Mon. Not. R. Astron. Soc. 000, 1–18 (2016) Printed 14 October 2018 (MN LATEX style file v2.2) Constraining ultra-compact dwarf galaxy formation with galaxy clusters in the local universe J. Pfeffer1,2⋆, M. Hilker3, H. Baumgardt1, B. F. Griffen4 1School of Mathematics and Physics, The University of Queensland, Brisbane, QLD 4072, Australia 2Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK 3European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany 4Massachusetts Institute of Technology, Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Avenue, Cambridge, MA 02139, USA ABSTRACT We compare the predictions of a semi-analytic model for ultra-compact dwarf galaxy (UCD) formation by tidal stripping to the observed properties of globular clusters (GCs) and UCDs in the Fornax and Virgo clusters. For Fornax we find the predicted number of stripped nuclei agrees very well with the excess number of GCs+UCDs 7.3 above the GC luminosity function. GCs+UCDs with masses >10 M⊙ are consistent with being entirely formed by tidal stripping. Stripped nuclei can also account for Virgo 7.3 UCDs with masses >10 M⊙ where numbers are complete by mass. For both Fornax and Virgo, the predicted velocity dispersions and radial distributions of stripped nuclei are consistent with that of UCDs within ∼50-100 kpc but disagree at larger distances where dispersions are too high and radial distributions too extended. Stripped nuclei are predicted to have radially biased anisotropies at all radii, agreeing with Virgo UCDs at clustercentric distances larger than 50 kpc. However, ongoing disruption is not included in our model which would cause orbits to become tangentially biased at small radii. We find the predicted metallicities and central black hole masses of stripped nuclei agree well with the metallicities and implied black hole masses of UCDs 6.5 for masses >10 M⊙. The predicted black hole masses also agree well with that of M60-UCD1, the first UCD with a confirmed central black hole. These results suggest that observed GC+UCD populations are a combination of genuine GCs and stripped nuclei, with the contribution of stripped nuclei increasing toward the high-mass end. Key words: methods: numerical – galaxies: dwarf – galaxies: formation – galaxies: interactions – galaxies: star clusters arXiv:1603.00032v1 [astro-ph.GA] 29 Feb 2016 1 INTRODUCTION (Mieske, Hilker & Infante 2002; Mieske, Hilker & Misgeld 2012) or the nuclei of tidally stripped dwarf galax- Ultra-compact dwarf galaxies (UCDs) are a new type of ies (Bekki, Couch & Drinkwater 2001; Bekki et al. 2003; galaxy discovered just over 15 years ago in spectroscopic Drinkwater et al. 2003; Pfeffer & Baumgardt 2013). surveys of the Fornax galaxy cluster with sizes (Re . 100 pc) and luminosities (−14 . MV . −12) interme- Through dedicated surveys many new UCDs have been diate between globular clusters (GCs) and dwarf galaxies found (Mieske, Hilker, & Infante 2004a; Firth et al. 2007; (Hilker et al. 1999a; Drinkwater et al. 2000). Follow-up high Firth, Drinkwater, & Karick 2008; Gregg et al. 2009), to resolution spectroscopy has found that UCDs have simi- the point where the confirmed objects now number in the lar internal velocity dispersions to dwarf elliptical nuclei hundreds in the Fornax cluster alone (Mieske et al. 2012). (σ ∼30 km s−1, Drinkwater et al. 2003). Initially labelled UCDs have since been discovered in many environments, UCDs (Phillipps et al. 2001) or dwarf-globular transition including other galaxy clusters (Abell 1689: Mieske et al. objects (Ha¸segan et al. 2005) their formation mechanism is 2004b; Virgo: Ha¸segan et al. 2005; Jones et al. 2006; still under debate, however two main scenarios have been Centaurus: Mieske et al. 2007; Coma: Price et al. 2009; suggested: they may be the high-mass end of the GC mass Chiboucas et al. 2010; Hydra I: Misgeld et al. 2011; Perseus: function observed around galaxies with rich GC systems Penny et al. 2012), galaxy groups (Dorado and NGC 1400: Evstigneeva et al. 2007; NGC 5128: Rejkuba et al. 2007; HCG 22 and HCG 90: Da Rocha et al. 2011; NGC 3923: ⋆ E-mail: j.l.pfeff[email protected] Norris & Kannappan 2011) and around isolated galaxies c 2016 RAS 2 J. Pfeffer et al. (NGC 7252: Maraston et al. 2004; Sombrero: Hau et al. mass functions, radial and velocity distributions, metallici- 2009; NGC4546: Norris & Kannappan 2011). As more ties and central black hole masses with the observed distri- UCDs are found there is growing evidence that no single for- butions. Throughout the paper we refer to objects formed mation mechanism is responsible for their formation, how- in the simulation by tidal stripping of nucleated galaxies as ever most UCDs either formed as giant GCs or by tidal stripped nuclei since such objects may resemble both GCs stripping of nucleated dwarf galaxies (Ha¸segan et al. 2005; and UCDs and because the observed UCD populations may Mieske et al. 2006; Brodie et al. 2011; Chilingarian et al. be the result of more than one formation channel. 