An updated survey of globular clusters in M31. II Newly discovered bright and remote clusters

TL;DR

This paper reports the discovery of five new remote globular clusters in M31's outskirts, expanding the known population and providing spectroscopic data on their metallicity and age, revealing differences from the Milky Way.

Contribution

It presents the first spectroscopic survey of candidate outer globular clusters in M31, discovering four new clusters and analyzing their properties.

Findings

Five remote globular clusters identified, four are new discoveries.

Clusters are located at 40-100 kpc from M31 center.

Spectroscopic analysis shows most are old, metal-poor, with one possibly young.

Abstract

We present the first results of a large spectroscopic survey of candidate globular clusters located in the extreme outskirts of the nearby M31 galaxy. We obtained low resolution spectra of 48 targets selected from the XSC of 2MASS, as in Galleti et al. (2005). The observed candidates have been robustly classified according to their radial velocity and by verifying their extended/point-source nature from ground-based optical images. Among the 48 observed candidates clusters we found 5 genuine remote globular clusters. One of them has been already identified independently by Mackey et al. (2007), their GC1; the other four are completely new discoveries: B516, B517, B518, B519. The newly discovered clusters lie at projected distance 40 kpc<~R_p<~100 kpc from the center of M31, and have absolute integrated magnitude -9.5<M_V<-7.5. For all the observed clusters we have measured the strongest Lick indices and we have obtained spectroscopic metallicity estimates. Mackey-GC1, Martin-GC1, B517 and B518 have spectra typical of old and metal poor globular clusters ([Fe/H]<~ -1.3); B519 appears old but quite metal-rich ([Fe/H]~-0.5); B516 presents very strong Balmer absorption lines: if this is indeed a cluster it should have a relatively young age (likely <2 Gyr). The present analysis nearly doubles the number of M31 globulars at R_p> 40 kpc. At odds with the Milky Way, M31 appears to have a significant population of very bright globular clusters in its extreme outskirts.