Globular clusters in the outer halo of M 31^⋆

¹ Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
e-mail: songw@bao.ac.cn
² College of Astronomy and Space Sciences, University of Chinese Academy of Sciences, Beijing 100049, PR China
³ WHU-NAOC Joint Center for Astronomy, Wuhan University, Wuhan, Hubei 430072, PR China

Received: 30 November 2018
Accepted: 29 January 2019

Abstract

In this paper, we present photometry of 53 globular clusters (GCs) in the M 31 outer halo, including the GALEX far-ultraviolet (FUV) and near-ultraviolet (NUV), SDSS ugriz, 15 intermediate-band filters of BATC, and 2MASS JHK_s bands. By comparing the multicolour photometry with stellar population synthesis models, we determine the metallicities, ages, and masses for these GCs, aiming to probe the merging/accretion history of M 31. We find no clear trend of metallicity and mass with the de-projected radius. The halo GCs younger than ∼8 Gyr are mostly located at the de-projected radii around 100 kpc, but this may be due to a selection effect. We also find that the halo GCs have consistent metallicities with their spatially associated substructures, which provides further evidence of the physical association between them. Both the disc and halo GCs in M 31 show a bimodal luminosity distribution. However, we should emphasise that there are more faint halo GCs which are not seen in the disc. The bimodal luminosity function of the halo GCs may reflect a different origin or evolution environment in their original hosts. The M 31 halo GCs include one intermediate metallicity group (−1.5 < [Fe/H] < −0.4) and one metal-poor group ([Fe/H] < −1.5), while the disc GCs have one metal-rich group more. There are considerable differences between the halo GCs in M 31 and the Milky Way (MW). The total number of GCs in M 31 is approximately three times greater than in the MW, however M 31 has about six times more halo GCs than the MW. Compared to the halo GCs of M 31, those of the MW are mostly metal-poor. Both the numerous halo GCs and the higher-metallicity component are suggestive of an active merger history of M 31.