rees 1978), in particular since they appear dominated by darkmatter (e.g. McGaugh et al. 2001, de Blok et al. 2001). CDMmodels predict the existence of low luminosity galaxies in allenvironments, but fLSBs seem to be present in higher num- ⋆ Based on observations obtained with MegaPrime / MegaCam, ajoint project of CFHT and CEA / DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National ResearchCouncil (NRC) of Canada, the Institut National des Sciences del’Univers of the Centre National de la Recherche Scientifique (CNRS)of France, and the University of Hawaii. The data processing were per-formed by the TERAPIX Data Centre. bers in clusters than in the field (see e.g. Sabatini et al. 2005,ASU06, and references therein).Many fLSBs are fainter than the night sky and clearly ex-tend toward fainter brightnesses than predicted by the Freemanlaw (1970), as shown for example by Bothun et al. (1997). Dueto their extreme faintness both in terms of surface brightnessand of total magnitude, fLSBs are therefore very di ffi cult todetect, hence their origin, physical properties and number den-sity are not well known in a statistical way over a large num-ber of clusters, despite numerous studies (e.g. Binggeli et al.1985; Schombert et al. 1992; Bothun et al. 1993; Bernstein etal. 1995; Impey et al. 1996; Sprayberry et al. 1996; Ulmer etal. 1996; Impey & Bothun 1997; O’Neil et al. 1997; Kuzio deNaray et al. 2004).In order to increase the number of fLSBs detected in clus-ters, our team has searched for Coma cluster fLSBs in the to-tal magnitude versus central surface brightness space (ASU06,Adami et al. 2009a) and found for example that these objectstended to be more concentrated in several areas (not alwayscentral). Furthermore, based on their position in the (B − R) ver-sus R plane, we found that we could identify three distinct typesof fLSBs. Those that fall on the color magnitude relation ex-
M. P. Ulmer et al.: Faint low surface brightness galaxies in Abell 496 trapolated from the bright normal galaxy population we called sequence fLSBs. We interpreted sequence fLSBs as galaxiesthat formed in small groups prior to the cluster assembly. Thenwe interpreted the reddest fLSBs as faint stripped ellipticalsand the blue fLSBs as galaxies made of material stripped fromspiral infalling galaxies. However, the Coma cluster is highlysubstructured (e.g. Adami et al. 2005) and we do not know howsubstructure could a ff ect the spatial distribution of the fLSBpopulation. We therefore decided to analyze in the same waythe distribution and properties of fLSBs in a more relaxed clus-ter where substructures will not complicate the picture.Abell 496 is one of the rare nearby nearly relaxed clusters(see e.g. Durret et al. 2000). Bou´e et al., (2008) reported the de-tailed analysis of the galaxy luminosity functions of Abell 496,based on deep CFHT Megacam images in four bands whichare ideal to search for fLSBs. They confirmed that this clusterappears very relaxed, with no particular structure at the clusterscale, though at larger scale an extended filament of galaxieswith redshifts close to that of Abell 496 was found to spreadfrom the north-west to the south-east of the cluster (see Fig. 10in Bou´e et al. 2008).The mean heliocentric velocity of Abell 496 iscz = − , corresponding to a redshift z = . − , assuming H =
72 km s − Mpc − , Ω M = . Ω Λ = .
7. It has an angular virial radius of 0 . ◦ (1.85 Mpc),obtained by extrapolating the radius of overdensity 500(Markevitch et al. 1999), measured relative to the criticaldensity of the Universe to the radius of overdensity 100. Wewill give magnitudes in the AB system.The paper is organized as follows. The data and method tosearch for fLSBs are described in Section 2. Results concerningthe color-magnitude relation, spatial distribution and luminos-ity function of fLSBs are presented in Section 3 and discussedin Section 4. We give in the Appendix the list of the 142 fLSBswith photo − z < .
