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Isolated Ellipticals and Their Globular Cluster Systems III. NGC 2271, NGC

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Isolated Ellipticals and Their Globular Cluster Systems III. NGC 2271, NGC Astronomy & Astrophysics manuscript no. salinas+15_arxiv c ESO 2018 October 5, 2018 Isolated ellipticals and their globular cluster systems III. NGC 2271, NGC 2865, NGC 3962, NGC 4240 and IC 4889 ⋆ R. Salinas1, 2, A.Alabi3, 4, T. Richtler5, and R. R. Lane5 1 Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, FI-21500 Piikkiö, Finland 2 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA 3 Tuorla Observatory, University of Turku, Väisäläntie 20, FI-21500 Piikkiö, Finland 4 Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, VIC 3122, Australia 5 Departamento de Astronomía, Universidad de Concepción, Concepción, Chile Accepted 21 Feb 2015 ABSTRACT As tracers of star formation, galaxy assembly and mass distribution, globular clusters have provided important clues to our under- standing of early-type galaxies. But their study has been mostly constrained to galaxy groups and clusters where early-type galaxies dominate, leaving the properties of the globular cluster systems (GCSs) of isolated ellipticals as a mostly uncharted territory. We present Gemini-South/GMOS g′i′ observations of five isolated elliptical galaxies: NGC 3962, NGC 2865, IC 4889, NGC 2271 and NGC 4240. Photometry of their GCSs reveals clear color bimodality in three of them, remaining inconclusive for the other two. All the studied GCSs are rather poor with a mean specific frequency S N 1.5, independently of the parent galaxy luminosity. Considering also previous work, it is clear that bimodality and especially the presence∼ of a significant, even dominant, population of blue clusters occurs at even the most isolated systems, casting doubts on a possible accreted origin of metal-poor clusters as suggested by some models. Additionally, we discuss the possible existence of ultra-compact dwarfs around the isolated elliptical NGC 3962. Key words. Galaxies: star clusters – Galaxies: individual: NGC 3962 – Galaxies: individual: NGC 2865 – Galaxies: individual: IC 4889 – Galaxies: individual: NGC 2271 – Galaxies: individual: NGC 4240 1. Introduction have been naturally focused on galaxy clusters (e.g. Peng et al. 2006; Harris et al. 2006; Strader et al. 2006; Liu et al. 2011). The globular cluster systems (GCSs) of old, massive ellip- tical galaxies present an almost ubiquitous optical color bi- In the current reigning paradigm of galaxy formation, accre- modality (e.g. Kundu & Whitmore 2001; Larsen et al. 2001), tion and merging play the central roles (e.g. Cole et al. 1994; thought as corresponding to a metallicity bimodality (e.g. De Lucia et al. 2006), finding strong observational support (e.g. Strader et al. 2007; Usher et al. 2012; Brodie et al. 2012), with Ibata et al. 1994; Tal et al. 2009; van Dokkum et al. 2010). Sim- caveats coming from possible non-linearities of the color- ulations predict a different accretion history for elliptical galax- metallicity transformations introduced by horizontal-branch ies in low-density environments compared to their high-density stars (Richtler 2006; Yoon et al. 2006; Chies-Santos et al. 2012; siblings (e.g. Niemiet al. 2010); but the impact that a low- Blakeslee et al. 2012; Richtler 2013). density environment may have on the properties of a GCS has This “universal” bimodality has been obviously one of seldom been investigated (e.g Gebhardt & Kissler-Patig 1999), the main aspects the theories of GCS formation need to ad- although it could give important evidence to discriminate be- dress. Many scenarios, not necessarily exclusive and some- tween accretion driven (e.g. Côté et al. 1998; Hilker et al. 1999) what overlapping, have been put forward over the years (e.g. and in situ (Forbes et al. 1997) scenarios. Ashman&Zepf 1992; Forbesetal. 1997; Côté et al. 1998; Only a handful of isolated elliptical galaxies (IEs) have their Beasley et al. 2002; Rhode et al. 2005; Muratov & Gnedin 2010; GCS thoroughly studied (e.g. Spitler et al. 2008), given their arXiv:1502.06148v1 [astro-ph.GA] 21 Feb 2015 Elmegreen et al. 2012; Tonini 2013). paucity in the local environment. No field ellipticals are listed, Since elliptical galaxies are known to inhabit mostly high- for example, in the big compilation of Brodie & Strader (2006, density environments (e.g. Dressler 1980), most of the GCS see their Table 1), and only a handful can be seen in the newer studies, and hence the observational constraints to these theories, compilation of Harris et al. (2013). Recently, Cho et al. (2012) studied 10 early-type galaxies in low density environments in the ⋆ Based on observations obtained at the Gemini Observatory, which magnitude range 18.5 < MB < 20 using HST/ACS, although is operated by the Association of Universities for Research in Astron- solely using the Tully− (1988) density− parameter as isolation cri- omy, Inc., under a cooperative agreement with the NSF on behalf of the terion. Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the This paper is part of an effort to understand the proper- National Research Council (Canada), CONICYT (Chile), the Australian ties of GCS in IEs and their connections to their parent galax- Research Council (Australia), Ministério da Ciência, Tecnologia e Ino- ies, collecting a large sample. This paper is a continuation of vação (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Pro- Lane et al. (2013, hereafter Paper I) who studied the GCS of the ductiva (Argentina). IE NGC 3585 and NGC 5812 using Washington photometry,and Article number, page 1 of 15 A&A proofs: manuscript no. salinas+15_arxiv Table 1. Basic data of the galaxies presented in this work. T Name RA Dec l b Type B0 (m M)0 MB fov (mag) (mag)− (mag) (kpc) NGC2271 06:42:53.0 –23:28:34.0 233:13:30.5 –12:14:45.8 E/S0 12.10 32.51 20.41 50.9 NGC 2865 09:23:30.2 –23:09:41.0 252:57:12.4 +18:56:29.9 E3 12.18 32.89 −20.71 60.5 NGC 3962 11:54:40.1 –13:58:30.0 282:39:10.7 +46:38:57.5 E1 11.59 32.74a −21.15 56.5 NGC 4240 12:17:24.3 –09:57:06.0 289:14:11.8 +52:00:46.3 E/cD? 14.01 32.07 −18.06 41.5 IC4889 19:45:15.1 –54:20:39.0 343:32:15.1 –29:25:10.4 E5-6/S0 11.91 32.33 −20.42 46.8 − Notes. Classification type comes from NED and HyperLeda databases, while total apparent B magnitudes are taken from RC3, except for NGC 4240, taken from HyperLeda. Distance moduli have been adopted from Tonry et al. (2001), except the one for NGC 4240 which comes from Reda et al. (2004). The last column gives the physical size covered by the GMOS FOV at each galaxy’s distance. (a) Considered as uncertain by Tonry et al. (2001). Richtler et al. (2015, Paper II) who studied the IE NGC 7796. It Table 2. Observations log is also complemented by the study of the field elliptical NGC 7507 (Caso et al. 2013) also conducted by our group. Name Obs.date Exp.time(s) FWHM(′′) (dd.mm.yyyy) g′ i′ g′ i′ 1.1. The galaxy sample NGC2271 01.02.2012 9 419 9 315 0.73 0.74 NGC2865 25.01.2012 6 × 480 6 × 340 0.63 0.50 As isolated galaxies give the opportunity to study galaxies 26.01.2012 6 × 490 6 × 350 without the interference of many of the processes that af- NGC3962 22.02.2012 6 × 610 6 × 440 0.73 0.60 fect their structure and evolution in more crowded environ- NGC4240 02.02.2012 5 × 422 5 × 275 0.80 0.58 ments, many samples of IEs have been constructed, using IC4889 17.04.2012 3 × 575 5 × 355 – 0.75 diverse isolation criteria (Kuntschner et al. 2002; Smith et al. 01.05.2012 3 × 575 1 × 355 0.49 – 2004; Stocke et al. 2004; Reda et al. 2004; Collobert et al. 2006; × × Nigoche-Netro et al. 2007; Fuse et al. 2012). Notes. The FWHM was measured on the combined images. In this work we present observations of five IEs taken from the catalogues of Smith et al. (2004) and Reda et al. (2004); by a mild age gradient. The existence of an also shallow closer than 50 Mpc and observable from the southern hemi- ∼ metallicity gradient is explained as indication of a past ma- sphere. The GCS of NGC 2865 and IC 4889 have been studied jor merger. It possesses a kinematically distinct core and a before, albeit with a smaller field of view (see below). In the mostly flat velocity dispersion profile (Hau & Forbes 2006). following we give their main characteristics as found in the lit- – IC 4889 is an E/S0 (Laurikainen et al. 2011) present in the erature. Smith et al. (2004) catalogue, showing isophotal twisting (Tal et al. 2009) as well as a dusty disk (Smith et al. 2004). – NGC 2271 isaE/S0 at a surface brightness fluctuation dis- Serra & Oosterloo (2010) estimate an age of 3.6 Gyr. Its tance of 31.8 Mpc (Tonryet al. 2001) present in the cata- GCS has been studied by Gebhardt & Kissler-Patig∼ (1999) logue of Reda et al. (2004). Reda et al. (2007) measured an using HST data, without finding evidence for bimodality. ageof11.5 0.5 Gyr, with no evidenceof radial age or metal- licity gradients,± while Hau & Forbes (2006) found strong ro- The rest of the paper is distributed as follows: in Sect. 2.1 we tation and kinematic signatures of an inner disk, which is describe the observations, as well as the data reduction and stel- also found on optical imaging (de Souza et al.
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