arXiv:1510.04274v1 [astro-ph.GA] 14 Oct 2015 op&Mtre(95 dnie v cutr"i h the in the "clusters" five of a ( " identified region 2008), cases (1985) al. others et Materne Castelli the & term Smith 1990), Hopp 1985; in the Materne Sandage used & example, & (Hopp authors (Ferguson cluster" for Some of group" as, clusters. galaxy "galaxy clear the Fornax as in or assembly Virgo not galaxy is the Antlia of character The Introduction 1. esplmn u aaysml ihltrtr aa endin data, Antl literature in with located sample galaxy of our pap velocities this supplement In We radial yet. new done present been we not has population galaxy a Antlia’s et Cantiello galaxies. both However, for moduli (2001). distance same al. the et quoted Blakeslee (2005) brightness surface by with nearer whic obtained tuations 2008), Mpc al. distances et few the Bassino with a 2003b; agrees al. For be luminosi et could (Dirsch like cluster 3268 3258 NGC globular clusters NGC than that The in indicated 2001). those function al. than et (Drinkwater higher nax Antlia, of population op&Mtre(95 n,mr eety ese l (2015) elliptica al. et giant of Hess recently, dispersions a more velocity of and, estimated 2 (1985) typical al. Materne et still & (Bassino clusters Hopp globular is 3268 5000 NGC almost but with of galaxy, poorer, system NG The somewhat ric of members. is system exhibit 6000 cluster about that globular contains 1997) 3258 The al. systems. et cluster Pedersen globular 2000; al. et kpc. (Nakazawa 220 elliptic of giant distance projected the a around in 3268 concentrated NGC and groups 3258 two NGC of ap that morphological Its is II". 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J. o h bright the for pearance 008). fluc- ABSTRACT eetorglx apewt ieauedt,edn pwit up ending data, literature with sample galaxy our lement 1 , als nd er, ia. 2 ty C l. h h g - iaai 97 13AJCua uóoad unsAires, Buenos de Autónoma Ciudad C1033AAJ 1917, Rivadavia , l n .Richtler T. and cpin Chile ncepción, csyGoíia el nvria ainld aPaa and Plata, La de Nacional Universidad la de Geofísicas y icas NP,PsodlBsu S Bosque del Paseo UNLP), h in litcl G 3258 NGC ellipticals giant the h usinaottentr fte"nlaglx cluster galaxy "Antlia the of nature the about question the o ue with duced lt fewrs h isadfltfil orcin eeap were corrections e flat-field for and identified bias were the lines Afterwards, 20 slit. spectral than and more calibrations Typically, was wavelength tortions. VMSPCALDISP the set recipe determine a The to from used VMSPFLAT exposures. recipe flat-field the VM- dome with recipe of created mas the was normalised field with The flat field exposures. ter bias each individual for five from obtained BIAS was bias master a iin)adaseta eouinof resolution spectral a and sitions) th wavelen Å into 3700 a split spanning implied h, coverage 1 configuration was This time exposures. integration individual the field, science each adrne l 05,w dp itneo 5Mc which Mpc, 2014; 35 pa- of 2013, distance pc al. earlier a 169.7 et of Caso adopt scale with a we 2012; means 2015), consistency al. et al. et Castelli Calderón Smith maintain (e.g. To pers simplicity. for bett to us help will Antlia. of data structure These the velocities. understanding galaxy 105 with up hw h orqarnsfrec n ftesxVMSfields, VIMOS six the 1 of Figure one 2008. each di and using for 2007 quadrants four of the semesters shows first Richtler), the Tom during (PI observed the 079.B-0480(B) under and out o 60.A-9050(A) carried part grammes were inner observations the in The located cluster. fields Antlia six of in VLT-VIMOS population galaxy with the spectroscopy multi-object performed We reductions and Observations 2. ea eotdi rvosppr.Hwvr efidtregrou three find we However, papers. previous in reported as le n ouainsubstructure. population and ntefloig eke h em"nlacluster" "Antlia term the keep we following, the In h rtn a Rbu,adtesi it a 1 was width slit the and blue, HR was grating The einrflcsacnieal xeso ln h ieof line the along extension considerable a reflects region a ff ypplto nteinrpr fteAti lse and cluster Antlia the of part inner the in population xy ff rne ntercsinvlcte ahrta peculiar than rather velocities recession the in erences rn colours. erent esorex 3 nteuulmne o IO aa First, data. VIMOS for manner usual the in / ,B90W aPaa Argentina Plata, La B1900FWA N, / arcsec. / 28rnefo bu 00km 1000 about from range 3268 − 60Å(eedn ntesi po- slit the on (depending Å 6600 ∼ ril ubr ae1o 11 of 1 page number, Article 2 . h aawr re- were data The . Å 5 0 galaxy 105 h

c S 2018 ESO ". ps / s ′′ For . plied the f pro- dis- ach gth ree er - A&A proofs: manuscript no. PrintProof_Manuscript