2. The data and method
This work is based on deep images obtained at the CFHTwith the Megaprime / Megacam camera (program 03BF12, P.I.V. Cayatte) in the four bands u ∗ , g ′ , r ′ , i ′ already describedin detail by Bou´e et al. (2008). The images are centered onthe cluster centre as defined by NED: J2000.0 equatorial coor-dinates 04 h mn s , − ◦ ′ ′′ . They were reduced by theTERAPIX pipeline. Since simple detection with SExtractor(Bertin & Arnouts 1996) is not always su ffi cient to measurefLSB magnitudes unambiguously, we applied the same elabo-rate technique as in ASU06, which is briefly described below. In order to make the comparison with the fLSBs in Comastraightforward, we detected fLSBs with the same method asdescribed in ASU06. In brief, we started with a catalog pro- duced by SExtractor from the Abell 496 CFHT Megacam im-ages. Then, since our fLSB dedicated software could not beapplied to such large images, we divided each image (and thecorresponding catalogue) in 25 subimages, each 0 . × . .The first cut was to eliminate bright objects (total mag-nitudes r ′ <
21) from our analysis in order to be consistentwith the ASU06 selection process. This selection criterion isbased on the fact that part of the cluster fLSBs could be tidaldwarf galaxies (see ASU06). Tidal dwarf galaxies have massesas low as 10 or 10 M ⊙ (Bournaud et al. 2003), and as shownin ASU06 this translates to magnitudes fainter than r ′ ∼ ff raction spikes and between CCDs, andall SExtractor objects in these areas were removed from furtheranalysis.As we had images in four bands and our software was de-signed to process only two bands at once, we first consideredthe r ′ and u ∗ bands, in order to encompass the 4000 Å break atthe redshift of Abell 496 ( z = . r ′ and u ∗ bands to generate a primary data set.First, we fit a Gaussian form plus a constant backgroundto the linear-scale surface brightness profiles on the images, asin ASU06. Although fLSBs have exponential surface bright-ness profiles, Ulmer et al. (1996) found that fLSB selectionbased on exponential profiles generates a large number of falsecandidates in rich environments, due to the proximity of neigh-boring objects. Instead of using exponential profiles, ASU06therefore selected fLSBs by χ -fitting of Gaussian curves tothe radial surface brightness profiles of fLSBs. This does notmean that an exponential is not the proper form of fLSB pro-file. Rather, the Gaussian profile is the result of the intrinsic (ex-ponential) shape convolved with instrumental e ff ects (the PSF,due to seeing, had typical values between 0.4 and 0.6 arcsec).Initially, we let the radial profiles extend to a maximum radius θ max = . σ and not as the FWHM of the profile (FWHM = σ ). Thesize threshold was chosen above the seeing radius in order tolimit contamination by globular clusters which at the distanceof Abell 496, appear as point sources. The r ′ central surfacebrightness was chosen fainter than µ r ′ =
24 mag arcsec − to beconsistent with ASU06.Third, we optimized the final value of θ max for all the se-lected candidates to ensure that none of their surface bright-ness profiles were contaminated by surrounding objects. Thisprocess is explained in more detail in ASU06. The optimized θ max for each candidate was determined by visual inspection.We then repeated the two previous steps. After inspecting allcandidates visually we selected as final fLSBs the candidatesthat yielded an acceptable Gaussian fit to a distance of θ max (seealso ASU06 for more details). By “acceptable” we mean thatthe probability of finding a better fit (by changing the parame-ters) is smaller than 10%. An example is shown in Fig. A.2. . P. Ulmer et al.: Faint low surface brightness galaxies in Abell 496 3 The convergence / non-convergence was done as follows: ifthe chi-squared changed by less than 0.1% within 20 itera-tions, this was called convergence. If the chi-squared failedto decrease by less than 0.1% in 20 iterations or if the chi-squared actually grew without bound, then this was called non-convergence.The final data set was defined requiring a good (i.e. con-verging) fit for both the r ′ and u ∗ bands simultaneously. Wethen computed magnitudes for this sample in the g ′ and i ′ im-ages. The automated analysis produced valid Gaussian fits mostof the time. For objects with a non converging process (for ex-ample only 3 cases for the g ′ band), we calculated the missing i ′ and g ′ band magnitudes by comparing the SExtractor mag-nitudes with the results from our dedicated code for the fLSBswith a converging process. Then we applied the relation de-duced in this way to the fLSBs with a non converging process. Independently, we calculated the photometric redshifts (here-after photo − z s) for all the galaxies detected in the images,based on the SExtractor magnitudes in the four bands by ap-plying the LePhare software (Ilbert et al. 2006). The zero pointof each band was adjusted using a spectroscopic catalog of 596galaxies brighter than i ′ ∼ ffi ciently discriminate between z ≥ < − z s < < − z below 0.2 (142 galaxies); (b) fLSBs witha photo − z above 0.2 (783 galaxies).The goal being to study cluster galaxies, we must estimatethe contamination of the sample of 142 fLSBs by non clustergalaxies. From Fig. 1, the expected contamination of the z < ≥ < ∼ < < = at photo − z < ≤ Fig. 1.
