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An updated survey of globular clusters in M 31. II. Newly discovered bright and remote clusters
Aims.We present the first results of a large spectroscopic survey ofcandidate globular clusters located in the extreme outskirts of thenearby M 31 galaxy. The survey is aimed at ascertaining the nature ofthe selected candidates to increase the sample of confirmed M 31clusters lying more that 2° away from the center of the galaxy.Methods: We obtained low resolution spectra (λ/Δλ≃ 800-1300) of 48 targets selected from the Extended SourceCatalogue of 2MASS, as in Galleti et al. (2005, A&A, 436, 535). Theobserved candidates have been robustly classified according to theirradial velocity and by verifying their extended/point-source nature fromground-based optical images. We have also obtained a spectrum and aradial velocity estimate for the remote M 31 globular discovered byMartin et al. (2006b, MNRAS, 371, 1983). Results: Among the 48 observedcandidates clusters we found: 35 background galaxies, 8 foregroundGalactic stars, and 5 genuine remote globular clusters. One of them hasbeen already identified independently by Mackey et al. (2007, ApJ, 655,L85), their GC1; the other four are completely new discoveries: B516,B517, B518, B519. The newly discovered clusters lie at projecteddistance 40 kpc ≲ R_p≲ 100 kpc from the center of M 31, andhave absolute integrated magnitude -9.5 ≲ MV ≲-7.5. For all the observed clusters we have measured the strongest Lickindices and we have obtained spectroscopic metallicity estimates.Mackey-GC1, Martin-GC1, B517 and B518 have spectra typical of old andmetal poor globular clusters ([Fe/H] ≲ -1.3); B519 appears old butquite metal-rich ([Fe/H]~≃ -0.5); B516 presents very strong Balmerabsorption lines: if this is indeed a cluster it should have arelatively young age (likely < 2 Gyr). Conclusions: The presentanalysis nearly doubles the number of M 31 globulars at R_p≥ 40 kpc.At odds with the Milky Way, M 31 appears to have a significantpopulation of very bright globular clusters in its extreme outskirts.Appendices A and B are only available in electronic form athttp://www.aanda.org Based on observations made with the ItalianTelescopio Nazionale Galileo (TNG) operated on the island of La Palma bythe Fundación Galileo Galilei of the INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias. Based on observations madewith the Cassini Telescope (Loiano, Italy). The Cassini telescope isoperated by INAF - Osservatorio Astronomico di Bologna.
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