2011; Da Rocha et al. 2011; Norris & Kannappan 2011; This paper is organized as follows. Section 2 describes Norris et al. 2014; Pfeffer et al. 2014). Although the forma- the criteria for selecting analogue galaxy clusters of the For- tion mechanism of a few peculiar objects can be determined nax and Virgo clusters and briefly summarises the method (e.g. Seth et al. 2014; Norris et al. 2015), disentangling the of P14 for identifying stripped nuclei in cosmological simu- individual formation mechanism of most UCDs is almost lations. Section 3 describes the compilation of observational impossible due to the similar predictions of internal UCD data of GCs, UCDs and dwarf galaxies. Section 4 presents properties from each formation scenario. Determining the the results comparing the simulation and observational data. origin of UCDs therefore requires detailed predictions of In Section 5 and 6 we discuss the implications of our work how much each possible formation mechanism contributes for UCD formation scenarios and summarize our results. to UCD populations. Tidal stripping of nucleated galaxies is a likely origin for many UCDs (and a confirmed origin for 2 SEMI-ANALYTIC MODELLING two objects, Seth et al. 2014; Norris et al. 2015) how- ever their contribution to the total UCD population is Here we summarize the stripped nucleus formation model of uncertain (and partly compounded by the various def- P14 and detail our selection criteria for comparing against initions of UCDs). Previous studies have shown that observed galaxy clusters. tidal stripping of nucleated galaxies can produce objects The model makes use of the semi-analytic galaxy forma- with similar properties to observed UCDs (Bekki et al. tion model (SAM) of Guo et al. (2011, hereafter G11) which 2003; Pfeffer & Baumgardt 2013). A number of studies was applied to the subhalo merger trees of the Millennium- presented estimates for the number of UCDs formed II simulation (Boylan-Kolchin et al. 2009, hereafter MS-II). due to tidal disruption (Bekki et al. 2003; Goerdt et al. The MS-II is a cosmological dark-matter only simulation 2008; Thomas, Drinkwater & Evstigneeva 2008). However which has a box size of 137 Mpc and a particle mass of 6 as these estimates were based on UCD formation in clusters 9.42 × 10 M⊙. The G11 SAM is constrained by low-redshift with static potentials they suffer from a number of problems. galaxy abundance and clustering in the Sloan Digital Sky Static models do not take into account UCD formation that Survey and is tuned to reproduce the z = 0 mass dis- 7.5 may have occurred within subclusters that later fell into tribution of galaxies down to stellar masses of 10 M⊙. the main cluster. Since galaxy clusters are expected to un- In the G11 SAM satellite galaxies (i.e. those currently or dergo many mergers during their formation, galaxies in clus- previously at the centre of non-dominant haloes orbiting ters may be on chaotic orbits providing a few close pericen- within a more massive halo) may have either resolved or tre passages necessary for UCD formation but far from the unresolved dark matter (DM) haloes under the assumption cluster centre at other times (Pfeffer & Baumgardt 2013)1. that the stellar component of a satellite galaxy is harder to Galaxies orbiting in clusters may also have interactions with disrupt than its halo. For the stellar component of a satel- other satellite galaxies thereby making tidal disruption more lite galaxy to be disrupted its DM halo must first be en- likely. tirely dissolved (i.e. become unresolved). Since tidal strip- In Pfeffer et al. (2014, hereafter P14) we presented the ping is not taken into account in the model, satellite galax- first model for UCD formation based on cosmological simu- ies do not lose stellar mass until they are completely dis- lations of galaxy formation. Our model uses a semi-analytic rupted. For all data associated with MS-II and the G11 galaxy formation model to select possible UCD progenitor SAM we assume a cosmology consistent with the Wilkinson galaxies and to determine when they become disrupted by Microwave Anisotropy Probe 1-year data (WMAP1) results tidal forces. Assuming that galaxies at high redshift have the (Spergel et al. 2003) and assume h = 0.73 for all masses 2 same distribution of nucleus-to-galaxy mass and nucleation and distances . The data associated with the MS-II and fraction as galaxies in the present day Universe we deter- G11 SAM are publicly provided by the Virgo-Millennium mined the numbers and masses of UCDs formed by tidal Database (Lemson & Virgo Consortium 2006)3. stripping. Some preliminary analysis was presented compar- In P14 a sample of galaxy clusters was chosen such 13 ing the number of UCDs predicted with the observed num- that Mvir > 10 M⊙/h.