Upper figure: i ′ band magnitude histogram of our spec-troscopic sample. Lower figure: photometric versus spectro-scopic redshifts.Along the line of sight to Abell 496, we detected 122 fLSBsbrighter than i ′ = . = field, so 21 of these should therefore notbe part of the cluster. Extrapolating this number to the completemagnitude range, 24 of the 142 detected fLSBs at photo − z < < ∼ ±
22 fLSBs at z ≤ − z < ff ectively located at z < . > . < .
3. Results
The list of our 142 z < M. P. Ulmer et al.: Faint low surface brightness galaxies in Abell 496
Fig. 2.
Color–magnitude relation for all the fLSBs in the direc-tion of Abell 496 (in black), and for the 142 galaxies for whichthe photo − z is less than 0.2 (in blue). The red line correspondsto the red sequence for bright galaxies (see text). Fig. 3.
Zoom on the color–magnitude relation for the fLSBsin the direction of Abell 496 with photo − z < . red and blue fLSBs. The separation between these two lines corresponds tothe approximate 1 σ loci, ± .
29 mag on either side of the redsequence.
The g ′ − i ′ versus i ′ color-magnitude relation obtained for ourfLSBs is plotted in Fig. 2, together with the color-magnitude re-lation found by Bou´e et al. (2008) for the “normal” galaxies ofAbell 496. The red sequence defined for the galaxies belongingto Abell 496 was computed by Bou´e et al. (2008) for galaxiesbrighter than i ′ ∼
21 to be: g ′ − i ′ = − . i ′ + .
75. We can seein Fig. 3 that most of the fLSBs with photo − z < . − z > . Fig. 4.
Positions of the fLSBs with photo − z < .
2. Black, cyanand red points correspond to fLSBs within ± σ of the red se-quence, below, and above this interval respectively. The blackline indicates the direction of the very large scale filament ofgalaxies found by Bou´e et al. (2008) – see their Figure 10. Thecontours correspond to the X-ray emission from the XMM-Newton EPIC MOS1 image (Lagan´a et al. 2008).are mostly redder and therefore background objects. This isnot surprising, as the photometric redshift selection is primar-ily based on colors and therefore defines relatively blue colorsat low redshift and relatively red colors for higher redshift.We show in Fig. 3 a zoom of the color-magnitude relationfor the 142 fLSBs with photo − z < .
2. We can define three sub-samples: the sequence fLSBs (within ± σ , or ± .
29 mag fromthe red sequence), the blue fLSBs (more than 1 σ below the redsequence), and the red fLSBs (more than 1 σ above the red se-quence). This classification is similar to that already proposedfor fLSBs in Coma (ASU06), suggesting that a large fraction(here about 2 /
3) of fLSBs follows an evolutionary path compa-rable to that of normal ellipticals in clusters. We will discussthis result in more detail in Section 4.