4000

3500

3000 ] -1

2500 [km s R,lit V

2000

1500

1000 1000 1500 2000 2500 3000 3500 4000 -1 VR,ours [km s ] Fig. 2. Comparison between heliocentric measurements from this paper and the literature for Antlia members. Fig. 1. Positions of the six VIMOS fields (each one composed of four quadrants). The black circle is centred on the midpoint between the pro- jected positions for the two gEs, and its radius is 20′. North is up, east to the left. the 67 galaxies with VR,h measurements, 45 are thus members of our sample. to each science exposure, together with the wavelength calibra- Additional VR,h measurements were collected from the liter- tion. This was done with the recipe VMMOSOBSSTARE. In- ature (Smith Castelli et al. 2008, 2012) and the NED1. The final dividual exposures were then combined with the iraf task IM- sample of Antlia members consists of 105 galaxies, which are COMBINE to achieve a higher S/N. The spectra were extracted listed in Table2. A fraction of the present galaxieshad been mea- with the task APALL, also within iraf. We measured the helio- sured earlier. In these cases, we find good agreement between centric radial velocities using the iraf task FXCOR within the our measurements and the literature values (Figure2). The mean −1 −1 NOAO.RV package. We used synthetic templates, which were VR,h difference and dispersion are 20kms and 50kms , re- selected from the single stellar population (SSP) model spec- spectively. In comparison, the uncertainties of our measurements tra at the miles library (http : //www.iac.es/proyecto/miles, are typically in the range 10 − 40kms−1. Sánchez-Blázquez et al. 2006). We selected SSP models with the metallicities [M/H]= -0.71 and [M/H]= -0.4, a unimodal initial Ferguson & Sandage (1990) provided the photometrical cat- mass function with slope 1.30, and an age of 10Gyr. The wave- alogue of galaxies in Antlia with the largest spatial coverage, length coverage of these templates is 3700Å − 6500Å,and their while Calderón et al. (2015) have carried out a deeper survey spectral resolution is 3 Å FWHM. of the early-type galaxies in a region that contains our VI- MOS fields. To supplement the Ferguson & Sandage (1990) cat- We also obtained GEMINI-GMOS multi-object spectra from alogue, we derived B magnitudes for the galaxies measured programme GS-2013A-Q-37 (PI J. P. Calderón). The grating by Calderón et al. (2015) in the Washington (C, T1) photomet- B600_G5303 blazed at 5000Å was used with a slit width of ric system (Canterna 1976). As a result, we applied equation 4 1arcsec. The wavelength coverage spans 3300Å − 7200Å, de- from Smith Castelli et al. (2008) to transform (C − T1)0 into pending on the position of the slits. The data were reduced (B − R) colours. Then we obtained B magnitudes, considering using the GEMINI.GMOS package within IRAF. We refer to 0 that R and T1 filters only differ in a small offset (Dirsch et al. Caso et al. (2014) for more information about the reduction. 2003b). Figure3 shows the completeness for those galaxies lo- We could determine heliocentric radial velocities (VR,h) for ′ ′ cated within 30 of the cluster centre (see Section3.2), which 67 galaxies located in the inner 20 of the (i.e., roughly matches the region observed with our VIMOS fields the inner 200kpc for our adopted distance). In previous studies and previous spectroscopic studies (Smith Castelli et al. 2008, (Smith Castelli et al. 2008, 2012; Caso et al. 2013), those galax- −1 −1 2012). The bin width is 1mag. When we exclude galaxies with ies with VR,h between 1200kms and 4200kms have been Ferguson & Sandage (1990) membership status ‘3’ (which are assigned to Antlia (which already raised doubts owing to the the less probable members), the 80% completeness is reached at large velocity interval). In our enhanced sample, we find the −1 BT = 17mag and the 60% at BT = 19mag (small red circles). lowest velocity to be VR,h = 1150kms , and there are no ve- locities between 4300kms−1 and ∼ 7600kms−1. Galaxies with higher VR,h than the latter limit were rejected from our sample and are listed in Table5. It can be noticed that several galaxies 1 This research has made use of the NASA/IPAC Extragalactic from the Ferguson & Sandage (1990) catalogue are indeed in the Database (NED), which is operated by the Jet Propulsion Laboratory, background. In these cases, Ferguson & Sandage (1990) classi- California Institute of Technology, under contract with the National fied them as “likely members” or “probable background”. From Aeronautics and Space Administration.

Article number, page 2 of 11 Caso & Richtler: Deconstructing the Antlia cluster core

ters like Virgo (Conselice et al. 2001). Nakazawa et al. (2000) obtained a gas temperature of 2keV for the surroundings of NGC3268, which does not exclude a dispersion of 617 km/s, considering the σ-T- relation of Xue & Wu (2000) and its scat- ter. However, whether this high velocity dispersion obtained for our Antlia sample represents a single virialized system, is an in- teresting question, which we try to answer here. Assuming that the individual VR,h uncertainties are the sigma values of a normal distribution, we simulated the VR,h for each Completeness Fraction

0.2 0.4 0.6 0.8 1.0 oneof the 105galaxies by a Monte-Carlomethod.The procedure 14 16 18 20 22 was repeated 100 times, in all cases obtaining a non-rejection B of the normality hypothesis. This demonstrates that the intrinsic T uncertainties of the measurements do not play a central role in Fig. 3. Completeness analysis for the spectroscopic sample considering the result. the entire photometric catalogue (large light-blue circles), and exclud- Several studies in galaxy clusters found different velocity ing galaxies with Ferguson & Sandage (1990) membership status ‘3’ dispersions for the dwarf and bright galaxies populations (e.g. (small red circles). The bin width is 1 mag. Drinkwater et al. 2001; Conselice et al. 2001; Edwards et al. 2002). To test whether this is also true for the Antlia cluster, we subdivided our sample in giants and dwarfs using the morpho- logical classification from Smith Castelli et al. (2008, 2012) and, when this is not available, from Ferguson & Sandage (1990). Normal distributions were fitted by least squares to the dwarf and bright galaxies, separately. The resulting mean values for the bright and dwarf populations were 2768 ± 104kms−1 and 2698 ± 108kms−1, respectively. The corresponding dispersions are 698 ± 122kms−1 and 682 ± 120kms−1. From a K-S test, we cannot reject the hypothesis that both samples are drawn from the same distribution with 90 % confidence.

