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The Outer Halo Globular Cluster System of M31-III. Relationship to The MNRAS 000, 1–33 (2019) Preprint 8 January 2019 Compiled using MNRAS LATEX style file v3.0 The outer halo globular cluster system of M31 – III. Relationship to the stellar halo A. D. Mackey1, A. M. N. Ferguson2, A. P. Huxor3, J. Veljanoski4, G. F. Lewis5, A. W. McConnachie6, N. F. Martin7,8, R. A. Ibata7, M. J. Irwin9, P. Cˆot´e6, M. L. M. Collins10, N. R. Tanvir11, N. F. Bate9 1Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia 2Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK 3HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL, UK 4Kapteyn Astronomical Institute, University of Groningen, PO Box 800, NL-9700 AV Groningen, The Netherlands 5Sydney Institute for Astronomy, School of Physics, A28, The University of Sydney, Sydney, NSW 2006, Australia 6NRC Herzberg Astronomy and Astrophysics, 5071 West Saanich Road, Victoria, B.C., V9E 2E7, Canada 7Universit´ede Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France 8Max-Planck-Institut fur¨ Astronomie, K¨onigstuhl 17, D-69117 Heidelberg, Germany 9Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK 10Department of Physics, University of Surrey, Guildford, GU2 7XH, UK 11Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK Draft version 8 January 2019. ABSTRACT We utilise the final catalogue from the Pan-Andromeda Archaeological Survey to inves- tigate the links between the globular cluster system and field halo in M31 at projected radii Rproj = 25 − 150 kpc. In this region the cluster radial density profile exhibits a power-law decline with index Γ = −2.37 ± 0.17, matching that for the stellar halo component with [Fe/H] < −1.1. Spatial density maps reveal a striking correspondence between the most luminous substructures in the metal-poor field halo and the posi- tions of many globular clusters. By comparing the density of metal-poor halo stars local to each cluster with the azimuthal distribution at commensurate radius, we re- ject the possibility of no correlation between clusters and field overdensities at 99.95% significance. We use our stellar density measurements and previous kinematic data to demonstrate that ≈ 35 − 60% of clusters exhibit properties consistent with hav- ing been accreted into the outskirts of M31 at late times with their parent dwarfs. Conversely, at least ∼ 40% of remote clusters show no evidence for a link with halo substructure. The radial density profile for this subgroup is featureless and closely mirrors that observed for the apparently smooth component of the metal-poor stellar halo. We speculate that these clusters are associated with the smooth halo; if so, their properties appear consistent with a scenario where the smooth halo was built up at arXiv:1810.10719v2 [astro-ph.GA] 6 Jan 2019 early times via the destruction of primitive satellites. In this picture the entire M31 globular cluster system outside Rproj = 25 kpc comprises objects accumulated from external galaxies over a Hubble time of growth. Key words: galaxies: individual (M31) – galaxies: halos – globular clusters: general – galaxies: formation – Local Group 9 1 INTRODUCTION with stellar masses greater than ∼ 10 M⊙ as well as many below this limit; observability, usually being both compact Globular clusters are widely used as key tracers of the main and luminous (with a typical size rh ∼ 3 pc, and bright- astrophysical processes driving the formation and evolution ness MV ∼ −7.5); and longevity, commonly surviving in ex- of galaxies (e.g., Brodie & Strader 2006; Harris 2010). Their cess of a Hubble time unless subjected to a disruptive tidal utility stems in part from a variety of convenient characteris- environment. However, their usefulness as tracers of galaxy tic properties: ubiquity, being found in essentially all galaxies © 2019 The Authors 2 Mackey et al. assembly is mainly a consequence of the apparently close, al- cently facilitated the derivation of full 6D phase-space in- though not necessarily straightforward, couplings found be- formation for many of the Milky Way’s globular clusters tween the features of a given globular cluster system and (e.g., Gaia Collaboration 2018b; Vasiliev 2018), adding fur- the overall properties of the host and its constituent stellar ther support for the idea that many are accreted objects populations. These connections can give rise to surprisingly (Myeong et al. 2018; Helmi et al. 2018). simple scaling relations, such as the nearly one-to-one linear Despite this vast array of indirect evidence, and despite correlation observed between the halo mass of a galaxy and the discovery of abundant substructure and numerous stel- the total mass in its globular cluster population spanning lar streams criss-crossing the Milky Way’s inner halo (e.g., more than five orders of magnitude (see e.g., Hudson et al. Belokurov et al. 2006; Bell et al. 2008; Bernard