A bi-dimensionnal Kolmogorov Smirnov (KS hereafter) testshows that the α , δ spatial distribution of fLSBs at z < ff erent at the 92% level from a uniform distribution; the sameKS test shows that the spatial distributions of the z < ≥ ff erent at the 99.9% level. The fLSBs witha high probability of belonging to Abell 496 are therefore notas uniformly distributed throughout the cluster as the galaxieslikely to be non-cluster members.The α , δ spatial distributions of the photo − z < . sequence , red , and blue fLSBs shown in Fig. 4 are also dif-ferent. The distribution of blue fLSBs is di ff erent from a uni-form spatial distribution only with a probability of less than 1%from a KS test, so we can say that blue fLSBs are relatively uni-formly distributed. On the other hand, sequence and red fLSBsare di ff erent from a uniform spatial distribution with respectiveprobabilities of 90 and 99%, based on a KS test. In Fig. 4, the . P. Ulmer et al.: Faint low surface brightness galaxies in Abell 496 5 Fig. 5.
Spatial distribution of galaxies of various types withthe following symbols: red hexagones for ellipticals, purplesquares for early-type spirals, green triangles for intermediatespirals, and blue diamonds for late-type spirals. The spectraltypes of these galaxies were determined in the photo − z com-putation process fixing the redshifts to their spectroscopic val-ues. The black open symbols (squares, triangles and circles)show the three main dynamically distinct groups. Contours cor-respond to the XMM-Newton X-ray emission. The black lineindicates the direction of the very large scale filament of galax-ies found by Bou´e et al. (2008). red fLSBs generally tend to be found preferentially along thelarge scale filament of galaxies found by Bou´e et al. (2008).This suggests that red fLSBs could be linked with this filamentmade up of groups infalling toward the Abell 496 center.Since the cluster Abell 496 is believed to be nearly relaxed(Durret et al. 2000), it is important to determine if it is still ex-periencing an infalling activity. We searched for substructuresin Abell 496 by applying the Serna & Gerbal (1996) method toour large spectroscopic redshift sample of 596 galaxies (Durretet al. 1999). We show in Fig. 5 the spatial distribution of galax-ies belonging to various independent dynamical structures in-side the Abell 496 cluster.We only detected three such structures and they are all lowmass structures of a few 10 M ⊙ . These masses are very smallcompared to the overall cluster mass of the order of 3.5 10 M ⊙ (e.g. Lagan´a et al. 2010) and do not prevent us from clas-sifying the cluster from being relatively well relaxed. The de-scription of the Serna & Gerbal method and the full analysis ofthe results thus obtained can be found in Appendix B.We also see from Fig. 5 that the two main substructures arelocated towards the northwest and southeast of the cluster, thatis roughly along the direction of the large scale filament feedingthe cluster. A third less massive structure is located towards thewest. Cold dark matter hierarchical structure formation models(e.g. Colberg et al 1999) predict that clusters of galaxies growvia group accretion. In this context, the cluster substructuresdetected along the path of the large scale filament are probablyrecent infallen groups.We also quantified the spatial distribution of galaxies withphoto − z < . Fig. 6.
Upper figure: number of galaxies per square arcmin ver-sus distance to the cluster center, considering the 142 fLSBswith photo − z < .
2. The vertical dashed line shows the clus-ter central galaxy radius. Lower figure: number of galaxies persquare arcmin versus distance to the cluster center, consider-ing the 5766 galaxies with photo − z < . ) thus obtained are drawn in Fig. 6.These distributions were fit by a King model and weadded a constant background to take into account non-clusterphoto − z < . I ( r ) = P (0) + P (1) / (1 + ( r / P (2)) ) . For the 142 fLSBs with photo − z < .
2, the best fit was obtainedfor the following parameters: P(0) = (6.8 ± − arcmin − ,P(1) = ± − , P(2) = ± M. P. Ulmer et al.: Faint low surface brightness galaxies in Abell 496
For all the galaxies, the corresponding numbers are:P(0) = ± .