3.2. Morphological types and spatial distribution Assuming that the groups of galaxies dominated by the gEs as the main substructures of the cluster and that both haloes present similar masses (Pedersen et al. 1997; Nakazawa et al. 2000), we consider the midpoint between them as its approximate centre (α = 10h 29m 27s, δ = −35o 27′ 58′′). The 105 galaxies in our sample occupy a region of ∼ 7 degree2. Therefore, we obtained the mean surface density, −2 Fig. 4. Histogram of the VR,h distribution for the galaxies in our Antlia ∼ 15 degree , and the mean distance between galaxies, ∼ sample. The bin width is 150 km s−1. Overplotted are the smooth ve- 9arcmin. And for each galaxy, we calculated the number of locity distribution, obtained with a Gaussian kernel (green solid curve), neighbours nearer than this value, and we used it to split the and the normal profile fitted by least squares (brown dashed curve). galaxies in four percentiles. The left-hand panel of Figure 5 shows the regions occupied by the 25, 50, and 75 percentiles of galaxies with a larger number of neighbours. The white as- 3. Results teriks correspond to the position of NGC3258 (south-west) and NGC3268 (north-east).The origin of the black circle is the bona 3.1. VR,h velocity distribution fide centre of the cluster, and its radius is 30′ (i.e. ∼ 300kpc at The histogram of the VR,h distribution for the galaxies in Antlia distance). The 25 percentile seems to clearly represent the our Antlia sample is shown in Figure 4 with a bin width of overdensities of galaxies around both gEs, which are also iden- 150kms−1. The green solid curve represents the smooth velocity tified in the 50th percentile. The lower percentile shows a more distribution, obtained with a Gaussian kernel. The distribution extended spatial distribution. resembles a Gaussian, which would be expected if the spatial In the middle panel of Figure5, the projected spatial distri- velocity for the Antlia members is described by a Maxwellian bution for bright galaxies (i.e., all those galaxies not classified distribution. as dwarfs) is plotted. Different symbols indicate the Hubble type A Shapiro-Wilk normality test returns a p value of 0.9 (i.e., spirals, lenticulars, or ellipticals). The colour palette ranges (Shapiro & Wilk 1965; Royston 1995, hereafter S-W test), from red to blue, spanning the VR,h range defined in Sect.2. The meaning that we cannot reject the hypothesis that our sample origin and radius of the black circle are identical to those in the is drawn from a normal distribution. Considering this, we fit- left-hand panel. The two groups around each of the gEs can be ted a normal profile to the VR,h distribution by least-squares, clearly identified. There are also several galaxies located in the assuming Poisson uncertainties for the bin values. The best fit extrapolation of the straight line joining the two gEs, mainly to corresponds to a Gaussian distribution with a mean velocity of the north-east.These galaxies,as well as those located in the cen- −1 −1 2708 ± 42kms and a dispersion of 617 ± 46kms (dashed tral part of the cluster, mainly present intermediate VR,h. There −1 brown curve in Fig.4), which agree with the values from opti- are a few galaxies with VR,h < 2000[kms ], whose projected cal data by Hess et al. (2015). This dispersion fits massive clus- positions agree with the general scheme of galaxies with inter-

Article number, page 3 of 11 A&A proofs: manuscript no. PrintProof_Manuscript

− − 4300 km s 1 4300 km s 1 Percentile 25 Ellipticals Percentile 50 Lenticulars Percentile 75 Spirals

− − 1 1100 km s 1 1100 km s DEC [º] DEC [º] DEC [º] −35.8 −35.6 −35.4 −35.2 −35.0 −34.8 −37.0 −36.5 −36.0 −35.5 −35.0 −34.5 −37.0 −36.5 −36.0 −35.5 −35.0 −34.5 158.5 158.0 157.5 157.0 156.5 158.5 158.0 157.5 157.0 156.5 158.2 158.0 157.8 157.6 157.4 157.2 157.0

RA [º] RA [º] RA [º] Fig. 5. Left panel: projected density distribution for galaxies, split into three percentile ranges. Middle panel: projected positions for bright galaxies. The colour palette ranges from red to blue, spanning the VR,h range defined in Sect. 2. Right panel: similar plot for dwarf galaxies in the sample. The black asterisks indicate the positions of NGC 3258 (south-west) and NGC 3268 (north-east). In both panels, the origin of the black circle is the bona fide centre of the cluster, and its radius is 30′.

of parameters. For the inner region, we proposed typical param- −1 eters for the Fornax cluster rvir = 0.7Mpc and σv = 374kms (Drinkwater et al. 2001). Considering the high mass derived for ] 1

− the Antlia cluster by Hess et al. (2015), we plotted the second region assuming the Virgo cluster virial radius, rvir = 1.8Mpc

[km s [km −1 R (Kim et al. 2014) and σ = 617kms , which were previously

V v derived in Section3.1. While the first set of parameters implies that a large number of galaxies lie outside of the caustic curves, for the second one all galaxies present VR,h-values lower than the

1000 2000 3000 4000 escape velocity at their corresponding projected distances from 0.0 0.5 1.0 1.5 2.0 the assumed centre of the cluster.

dp [º] Dwarf galaxies seem to be spread all over the VR,h range for ′ dp < 20 . Just a few dwarf galaxies outside this limit were mea- Fig. 6. Heliocentric radial velocities (VR,h) as a function of projected dis- sured, but all of them present intermediate values of VR,h. The tances to the galaxy centre (dp). Different symbols represent dwarf (red ff circles), ellipticals (blue squares), lenticulars (blue-violet triangles), and picture is quite di erent when we look at the bright galaxy popu- ′ lation. As expected, spiral galaxies are mainly at larger distances spirals (green diamonds). The dashed line indicates dp = 30 , and the ′ −1 than 20 from the bona fide cluster centre. Moreover, their ve- dotted one represents the VR,h for both gEs, ∼ 2800 km s . The grey re- gions indicate the caustic curves obtained with the Praton & Schneider locity distribution also distinguishes them from the rest of the ′ (1994) infall model for two sets of parameters. galaxies in the sample. From the thirteen spirals with dp > 30 , −1 seven of them present VR,h around3200kms and relatively low dispersion. In fact, the mean VR,h and its dispersion for these spi- −1 −1 mediate velocities. However, the galaxies with the higher VR,h rals are 3190 ± 40kms and 110kms . ′ seem to present a different projected spatial distribution. The If we consider all the spirals with dp > 30 , their mean VR,h is majority of them were classified as spirals, despite the relatively 2980 ± 120kms−1. For comparison, the eleven lenticulars in the isolated elliptical FS90-152 (see Table2). On the other hand, the same radial regime have a mean velocity of 2550 ± 125kms−1. few other ellipticals have a projected distance to the centreof the A Student’s test between the two samples rejects the hypothesis ′ −1 cluster (dp) lower than 25 and present 1775 < VR,h [kms ] < that both samples belong to the same population with 90% of 3000. confidence. The right-hand panel of Figure5 shows the projected spatial To study the central part of the cluster, which is expected distribution for dwarf galaxies in the sample and the positions to be dominated by the gEs haloes, we restricted our sample to of NGC3258 and NGC3268. The origin and radius of the black include the 50th percentile of galaxies with larger number of circle are identical to the previous panels. Their concentration neighbours. The upper panel of Figure7 shows the V distri- ′ R,h within the region restricted by dp = 30 is due to a bias in the bution for these galaxies, adopting a bin width of 180kms−1. spectroscopic survey. Therefore, we cannot arrive at any conclu- It seems to represent three groups of galaxies with V around ′ R,h sion regarding their distribution when dp > 30 . The dwarfs near ∼ 2000kms−1, ∼ 2800kms−1, and ∼ 3700kms−1. To the the centre present a wide range of VR,h. They seem to be more last group belong the three bright lenticulars NGC3267 (FS90- concentrated towards the two gEs and do not seem to be aligned 168), NGC3269 (FS90-184), and NGC3271 (FS90-224), near with the projected position of the two gEs. to NGC3268 in projected distance. Considering the VR,h for Figure 6 shows the VR,h as a function of dp for the galaxies both gEs, the galaxies in the group with intermediate veloci- in the sample, discriminated by morphological types. The grey ties have the higher probability of belonging to the Antlia clus- regions represent caustic curves obtained with the infall model ter. We applied a S-W test to this sample and obtained a p of Praton & Schneider (1994), assuming Ω0 = 0.3 and two sets value of 0.32. Then, we ran GMM (Muratov & Gnedin 2010)

