Deconstructing the Antlia Cluster Core

Astronomy & Astrophysics manuscript no. PrintProof_Manuscript c ESO 2018 March 4, 2018 Deconstructing the Antlia cluster core J. P. Caso1, 2 and T. Richtler3 1 Grupo de Investigación CGGE, Facultad de Ciencias Astronómicas y Geofísicas de la Universidad Nacional de La Plata, and Instituto de Astrofísica de La Plata (CCT La Plata – CONICET, UNLP), Paseo del Bosque S/N, B1900FWA La Plata, Argentina 2 Consejo Nacional de Investigaciones Científicas y Técnicas, Rivadavia 1917, C1033AAJ Ciudad Autónoma de Buenos Aires, Argentina 3 Departamento de Astronomía, Universidad de Concepción, Concepción, Chile Received ...; accepted ... ABSTRACT Context. The present literature does not give a satisfactory answer to the question about the nature of the "Antlia galaxy cluster". Aims. The radial velocities of galaxies found in the region around the giant ellipticals NGC 3258/3268 range from about 1000 km/s to 4000 km/s. We characterise this region and its possible kinematical and population substructure. Methods. We have obtained VLT–VIMOS multi-object spectra of the galaxy population in the inner part of the Antlia cluster and measure radial velocities for 45 potential members. We supplement our galaxy sample with literature data, ending up with 105 galaxy velocities. Results. We find a large radial velocity dispersion for the entire sample as reported in previous papers. However, we find three groups at about 1900 km/s, 2800 km/s, and 3700 km/s, which we interpret as differences in the recession velocities rather than peculiar velocities. Conclusions. The high radial velocity dispersion of galaxies in the Antlia region reflects a considerable extension along the line of sight. Key words. Galaxies: clusters: individual: Antlia cluster 1. Introduction up with 105 galaxy velocities. These data will help us to better understanding the structure of Antlia. The character of the galaxy assembly in the constellation of In the following, we keep the term "Antlia cluster" Antlia is not as clear as, for example, in the cases of the for simplicity. To maintain consistency with earlier pa- Virgo or Fornax galaxy clusters. Some authors used the term pers (e.g. Smith Castelli et al. 2012; Caso et al. 2013, 2014; "galaxy group" (Ferguson & Sandage 1990), others "galaxy Calderón et al. 2015), we adopt a distance of 35 Mpc, which cluster" (Hopp & Materne 1985; Smith Castelli et al. 2008), and means a scale of 169.7 pc/arcsec. Hopp & Materne (1985) identified five "clusters" in the Antlia region (α = 10h − 10h 50m, δ = −42o −−30o). The most striking central cluster is called "Antlia II". Its morphologicalappearance 2. Observations and reductions is that of two groups concentrated around the giant ellipticals NGC3258 and NGC3268 in a projected distance of 220kpc. We performed multi-object spectroscopy with VLT-VIMOS of Both dominant galaxies are extended X-ray sources the galaxy population in six fields located in the inner part of the (Nakazawa et al. 2000; Pedersen et al. 1997) that exhibit rich Antlia cluster. The observations were carried out under the pro- globular cluster systems. The globular cluster system of NGC grammes 60.A-9050(A) and 079.B-0480(B) (PI Tom Richtler), 3258 contains about 6000 members. The system of NGC3268 observed during the first semesters of 2007 and 2008. Figure1 is somewhat poorer, but is still typical of a giant elliptical shows the four quadrants for each one of the six VIMOS fields, galaxy, with almost 5000 globular clusters (Bassino et al. 2008). using different colours. arXiv:1510.04274v1 [astro-ph.GA] 14 Oct 2015 Hopp & Materne (1985) and, more recently, Hess et al. (2015) The grating was HR blue, and the slit width was 1′′. For estimated velocity dispersions of ∼ 500kms−1 for the bright each science field, the integration time was 1 h, split into three population of Antlia, higher than those in clusters like For- individual exposures. This configuration implied a wavelength nax (Drinkwater et al. 2001). The globular cluster luminosity coverage spanning 3700 Å − 6600Å (depending on the slit po- function indicated that NGC3258 could be a few Mpc nearer sitions) and a spectral resolution of ∼ 2.5Å . The data were re- than NGC3268 (Dirsch et al. 2003b; Bassino et al. 2008), which duced with esorex in the usual manner for VIMOS data. First, agrees with the distances obtained with surface brightness fluc- a master bias was obtained for each field with the recipe VM- tuations by Blakeslee et al. (2001). However, Cantiello et al. BIAS from five individual bias exposures. The normalised mas- (2005) quoted the same distance moduli for both galaxies. ter flat field was created with the recipe VMSPFLAT from a set Despite being nearby, a thorough radial velocity survey of of dome flat-field exposures. The recipe VMSPCALDISP was Antlia’s galaxy population has not been done yet. In this paper, used to determine the wavelength calibrations and spectral dis- we present new radial velocities of galaxies located in Antlia. tortions. Typically, more than 20 lines were identified for each We supplement our galaxy sample with literature data, ending slit. Afterwards, the bias and flat-field corrections were applied Article number, page 1 of 11 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 radial velocity 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 projected 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 Antlia cluster (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 velocities 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”.

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