2007A&A...471..127G

We present the first results of a large spectroscopic survey of candidate globular clusters located in the extreme outskirts of the nearby M 31 galaxy. The survey is aimed at ascertaining the nature of the selected candidates to increase the sample of confirmed M 31 clusters lying more that 2° away from the center of the galaxy. We obtained low resolution spectra (λ/Δλ≃800-1300) of 48 targets selected from the Extended Source Catalogue of 2MASS, as in Galleti et al. (2005A&A...436..535G). 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. We have also obtained a spectrum and a radial velocity estimate for the remote M 31 globular discovered by Martin et al. (2006MNRAS.371.1983M). Among the 48 observed candidates clusters we found: 35 background galaxies, 8 foreground Galactic stars, and 5 genuine remote globular clusters. One of them has been already identified independently by Mackey et al. (2007ApJ...655L..85M), their GC1; the other four are completely new discoveries: B516, B517, B518, B519. The newly discovered clusters lie at projected distance 40kpc≲R_p≲100kpc from the center of M 31, 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 <2Gyr). The present analysis nearly doubles the number of M 31 globulars at R_p≥40kpc. At odds with the Milky Way, M 31 appears to have a significant population of very bright globular clusters in its extreme outskirts.