Article number, page 4 of 11 Caso & Richtler: Deconstructing the Antlia cluster core

our galaxy velocities with the results from X-ray observations. We excluded spirals because of the differences that we found be- tween their VR,h distribution and those of early-type bright galax- ies in Section3.2. Two subsets of our sample were made, select- ing from the galaxies contained in the 50th percentile those with shorter projected distance to NGC3258 or NGC3268. These re- sults in samples of 17 (up to 10arcmin) and 24 members (up to 18 arcmin), respectively. We applied the "tracer mass estimator" (MTr) from Evans et al. (2003),

C M = V2 R , (1) GN X LOS,i i i

where Ri and VLOS,i are the projected distances from the corre- sponding gE and velocities relative to it, respectively. Here, G is the constant of gravitation, and N the number of tracers. In the case of isotropy, the constant C is calculated through

4(α + γ) 4 − α − γ 1 − (r /r )3−γ = in out . C 4−α−γ (2) π 3 − γ 1 − (rin/rout) We assume that the three-dimensional profile of the tracer population is represented well by a power law between an inner radius rin and outer radius rout with exponents γ and α, respec- Fig. 7. Upper panel: VR,h distribution for galaxies belonging to the 50 tively (e.g. see Mamon & Łokas 2005a,b). Under the assump- percentile with larger number of neighbours. Middle panel: VR,h dis- tion of constant circular velocity, α = 0. In the case of the tracer tribution for galaxies belonging to the 75 percentile with larger number population, it is difficult to measure the power law exponent pre- of neighbours. Lower panel: VR,h distribution for Antlia galaxies, sep- cisely, but it should not differ too much from γ = 3 (Dekel et al. arated by morphology. 2005; Agnello et al. 2014; Courteau et al. 2014). Taking this into account, we obtain the mass that corresponds to γ ranging from 2.75 to 3.25. In the case of a shallower profile, the resulting for a trimodal case. GMM calculated three peaks with means of masses are somewhat lower, but not by an order of . 2020 ± 160kms−1, 2750 ± 100kms−1, and 3650 ± 50kms−1. For the sample surrounding NGC3258 we obtained M = For the unimodal hypothesis, we obtained χ2 = 13.9 with eight Tr (8−11)×1013 M . This has to be compared with the X-ray mass degrees of freedom (pχ2 = 0.1) and a distribution kurtosis ⊙ of NGC3258. Pedersen et al. (1997) obtained kT ≈ 1.7 keV, k = −0.9. These results point to a multi-modal distribution. The e assuming an isothermal gas. The parameters of their beta model parameter DD, which measures the relevance of the peak de- are r = 8.2arcmin, corresponding to 83.5 kpc, and β = 0.6. tections, was 2.8 ± 0.7 ((DD > 2 is required for a meaningful c With the assumption of spherical symmetry, we apply the detection ( Ashman et al. 1994; Muratov & Gnedin 2010). expression (Grego et al. 2001) In the middle panel we present the VR,h distribution for galax- iesup tothe 75thpercentilewith the samebin widthasin thepre- vious plot. The three different groups also seem to be present, but 3βkT r3 M(r) = e (3) smoothed. In this case, the p value obtained from the S-W test is 2 2 Gµmp rc + r higher, ∼ 0.80. For the trimodal case, the mean from GMM were 2060 ± 200kms−1, 2780 ± 100kms−1 and 3600 ± 130kms−1. In Adopting µ = 0.6, the derived mass within the sphere with 2 2 ′ 12 this case, χ = 7 (pχ = 0.5), k = −0.5, and DD = 2.5 ± 0.8. radius 10 is ∼ 7×10 M⊙. In the case of the sample surrounding The parameters are less conclusive, but point to a multi-modal NGC3268, the result from the Evans et al. (2003) estimator is 13 distribution in VR,h. MTr = (5 − 8) × 10 M⊙. The lower panel of Fig.7 shows the VR,h distribution for We again applied Eq. 3 with the parameters derived by Antlia galaxies, separated between dwarf and bright galaxies, Nakazawa et al. (2000): kTe = 2 keV, rc = 5arcmin, and β = and the latter group between early-types (ellipticals and lentic- 0.38. Then the mass enclosed within 18′ around NGC3268 is ulars) and late-types (spirals). Spirals fill a wide range of V , 13 R,h ∼ 1.4 × 10 M⊙. In both cases, the mass derived from X-ray ob- but seem to avoid velocities similar to those of the gEs (∼ servations is significantly lower than the mass estimated from 2800kms−1). the VR,h measurements. It is unlikely that the mass enclosed within ∼ 100kpc around both gEs reaches values of a few times 13 3.3. Mass estimations 10 M⊙. We therefore conclude that the galaxies with the most deviant velocities in the two samples are probably not gravita- Pedersen et al. (1997) used X-ray observations (ASCA) to tionally bound to either of the two groups. measure the mass enclosed within a radius of 240kpc from From the VR,h histograms in Fig7, a large number 13 NGC3258 and obtained 0.9 −−2.4 × 10 M⊙. NGC3268 was of the galaxies is symmetrically grouped around 2750 − studied in X-rays (ASCA) by Nakazawa et al. (2000), who de- −2800[kms−1], spanning approximately the velocity range 13 −1 termined ∼ 2 × 10 M⊙ internal to ∼ 260kpc. Here we compare 2200 < VR,h [kms ] < 3400. This group corresponds to ∼ 50%

