Exploring the properties of the M31 halo globular cluster system

ABSTRACT Following on from our discovery of a significant population of M31 outer halo globularclusters (GCs), and updates to the Revised Bologna Catalogue of M31 GCs, we inves-tigate the GC system of M31 out to an unprecedented radius (≈ 120 kpc). We derivevarious ensemble properties, including the magnitude, colour and metallicity distribu-tions, as well as the GC number density profile. One of our most significant findingsis evidence for a flattening in the radial GC number density profile in the outer halo.Intriguingly, this occurs at a galactocentric radius of ∼ 2 degrees (∼ 30 kpc) which isthe radius at which the underlying stellar halo surface density has also been shown toflatten. The GCs which lie beyond this radius are remarkably uniform in terms of theirblue (V−I) 0 colours, consistent with them belonging to an ancient population withlittle to no metallicity gradient. Structural parameters are also derived for a sample of13 newly-discovered extended clusters (ECs) and we find the lowest luminosity ECshave magnitudes and sizes similar to Palomar-type GCs in the Milky Way halo. Weargue that our findings provide strong support for a scenario in which a significantfraction of the outer halo GC population of M31 has been accreted.Key words: galaxies: star clusters – galaxies: interactions – galaxies: formation –galaxies: evolution – galaxies: individual (M31) – galaxies: haloes