The outer halo globular cluster system of M31 – I. The final PAndAS catalogue

Abstract

We report the discovery of 59 globular clusters (GCs) and two candidate GCs in a search of the halo of M31, primarily via visual inspection of CHFT/MegaCam imagery from the Pan-Andromeda Archaeological Survey (PAndAS). The superior quality of these data also allow us to check the classification of remote objects in the Revised Bologna Catalogue (RBC), plus a subset of GC candidates drawn from SDSS imaging. We identify three additional new GCs from the RBC, and confirm the GC nature of 11 SDSS objects (8 of which appear independently in our remote halo catalogue); the remaining 188 candidates across both lists are either foreground stars or background galaxies. Our new catalogue represents the first uniform census of GCs across the M31 halo -we find clusters to the limit of the PAndAS survey area at projected radii of up to R proj ∼ 150 kpc. Tests using artificial clusters reveal that detection incompleteness cuts in at luminosities below M V = −6.0; our 50% completeness limit is M V ≈ −4.1. We construct a uniform set of PAndAS photometric measurements for all known GCs outside R proj = 25 kpc, and any new GCs within this radius. With these data we update results from Huxor et al. (2011), investigating the luminosity function (LF), colours and effective radii of M31 GCs with a particular focus on the remote halo. We find that the GCLF is clearly bimodal in the outer halo (R proj > 30 kpc), with the secondary peak at M V ∼ −5.5. We argue that the GCs in this peak have most likely been accreted along with their host dwarf galaxies. Notwithstanding, we also find, as in previous surveys, a substantial number of GCs with above-average luminosity in the outer M31 halo -a population with no clear counterpart in the Milky Way.

The Extended Baryonic Halo of NGC 3923

“…This process resulted in the selection of 500 candidate GCs detected by DECam within a projected radius of 0.5 degree, which corresponds to 185 kpc, about 37R e (Figure 4a). This makes the NGC 3923 globular cluster system (GCS) one of the most extended known, comparable to the GCSs of the Milky Way and M31 [16]. This is mainly due to the very wide-field imaging used here.…”

Gas expulsion in massive star clusters?

“…Associations of XGCs with tidal streams suggest that the majority of globular clusters was formed in dwarf galaxies and accreted with them to the haloes of larger galaxies (Mackey et al 2010(Mackey et al , 2013Huxor et al 2014;Veljanoski et al 2014). Old GCs in the Large Magellanic Cloud have been shown to exhibit the O-Na anti-correlation (Mucciarelli et al 2009).…”

Accretion in action: phase space coherence of stellar debris and globular clusters in Andromeda's South-West Cloud

“…The population of known M31 dwarf satellites has grown to over thirty (see e.g., McConnachie 2012; Martin et al 2013a,b,c, and references therein), sixteen of which were discovered in PAndAS data (e.g., Martin et al 2006;McConnachie et al 2008;Richardson et al 2011). The population of known halo globular clusters has also grown very substantially (Huxor et al 2008(Huxor et al , 2014. Of particular interest here, M31 outer halo globular clusters exhibit a striking spatial coincidence with diffuse stellar streams and overdensities in the field (Mackey et al 2010(Mackey et al , 2014.…”