Article number, page 5 of 11 A&A proofs: manuscript no. PrintProof_Manuscript of the galaxies that belong to the 50th percentile, and ∼ 60% of higher than the value obtained from the observations, and this the galaxies for the 75th percentile. This velocity constraint re- percentage is reduced to 6% for the κ15 test. Therefore, the tests duces the size of the samples around NGC3258 and NGC3268 point to underlying substructure, most probably due to the exis- to 8 and 13 members, respectively. Applying the mass estima- tence of non-member galaxies in our sample. tors to these limited samples, we obtain, for the galaxies, around 13 ′ NGC3258 and NGC3268 MTr = (1.4 − 2) × 10 M⊙ up to 9 13 ′ and MTr = (1.7 − 2.4) × 10 M⊙ up to 15 , respectively. 4. Discussion These estimations are in reasonable agreement with the 4.1. Extreme radial velocities masses derived from X-ray observations. The mass that we de- rived from the analysis of an UCD sample around NGC3268 is To use radial velocities as a discriminant for cluster member- 12 2.7×10 M⊙ within 47 kpc. Assuming a constant circular veloc- ship is often difficult if the depth along the line of sight is con- ity, the extrapolated mass within 101 kpc (corresponding to 10′) siderable. An extreme example is the interesting pair of Virgo 12 is 6×10 M⊙, and therefore also is in reasonable agreement with galaxies IC3492 with a radial velocity of -575 km/s and IC3486 our mass estimation from both companion galaxies and X-rays. with 1903 km/s, having a separation of only 1.4 arc min. The To avoid the need to explain peculiar radial velocities of the order deviation from a strict Hubble law may be a mix of peculiar of 1500kms−1, we conclude that the large velocity dispersion of velocities resulting from large scale structure and local gravi- our complete sample is caused by recessional velocities of galax- tational fields. Is it possible that the extreme radial velocities in ies in the near foreground or background, which are mixed in. Fig.6 are infall velocities? To answer this, it is useful to con- sider the highest possible velocities. We can represent the total mass distribution around NGC3268 quite well by an NFW-mass 3.4. Tests for cluster substructure distribution with a scale length of 22 kpc and a characteristic 3 In this section, we try a different approach. The nearest- density of 0.05M⊙/pc . A mass probe on an exactly radial or- neighbour tests are commonly used for detecting subgroups in bit, initially at rest and falling in from a distance of 1.5 Mpc, the environment of clusters of galaxies (e.g. Boschin et al. 2010, crosses the centre after 6.8 Gyr with a velocity of 1550kms−1. 2006; Burgett et al. 2004; Owers et al. 2009; Hou et al. 2012). However, after an additional 0.06 Gyr, its velocity has already −1 Our aim is to search for substructure in VR,h besides the obvi- declined to 1000kms . It is obvious that such a configuration ous existence of the two groups dominated by NGC3258 and is highly artificial and not suitable to explaining the broad veloc- NGC 3268. ity interval. The more plausible explanation is therefore the mix The ∆ test (Dressler & Shectman 1988) was intended for of recessional and Doppler velocities. probing deviations from the local mean velocities and disper- sions compared with the global cluster values. To achieve this, 4.2. Internal structure of the two dominant groups the local mean velocity and dispersion are calculated for each galaxy, restricting the sample to the galaxy itself and its ten near- What we called the Antlia cluster seems to be mainly formed est neighbours. Then, their deviation from the global cluster val- by two subgroups, each one dominated by a gE. Despite that, ues are computed, and their sum is defined as the ∆ parameter. both galaxies present similar VR,h, it is uncertain whether they Colless & Dunn (1996) proposed the κ test, similar in intent to are located at the same distance (e.g. Blakeslee et al. 2001; the ∆ test. The possibility that the scale in which the substructure Cantiello et al. 2005; Bassino et al. 2008). A recent study in is more obvious differs from what is imposed by the ten nearest HI (Hess et al. 2015) indicates that the subgroup dominated by neighbourscondition is considered, leaving the number of neigh- NGC3258 could be falling in to NGC3268, which they es- bours n as a free parameter. Arguing that distributions cannot timated is the actual cluster centre. It is clear that both gEs be characterised by the first two moments in general, they pro- dominate their own subgroups, and their peculiar velocities rel- posed a statistic ruled by the probability of the K-S two-sample ative to the group systemic velocity should not differ signifi- distribution (Kolmogorov 1933; Smirnov 1948). For both tests, cantly. For this reason, if the difference in distance between both the significance of their statistics are estimated by Monte Carlo galaxies was 2 − 4 Mpc, we would expect a VR,h difference of −1 simulations, in which the velocities of the cluster galaxies are ∼ 140 − 280kms . As a result, the measured VR,h might agree shuffled randomly. The significance level p was obtained after with the scenario suggested by Hess et al. (2015), where both repeating the previous procedure 1 000 times. subgroups are in a merging process. Pinkney et al. (1996) found that the existence of radial gra- It is interesting to have a closer look at the group around dients in the velocity dispersion of galaxies could produce arti- NGC3268. Strikingly, all three S0’s around the central galaxy ficial substructure when applying 3D statistics. We therefore ran (FS90-224, FS90-168, and FS90-184) have positive velocity the tests on galaxies whose projected distances to the equidistant offsets in the range of 900 − 1000kms−1. If these velocities point to both gEs was less than 20′, because velocity dispersions were due to the potential of NGC3268 (and its associated dark seem to remain constant up to that limit (see Fig.6). This re- matter), very radial orbits would be needed for surpassing the striction reduced our sample to 57 members. The area matches circular velocity that is about 300kms−1 and probably con- the field of view of our VIMOS survey, which guarantees a ho- stant (Caso et al. 2014). The high velocity offsets are, more- mogeneous sample. For the κ test, we selected n = 10, 15. The over, related to fairly large projected spatial offsets (which in results were ∆= 66.9 and p∆ = 0.12, κ10 = 14.4 and p10 = 0.16, the case of FS90-224 is about 65kpc), whereas the highest κ15 = 15.6 and p15 = 0.06. velocities are found at the bottom of the potential well. Fur- To investigate the significance of these numbers, we per- thermore, the galaxies do not show any morphological indica- formed 1000 Monte Carlo simulations. We randomly generated tion of being tidally disturbed. (We comment separately on the the VR,h for the galaxies in the sample, assuming a normal distri- special case NGC3269.) Moreover, there is no indication that bution with mean and dispersion of 2800kms−1 and 275kms−1, the globular cluster system of NGC 3268 is tidally disturbed respectively. Then, we applied the tests to these samples. For (Dirsch et al. 2003a), nor does the X-ray structure show any ab- the ∆ test, fewer than 15% of the cases produced a ∆-parameter normality (Nakazawa et al. 2000). We therefore prefer the inter-