et al. 2016; 2014; Harris et al. 2015; Forbes et al. 2018). Grillmair & Carlin 2016; Shipp et al. 2018; Malhan et al. While it was once thought that globular clusters 2018), surveys targeting the outskirts of globular clus- formed as a result of special conditions found only in the ters in search of the expected debris from their now- high-redshift universe (e.g., Peebles & Dicke 1968; Peebles defunct parent systems have proven largely fruitless (e.g., 1984; Fall & Rees 1985), more recent work has shown Carballo-Bello et al. 2014, 2018; Kuzma et al. 2016, 2018; that the simple assumption that globular clusters form Myeong et al. 2017; Sollima et al. 2018). Indeed, apart from wherever high gas densities, high turbulent velocities, and several Sagittarius members, there is no unambiguous exam- high gas pressures are found – i.e., in intense star form- ple of a Milky Way globular cluster that is embedded in a ing episodes – leads self-consistently to many of the ob- coherent tidal stream from a disrupted dwarf galaxy. While served properties of globular cluster systems at the present this observation might find a natural explanation if the ma- day (e.g., Kravtsov & Gnedin 2005; Muratov & Gnedin jority of significant accretion events occurred very early in 2010; Elmegreen 2010; Griffen et al. 2010; Tonini 2013; the Galaxy’s history (cf. Myeong et al. 2018; Helmi et al. Li & Gnedin 2014; Katz & Ricotti 2014; Kruijssen 2014, 2018; Kruijssen et al. 2018), the lack of any obvious as- 2015; Pfeffer et al. 2018). This provides a natural explana- sociation between the supposedly-accreted subset of Milky tion for the tight links observed between cluster systems Way clusters and substructures in the stellar halo inevitably and their host galaxies, and motivates the empirically suc- places some doubt on the fidelity with which the proper- cessful use of globular clusters as tracers of galaxy devel- ties of the globular cluster system reflect the accretion and opment across all morphological types (e.g., Strader et al. merger history inferred directly from the field. 2004, 2006; Peng et al. 2008; Georgiev et al. 2009, 2010; The Andromeda galaxy (M31) provides the next nearest Forbes et al. 2011; Romanowsky et al. 2012; Harris et al. example of a large stellar halo beyond the Milky Way, and 2013; Brodie et al. 2014). constitutes an ideal location to explore in detail the links be- Globular clusters played a central role in helping de- tween the field halo populations and the globular cluster sys- velop our understanding of the formation of the Milky tem in an L∗ galaxy. Indeed, in many ways M31 offers clear Way, providing some of the first experimental evidence advantages for such study relative to our own Milky Way that the hierarchical accretion of small satellites might (as outlined in e.g., Ferguson & Mackey 2016), and we ar- represent an important assembly channel (Searle & Zinn guably possess a significantly more complete understanding > 1978; Zinn 1993). This picture was spectacularly veri- of both its periphery (i.e., at projected radii Rproj ∼ 40 kpc) fied with the discovery of the disrupting Sagittarius dwarf and its low-latitude regions. Considering the stellar halo as galaxy (Ibata et al. 1994, 1995), presently being assim- a whole, it is well established that the system belonging to ilated into the Milky Way’s halo along with its ret- M31 contains a higher fraction of the overall galaxy lumi- inue of globular clusters (e.g., Da Costa & Armandroff nosity, is significantly more metal-rich, and is apparently 1995; Mart´ınez-Delgado et al. 2002; Bellazzini et al. 2003; more heavily substructured than that of the Milky Way Carraro et al. 2007). It is now known that the extended (e.g., Mould & Kristian 1986; Pritchet & van den Bergh low surface brightness stellar halos that surround large 1988; Ibata et al. 2001, 2007, 2014; Ferguson et al. 2002; galaxies are a generic product of the mass assembly pro- Irwin et al. 2005; McConnachie et al. 2009; Gilbert et al. cess in ΛCDM cosmology (e.g., Bullock & Johnston 2005; 2009, 2012, 2014), while the M31 globular cluster population Cooper et al. 2010); this accreted material typically includes is more numerous than that of the Milky Way by at least a substantial portion of the associated globular cluster sys- a factor of three (e.g., Galleti et al. 2006, 2007; Huxor et al. tem (cf. Beasley et al. 2018). 2008, 2014; Caldwell & Romanowsky 2016). Additional evidence in favour of the idea that a signif- These observations all suggest that the accretion his- icant fraction of globular clusters in the Milky Way is ac- tory of M31 is quite different from that of our own Galaxy, creted comes from precision stellar photometry with HST, in that M31 has likely experiened more accretions and/or which revealed that the Galactic globular clusters follow a a more prolonged history of accretion events.
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