II. Newly discovered bright and remote clusters
Università di Bologna, Dipartimento di Astronomia Via Ranzani 1, 40127 Bologna, Italy e-mail: email@example.com
2 INAF – Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127 Bologna, Italy e-mail: [michele.bellazzini;luciana.federici;alberto.buzzoni;flavio.fusipecci]@oabo.inaf.it
Accepted: 22 May 2007
Aims.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.
Methods.We obtained low resolution spectra ( 800–1300) of 48 targets selected from the Extended Source Catalogue of 2MASS, as in Galleti et al. (2005, A&A, 436, 535). 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. (2006b, MNRAS, 371, 1983).
Results.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. (2007, ApJ, 655, L85), their GC1; the other four are completely new discoveries: B516, B517, B518, B519. The newly discovered clusters lie at projected distance 40 kpc 100 kpc from the center of M 31, and have absolute integrated magnitude -9.5 ≲ MV ≲ -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).
Conclusions.The present analysis nearly doubles the number of M 31 globulars at 40 kpc. At odds with the Milky Way, M 31 appears to have a significant population of very bright globular clusters in its extreme outskirts.
Key words: galaxies: individual: M 31 / galaxies: star clusters / catalogs / galaxies: Local Group
Based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Based on observations made with the Cassini Telescope (Loiano, Italy). The Cassini telescope is operated by INAF – Osservatorio Astronomico di Bologna.
© ESO, 2007