Article number, page 6 of 11 Caso & Richtler: Deconstructing the Antlia cluster core pretation that the three S0s have to be placed in the background ies of the cluster. Together with the literature data, our list of of NGC3268. As Fig.7 suggests, more dwarf galaxies can be as- Antlia galaxies with measured radial velocities now embraces sociated with this group, but some cases are ambiguous. There 105 galaxies. Large numbers of these objects are projected onto is the dwarf FS90-195 that hosts about ten point sources, which either of the two subgroups around NGC3258 or NGC3268. may be globular clusters. The radial velocity is 3495kms−1, but Because the gravitational potentials of these galaxies are con- it is located close to NGC3268 (projected distance 15kpc), so it strained by X-ray studies and stellar dynamical tracers, we could may be bound and demonstrates how globular clusters can be do- compare the observed radial velocities with those expected. It nated to the system of NGC3268. In that sense it could resemble turned out that the total range of velocities seems to be too SH2 in NGC1316 (Richtler et al. 2012). high to be generated by the gravitational potential of NGC3258 We note that NGC3269 (FS90-184), together with or NGC3268. There are three groups of galaxies (not always NGC3267 (FS90-168) and NGC3271 (FS90-224), has been clearly separated) characterised by radial velocities of about identified as the "Lyon Group of Galaxies (LGG)" 202 of Garcia 1800kms−1,2700kms−1,and3700kms−1. We interpreted these (1993, 1995) and Barnes & Webster (2001), who already sus- velocities as recession velocities, which places the bright S0s pected its location behind the Antlia cluster. around NGC3268, in particular, into the background. Therefore, Galaxies of the low velocity group are mostly dwarf-like NGC3268 might qualify as a fossil group, resembling, for ex- galaxies and might be bona fide in the foreground.All look quite ample, NGC4636 in the Virgo cluster in its properties.The inter- −1 discy, which may also indicate a less dense environment. The mediate velocity group (i.e.VR,h around 2700kms ) is the most situation is somewhat different for the group around NGC3258. populated one. The distances between NGC3258 and NGC3268 There are also dwarfs (ANTLJ102914-353923.6 and FS90- are not well constrained in the literature, and we cannot discard 137) with positive velocity offsets of more than 1000kms−1 to that the groups dominated by them are in the early stages of a NGC3258, but the brighter galaxies do not show such extreme merging process. radial velocities. There is also a group around FS90-226 that is What we originally called the Antlia cluster is therefore char- less striking than the two dominant groups, which is also identi- acterised by several groups that, at least in some cases, are not fied as a subgroup by Hess et al. (2015). gravitationally bound. We may be looking along a filament of Thereforethe likely explanation for Fig.6 is that we are look- the cosmic web. ing along a filament of galaxies and that at least a fraction of the Acknowledgements. We thank Ylva Schuberth for discussions about the radial galaxies in the three velocity groups apparent in Fig.6 are sepa- velocities of Virgo galaxies, Mike Fellhauer for permission to use his orbit rated by their recession velocities. program, Francisco Azpillicueta for discussions of statistical issues, and Lilia Bassino for discussions about the Antlia cluster. We thank the refereee for sug- gestions that improved this article. 4.3. NGC 3268/3258 as fossil groups? This work was based on observations made with ESO telescopes at the La Silla Paranal Observatory under programmes ID 60.A-905 and ID 079.B-0480, and If the crowding of galaxies around NGC 3268 is mainly a pro- on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a coopera- jection effect, this galaxy fulfils the criteria for being a fossil tive agreement with the NSF on behalf of the Gemini partnership: the National group. After applying the definition of Jones et al. (2003), it has Science Foundation (United States), the National Research Council (Canada), the required X-ray luminosity of more than 1042 erg s−1, and the CONICYT (Chile), the Australian Research Council (Australia), Ministério da next galaxy in a luminosity ranking would be FS90-177, which Ciência, Tecnologia e Inovação (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and on observations acquired through the is fainter than 2mag in the R band. Gemini Science Archive. This research has made use of the NASA/IPAC Extra- The brighter galaxies around NGC3258 (FS90-105 and galactic Database (NED), which is operated by the Jet Propulsion Laboratory, FS90-125) may be companions. FS90-125 is perhaps a magni- California Institute of Technology, under contract with the National Aeronautics tude fainter than NGC3258, so only the X-ray criterium applies, and Space Administration. This work was funded with grants from Consejo Nacional de Investigaciones and it cannot be called a fossil group. However, its globular clus- Científicas y Técnicas de la República Argentina, Agencia Nacional de Promo- ter system is even richer than that of NGC3268, so the idea that ción Científica y Tecnológica (BID AR PICT-2013-0317), and Universidad Na- a collapsed at very early times is plausible. cional de La Plata (Argentina). TR is grateful for financial support from FONDE- CYT project Nr. 1100620, and from the BASAL Centro de Astrofísica y Tec- nologías Afines (CATA) PFB-06/2007. 4.4. Emission line galaxies Only three galaxies from our VIMOS sample present emis- sion lines. One of them, FS90-131, was classified as a spiral. References The other galaxies, FS90-220 and FS90-222, were classified as Agnello, A., Evans, N. W., & Romanowsky, A. J. 2014, MNRAS, 442, 3284 lenticulars. Evidence of strong formation would not be ex- Ashman, K. M., Bird, C. M., & Zepf, S. 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P., Raimondo, G., et al. 2005, ApJ, 634, 239 Caso, J. P., Bassino, L. P., Richtler, T., Calderón, J. P., & Smith Castelli, A. V. We have presented new radial velocities, measured with VLT- 2014, MNRAS, 442, 891 Caso, J. P., Bassino, L. P., Richtler, T., Smith Castelli, A. V., & Faifer, F. R. 2013, VIMOS and Gemini GMOS-S, for galaxies in the region nor- MNRAS, 430, 1088 mally addressed as the Antlia cluster. The fields are located in the Colless, M. & Dunn, A. M. 1996, ApJ, 458, 435 surroundings of NGC3258 and NGC3268, the dominant galax- Conselice, C. J., Gallagher, III, J. S., & Wyse, R. F. G. 2001, ApJ, 559, 791