09 arcmin − , P(1) = ± .
54 arcmin − ,P(2) = ± .
51 arcmin.As can be seen in Fig. 6, fLSBs in Abell 496 are not uni-formly distributed, but are preferentially found toward the clus-ter center, except for a decrease of the number of fLSBs perarcmin in the cluster innermost point. This data point is mostlikely low because it is located inside the central galaxy radius,thus fLSBs in this region would have been missed by our anal-ysis. Note that although the fLSBS are concentrated toward thecluster center they are less concentrated than the whole galaxypopulation, as discussed in Section 4. Before computing a luminosity function for fLSBs, it is im-portant to investigate the completeness level of our sample.We show in Fig. 7 the magnitude histogram of the 783 fLSBswith available photo − z s and the luminosity function of the 142fLSBs with photo − z < .
2. The peak of the magnitude his-togram for the 783 fLSBs is located close to r ′ = .
7. Thisgives a first estimate of the completeness limit of the sample.We also performed simulations in order to have an independentestimate of the completeness in the r ′ images.The simulation adds artificial objects of di ff erent shapesand magnitudes to the CCD images and then attempts to re-cover them by running SExtractor again with the same param-eters used for primary object detection (see Adami et al. 2006for more details). In this way, the completeness is measuredon the original CCD frames. We estimated the completeness ofour catalog for fLSBs using simulated point-like objects with aGaussian profile of FWHM 3.3 arcsec ( σ = ff erent sub-regions to have the completeness at di ff erent locations in thecluster. The percentage of recovered fLSBs as a function of the r ′ magnitude is shown in Fig. 9, where error bars show the vari-ation among these 100 regions. We can see that we reach a 50%completeness at r ′ ∼ − . ± .
1. This is significantly shal-lower than the global luminosity function of Bou´e et al. (2008),who found a slope of − . ± .
05. This means that the fLSBscannot be responsible for all the increase of the global galaxyluminosity function of the cluster at faint magnitudes. We prob-ably start missing fLSBs for r ′ magnitudes fainter than ∼ r ′ ∼ − .
4. Discussion
As described above, we have found 142 fLSBs in the directionof Abell 496 with photo − z < .
2, out of which about 80% are
Fig. 7.
Upper figure: r ′ -band magnitude histogram of the 783fLSBs with available photo − zs . Lower figure: luminosity func-tion for the 142 fLSBs with photo − z < . r ′ -band magnitude (assuming these objects are clustermembers). The vertical line shows the approximate complete-ness level of the fLSB sample derived from our simulations.The oblique line shows the mean slope of the luminosity func-tion for absolute r ′ -band magnitude brighter than − . Fig. 8.
Dispersion σ (in arcsec) of the photo − z < . . P. Ulmer et al.: Faint low surface brightness galaxies in Abell 496 7 Fig. 9.
Percentage of recovered fLSBs in our simulations as afunction of the r ′ magnitude. The dotted lines show the 50%completeness level.probably cluster members. Their angular density profile is wellfit by a King model with a core radius about twice as large as fornormal galaxies. The King distribution of fLSBs in Abell 496is very di ff erent from what was observed in Coma by ASU06,where fLSBs do not follow any King-like distribution. This dif-ference is consistent with the idea that Abell 496 is relaxedwhile Coma is not. Furthermore, the wider radial distributionof the fLSBs versus normal galaxies in Abell 496 is consistentwith the idea that mass segregation has occurred in Abell 496.The detected fLSBs fall reasonably well on the extensionof the bright end of the color-magnitude relation established byBou´e et al. (2008). The fact that we have found (see section 3.1)the ± σ interval for the fLSBs around the red sequence similarin Abell 496 and Coma, one a relaxed cluster, the other not,fits with the idea that the relaxation state of the cluster does notinfluence the position of the fLSBs on the red-sequence. Thesimilar red-sequence width in both clusters could be attributedto sequence fLSBs having evolved in similar groups that fellinto the clusters later, as suggested by ASU06.On scales of ≥ < . red fLSBs (with redshifts < .