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Table 1. Heliocentric radial velocities for background galaxies measured in this paper.

ID RA(J2000) DEC(J2000) VR,h hhmmss ddmmss kms−1

FS90-75 102812.0 -353220.4 12333±07 FS90-205 103018.5 -352443.2 45907±30 ANTL102823-352754 102823.8 -352754.0 40289±25 ANTL10294-352320 102904.6 -352320.4 51147±18 ANTL10295-352146 102905.5 -352146.8 52928±23 ANTL102817-353552 102817.0 -353552.8 44240±85 ANTL102937-353552 102937.2 -353552.8 7682±55 ANTL102936-353336 102936.7 -353336.0 24420±50 ANTL102951-351210 102951.6 -351210.8 32797±24 ANTL10303-3571 103003.8 -35701.2 28192±21 ANTL103042-35914 103042.2 -35914.4 15875±14 ANTL103049-352031 103049.4 -352031.2 32830±23 ANTL103045-35161 103045.8 -351601.2 32920±21 ANTL102945-35374 102945.4 -353704.8 8900±19 ANTL102936-353336 102936.7 -353336.0 24337±33 ANTL102926-351051 102926.9 -351051.6 20041±10 ANTL103041-351250 103041.0 -351250.4 9833±65 ANTL102940-35258 102940.6 -352508.4 53700±42 ANTL102936-35266 102936.0 -352606.0 24243±10 FS90-83 102823.0 -353057.6 19670±18 FS90-88 102828.1 -353104.8 19624±74 ANTL102838-352027 102838.2 -352027.6 46732±54

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Article number, page 8 of 11 Caso & Richtler: Deconstructing the Antlia cluster core

Table 2. Heliocentric radial velocity for Antlia members up to date.

ID NGC RA(J2000) DEC(J2000) HubbleType VR,h hhmmss ddmmss kms−1

FS90-01 102505.04 -355858.8 SmV 3220±051 FS90-18 102644.88 -365150.4 SmIV 2331±051 FS90-28 102702.16 -345750.4 SbII 3402±082 FS90-29 102702.40 -361330.0 Sc 3122±062 FS90-44 102721.12 -351630.0 S0 2931±453 FS90-50 102732.64 -355909.6 S0 2722±313 FS90-64 102757.84 -354919.2 dSB0 2261±453 FS90-68 102803.12 -352631.2 SBab 3219±18a 3188±453 FS90-70 102806.96 -353520.4 dE 2864±70b FS90-72 102807.92 -353820.4 S0 2986±38b FS90-77 102815.12 -353202.4 dE,N 2396±17a 2382±49b FS90-79 102819.2 -352721.6 S0 2772±15a 2930±60b FS90-80 102818.96 -354528.8 dS0 2519±313 ANTLJ102820-354236 102819.68 -354236.0 dE,N 2904±40a FS90-84 102824.00 -353140.8 E 2489±26a 2428±304 FS90-85 102824.0 -353422.8 dE 2000±200b FS90-87 102825.2 -351434.8 dE,N 3429±13a ANTLJ102829-351510.8 102829.3 -351510.8 dE,N 3226±40a FS90-93 102831.92 -354040.8 SmV 3608±57a FS90-94 102831.92 -354221.6 S0 2791±24a 2786±453 FS90-98 102835.04 -352739.6 BCD 2890±94b FS90-103 102845.12 -353440.8 dE 2054±29b FS90-105 3257 102848.0 -353928.8 SB01 3237±15a 3200±26b FS90-106 102851.36 -350939.6 BCD 2409±115b FS90-108 102853.28 -351912.0 dE,N 2611±39b FS90-109 102853.04 -353252.8 dE 1632±32a 1618±24b FS90-110 102853.04 -353534.8 M32 2911±7b FS90-111 3258 102854.00 -353621.6 E 2792±50a 2792±28b FS90-120 102902.16 -353404.8 ImV 2721±30a 2634±13b FS90-123 102903.12 -354030.0 dE,N 1865±25b FS90-125 3260 102906.24 -353534.8 S02 2439±46b ANTLJ102910-353920.1 102910.32 -353921.6 dE,N 1940±155b FS90-131 102911.04 -354124.0 Sb(r) 2158±10a 2104±60b FS90-133 102912.00 -353928.8 dE,N 2219±15a 2205±24b FS90-134 102913.20 -352924.0 S0 1355±60b ANTLJ102914-353923.6 102914.40 -353925.2 dE,N 4067±115b FS90-136 102915.36 -352558.8 dE,N 2995±16a 2989±10b FS90-137 102915.12 -354134.8 ImV 3987±36b FS90-139 102915.60 -350404.8 dE,N 1950±553 FS90-140 102918.24 -35356.0 dE,N 1924±31b FS90-142 102920.16 -353509.6 dS0 2245±13b FS90-152 102928.86 -344022.8 E 4093±453 FS90-153 102931.44 -351539.6 S0 1785±13a

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Table 2. continued.