2) seem to have an anisotropic distribution similar to thefilament found by Bou´e et al. However, blue fLSBs show noobvious anisotropic distribution, suggesting they had a di ff erentevolutionary history. Blue fLSBs are perhaps the remnants oftidally disrupted late-type galaxies as hypothesized by ASU06for Coma.In terms of tidal disruption, we note that the spatial distri-bution of fLSBs seems to show no holes in the cluster center,which is not the case for Coma (ASU06). For Coma the fLSBscould have been destroyed by tidal disruption due to the mas-sive D galaxies in the Coma core. In contrast, there is only onecentral galaxy in the center of Abell 496, which could producemuch less tidal disruption. It is beyond the scope of this work,though, to carry out numerical simulations to verify or falsify the idea that fLSBs are tidally destroyed in the core of Comaand not in Abell 496.
Acknowledgements. We thank the referee for useful comments. Weacknowledge our collaboration with G. Bou´e and V. Cayatte duringthe first stages of this project and are grateful to T. Lagan´a for giv-ing us her XMM-Newton images. M. Ulmer thanks UPMC, IAP, Aix-Marseille I University, and LAM for their hospitality during the di ff er-ent stages of this project, and Bryant Smith and Nicholas Logenbaughfor software support. References
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M. P. Ulmer et al.: Faint low surface brightness galaxies in Abell 496
Fig. A.2. r ′ band surface brightness profile (in mag arcsec − asa function of radius in arcsec for the galaxy of Fig. A.1). Thedot-dashed and dotted lines show the Gaussian and exponentialfits respectively.McGaugh S.S., Rubin V.C. & de Blok W.J.G. 2001, AJ 122,2381O’Neil K., Bothun G. D., Cornell M. E., 1997, AJ 113, 1212Sabatini S., Davies J., van Driel W. et al. 2005, MNRAS 357,819Schombert J. M., Bothun G. D., Schneider S. E., McGaugh S.S., 1992, AJ 103, 1107Serna A. & Gerbal D. 1996, A&A 309, 65Sprayberry D., Impey C. D., Irwin M. J., 1996, ApJ 463, 535Ulmer M. P., Bernstein G. M., Martin D. R., et al., 1996, AJ112, 2517White S.D.M. & Rees M.J. 1978, MNRAS 183, 341Wojtak R. & Łokas E. L. 2010, MNRAS 408, 2442 Appendix A: Example of postage stamp images
Postage stamps images in the four bands are shown for oneof the 142 fLSB candidates with photo − z < . Appendix B: Search for substructures in Abell 496
Based on the large spectroscopic and photometric cataloguesacquired for Abell 496 (Bou´e et al. 2008), we have estimatedthe spectral type of each galaxy with the Le Phare photometricredshift software. Galaxies are then assigned a spectral type:type 1 for ellipticals, type 2 for early type spirals, type 3 forintermediate type spirals and type 4 for late type spirals.In order to search for substructures, we applied the Serna& Gerbal (1996) software to galaxies with measured spectro-scopic redshifts and magnitudes. This hierarchical method al-lows to extract galaxy substructures or groups from a cataloguecontaining positions, magnitudes and redshifts, based on thecalculation of their relative (negative) binding energies. Themethod gives as output a list of galaxies belonging to eachgroup, as well as the information on the binding energy of thegroup itself, and on the mass of each substructure, assuming a mass to luminosity ratio (M / L). We used here a M / L ratio inthe r ′ band of 200, as previously assumed for the Coma clusterby Adami et al. (2005), based on the Coma cluster M / L ratiogiven by Łokas & Mamon (2003).The Serna & Gerbal analysis shows the existence of threesubstructures (also see Section 4). These all have low masses(smaller than a few 10 M ⊙ ) and therefore their existence doesnot contradict the overall relaxed structure of the cluster.If we analyze the morphological type distribution of thegalaxies belonging to these three substructures (also see Fig. 5),we find that only one galaxy is of type 4 (late type spiral), cor-responding to ∼
1% of all the galaxies in substructures. If weestimate the percentage of type 4 galaxies in the cluster (i.e.in the [0.0229,0.0429] redshift range) that are not included insubstructures, we find a value of 23%. The di ff erence betweenthese two values could be interpreted as indicating that late typespirals tend to avoid substructures and fall individually into thecluster, while earlier type galaxies fall into the cluster insidegroups. . P. Ulmer et al.: Faint low surface brightness galaxies in Abell 496 9 Table A.1.