ID NGC RA(J2000) DEC(J2000) HubbleType VR,h hhmmss ddmmss kms−1

1733±39b FS90-159 102941.52 -351731.2 dE,N 2821±21a FS90-162 102943.44 -352949.2 dE,N 2933±31a FS90-165 102946.08 -354225.2 S0 2695±14a 2604±453 FS90-168 3267 102948.48 -351922.8 SB01/2 3709±33b FS90-169 102948.48 -352512.0 E 2999±37b FS90-172 102951.84 -345436.0 S0 2549±195 FS90-173 102951.60 -351004.8 dE 2677±32a 2609±453 FS90-175 102953.52 -352237.2 dSB01 1834±25a 1781±66b FS90-176 102954.48 -351716.8 dE,N 1751±34a FS90-177 102954.48 -351919.2 dE,N 3540±9a 3505±45b FS90-184 3269 102957.60 -351330.0 S0/a 3754±33b FS90-185 3268 102958.56 -351930.0 E 2800±21b FS90-186 102959.52 -351810.8 dE 3721±45a FS90-187 103001.20 -354854.0 dS0 1960±553 FS90-188 103002.40 -352428.8 dE 2673±17b FS90-192 103004.56 -352031.2 M32 2511±18a 2526±4b FS90-195 103006.48 -351825.2 dE 3495±64a FS90-196 103006.48 -352331.2 dE 3593±9b ANTLJ103013-352458.3 103013.92 -352457.6 dE,N 2613±200b FS90-208 103018.72 -351149.2 S0 1774±100b FS90-209 103019.44 -353448.0 dE 3065±13b FS90-212 103021.36 -353531.2 SmIII 2364±27b ANTLJ103021-35314.8 103021.38 -353104.8 dE,N 2707±19a FS90-213 103021.60 -351214.4 dE 2151±60a 2185±21b ANTLJ103022-353806 103022.08 -353806.0 dE,N 3405±40a FS90-216 103022.56 -351026.4 E 2957±22a 2944±103b FS90-219 103024.72 -350632.4 Sb 1781±453 FS90-220 103024.72 -351518.0 S0/a 1160±8a 1182±453 FS90-222 103025.44 -353343.2 S0/a 2077±6a 2140±453 FS90-223 103025.68 -351319.2 dE,N 2701±29a 2661±9b FS90-224 3271 103026.64 -352136.0 Sb02 3737±27b FS90-226 3273 103029.28 -353636.0 S0/a 2660±08a 2503±206 FS90-227 103031.44 -352306.0 dE 2948±34a 2921±60b FS90-228 103031.68 -351438.4 dE,N 2450±18a 2417±13b ANTLJ103033-352638.6 103033.36 -352638.4 dE,N 2311±130b FS90-231 103034.56 -352313.2 dE,N 2915±20a 2909±38b ANTLJ103036-353046.8 103036.48 -353046.8 dE,N 3394±27a ANTLJ103037-352708.8 103037.44 -352707.2 dE,N 2400±100b FS90-238 103045.6 -352132.4 Sm 3078±37a ANTLJ103047-353918 103046.8 -353918.0 dE,N 3412±32a FS90-241 103048.48 -353220.4 dE,N 3518±36a

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Table 2. continued.

ID NGC RA(J2000) DEC(J2000) HubbleType VR,h hhmmss ddmmss kms−1

FS90-244 103051.60 -364413.2 SBb(r)II 3161±453 FS90-253 103100.24 -343350.4 SB0 2111±185 FS90-258 103103.12 -344015.6 dE,N 2450±553 FS90-277 103124.72 -351315.6 SBb(rs)II 2597±195 FS90-298 103148.48 -360144.4 Sd 3168±384 FS90-300 103152.08 -345114.4 Sa 3200±222 FS90-301 103151.84 -351214.4 S0 2423±453 FS90-304 103155.92 -352432.4 Sa 2476±162 FS90-306 103156.16 -345927.6 SB0 1981±453 FS90-307 103157.12 -345345.6 dE 2784±30a FS90-309 103159.04 -351145.6 SB0 2289±195 FS90-318 103208.40 -344015.6 dE,N 2588±453 FS90-321 103212.24 -344008.4 SB0 2129±453 FS90-323 103214.88 -351528.8 Sm 3825±453 FS90-325 103225.20 -350000.0 Sd 3058±107 FS90-331 103259.28 -345258.8 S0 2779±195 FS90-341 103401.20 -351655.2 Sapec 2573±088 FS90-343 103407.20 -351926.4 S0 2754±099 FS90-345 103413.68 -361355.2 S0 3372±453 FS90-353 103455.44 -352819.2 Sd 2662±453 FS90-373 103722.80 -352136.0 dE,N 2349±553

The upper index in column 6 indicates the reference for the VR,h measurement: athis paper, bSmith Castelli et al. (2008, 2012) The rest of the measurements were obtained from NED, and their references are: 1 Matthews et al. (1995), 2 Theureau et al. (1998), 3 Jones et al. (2009) 4 Huchra et al. (2012), 5 de Vaucouleurs et al. (1991), 6 Ogando et al. (2008), 7Mathewson et al. (1992), 8Lauberts & Valentijn (1989), 9Longhetti et al. (1998)

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