Properties of the fLSBs with photo − z < . Nb RA (J2000.0) DEC (J2000.0) u ∗ err( u ∗ ) g ′ err( g ′ ) r ′ err( r ′ ) i ′ err( i ′ ) photo − z Fig. A.1.
Postage stamp images of one fLSB in the four photometric bands (galaxy . P. Ulmer et al.: Faint low surface brightness galaxies in Abell 496 11
Table A.2.
Same as Table A.1 for objects
56 68.3343 -13.0432 23.22 0.04 22.59 0.02 22.25 0.03 23.17 88.00 0.0557 68.3349 -13.0424 24.89 0.08 24.73 0.05 23.87 0.03 23.56 0.03 0.0558 68.3396 -13.1014 23.65 0.04 22.29 0.02 21.94 0.02 21.61 0.01 0.0259 68.3412 -13.1677 26.71 0.43 24.87 0.06 25.86 0.21 26.06 88.00 0.0860 68.3429 -13.2528 23.62 0.04 22.43 0.02 21.88 0.02 21.17 88.00 0.1661 68.3435 -13.1890 23.33 0.04 22.27 0.02 21.88 0.02 20.59 88.00 0.0962 68.3508 -13.5357 24.48 0.10 23.31 0.04 22.81 0.03 22.49 88.00 0.0963 68.3589 -13.6966 25.05 0.26 24.44 0.13 24.02 0.10 23.61 0.05 0.0964 68.3623 -13.1966 25.82 0.13 24.74 0.06 24.43 0.05 23.58 88.00 0.0465 68.3642 -13.3606 24.04 0.06 22.70 0.02 22.13 0.02 22.02 0.02 0.1266 68.3797 -12.9096 24.20 0.06 23.23 0.03 22.72 0.02 22.19 88.00 0.1767 68.3830 -13.3300 24.00 0.05 22.85 0.03 22.40 0.02 22.09 88.00 0.1068 68.3896 -13.3450 24.09 0.07 22.96 0.03 22.42 0.02 21.65 88.00 0.1069 68.3967 -13.0617 24.55 0.07 23.39 0.03 23.06 0.03 22.55 0.02 0.1170 68.4025 -13.3745 23.14 0.03 23.19 0.03 22.84 0.03 14.50 88.00 0.1371 68.4032 -12.9952 25.63 0.21 25.53 0.22 27.65 3.41 24.80 0.34 0.1372 68.4050 -12.9890 23.65 0.03 23.22 0.03 22.64 0.02 22.13 0.01 0.1873 68.4103 -13.1480 23.79 0.05 22.40 0.02 21.94 0.02 21.72 0.01 0.0874 68.4251 -12.8447 23.54 0.04 22.49 0.02 22.25 0.02 22.09 0.01 0.1775 68.4301 -13.6645 22.84 0.04 22.04 0.02 21.76 0.02 21.57 0.01 0.1976 68.4366 -13.3139 23.47 0.03 22.68 0.02 22.30 0.02 21.90 0.01 0.0377 68.4389 -13.1513 24.57 0.06 24.07 0.04 23.49 0.03 23.20 0.03 0.1178 68.4415 -13.2490 23.77 0.04 22.88 0.02 22.36 0.02 22.07 0.01 0.1279 68.4465 -13.3644 24.81 0.11 23.54 0.04 23.09 0.03 21.89 88.00 0.0580 68.4475 -13.6480 24.94 0.16 26.39 0.43 26.11 0.65 24.47 0.17 0.1681 68.4486 -13.1831 24.30 0.07 23.07 0.03 22.45 0.02 22.90 88.00 0.1882 68.4502 -12.8030 24.36 0.04 23.69 0.04 23.29 0.03 22.60 0.02 0.0883 68.4568 -13.2141 23.75 0.04 22.81 0.02 22.16 0.02 22.17 88.00 0.0184 68.4716 -13.2397 26.34 0.45 24.87 0.11 23.57 0.04 22.32 0.02 0.0685 68.4732 -13.5865 23.04 0.05 21.91 0.02 21.67 0.02 21.41 0.02 0.0186 68.4996 -13.4164 24.83 0.10 23.19 0.03 22.58 0.02 22.66 0.02 0.0987 68.5051 -13.2755 24.31 0.07 22.86 0.03 22.38 0.02 22.05 88.00 0.1388 68.5112 -13.2345 23.69 0.05 22.59 0.02 22.16 0.02 21.99 88.00 0.1589 68.5113 -13.1664 23.29 0.04 21.97 0.02 21.56 0.01 21.26 0.01 0.0990 68.5196 -13.3452 24.48 0.09 23.14 0.03 22.75 0.03 22.45 0.02 0.0991 68.5219 -13.2690 23.90 0.04 23.31 0.03 22.55 0.02 22.35 0.02 0.1692 68.5226 -13.1325 24.33 0.06 23.00 0.03 22.49 0.02 22.34 0.02 0.1293 68.5261 -13.6989 23.24 0.06 22.29 0.03 22.03 0.02 22.25 88.00 0.0894 68.5351 -13.3788 23.81 0.05 23.34 0.03 23.23 0.03 23.28 88.00 0.1195 68.5359 -13.1588 24.54 0.08 23.46 0.03 23.03 0.03 22.94 0.03 0.1096 68.5476 -13.3125 23.59 0.05 22.36 0.02 21.97 0.02 20.84 88.00 0.0997 68.5579 -13.3335 24.61 0.11 23.53 0.04 23.08 0.03 22.67 0.02 0.0998 68.5614 -13.1666 25.18 0.12 24.16 0.05 23.20 0.03 22.44 0.02 0.0499 68.5682 -13.1921 23.85 0.05 22.71 0.02 22.05 0.01 22.02 0.02 0.09100 68.5697 -13.0574 23.27 0.03 22.40 0.02 22.08 0.01 21.99 0.01 0.06101 68.5797 -12.9160 24.32 0.05 23.23 0.03 22.73 0.02 22.44 88.00 0.12102 68.5885 -13.2804 24.30 0.07 22.87 0.03 22.38 0.02 22.24 0.02 0.10103 68.5979 -13.6359 23.92 0.09 23.55 0.05 23.40 0.04 23.26 0.04 0.08104 68.6017 -13.3695 24.04 0.06 22.83 0.02 22.30 0.02 22.07 0.01 0.12105 68.6066 -13.2390 23.47 0.03 22.66 0.02 22.21 0.02 22.03 0.02 0.12106 68.6121 -13.7463 23.11 0.06 22.15 0.02 21.85 0.02 22.08 88.00 0.16107 68.6190 -13.2040 24.59 0.07 23.01 0.02 22.64 0.02 22.37 0.02 0.05108 68.6193 -13.7032 22.97 0.04 22.10 0.02 21.77 0.02 21.66 0.02 0.01109 68.6264 -13.4053 23.82 0.07 22.34 0.02 21.82 0.02 20.75 88.00 0.13110 68.6380 -13.2445 24.34 0.06 23.12 0.03 22.60 0.02 22.53 0.02 0.122 M. P. Ulmer et al.: Faint low surface brightness galaxies in Abell 496
Table A.3.