The Globular Cluster System of the Andromeda galaxy

People involved at OAB: Bellazzini, Buzzoni, Cacciari, Federici, Fusi Pecci, Parmeggiani, Perina, Galleti.

The M31 globular cluster system is the largest found in the Local Group, sufficiently close to allow detailed observations and little affected by reddening, at least for a large outer sub-set. Since the intrinsic depth of the spheroid is small compared to the distance to M31, in the study of the basic properties and comparisons one can remove the degeneracies introduced by the uncertain knowledge of the individual distances. In practice, M31 offers the unique opportunity of studying in very good detail the GC system of a spiral galaxy that is similar to the MW, without some of the limitations that affect the Galactic GC system.

The study of the M31 GC system is a long-term project started at the OAB in the early 1980s, and recently revived by new observational programmes (Galleti et al. 2004, 2005, 2006a,b, 2007; Federici et al. 2007; Fusi Pecci et al. 2005). The OAB M31 team, in collaboration with scientists of other Italian and foreign institutions, is carrying out the exhaustive systematic census of M31 GC candidates using both photometry (from the UV to the IR bands) and spectroscopy. The purpose of the project is to use the globular cluster system to improve our knowledge of the mass, dynamics and chemical evolution of the parent galaxy.

Figure: CMDs of different circular annuli around the centre of the cluster VdB0 in the PC field, all having the same area ( upper panels and lower left panel), and of the whole WF2 field, whose area is 32 times that of the PC annuli ( lower right panel). The thin line is an isochrone of age 25 Myr; the heavy lines at F450W 24.0 are 12Gyr old isochrones of metallicity Z= $6\times 10^{-4}$ and Z= $6\times 10^{-3}$, from blue to red, respectively. The additional isochrone plotted in the lower right panel has Z=0.008 and age 125 Myr. All the isochrones are from Girardi et al. (2002). From Perina et al. (2009).

The main ongoing programmes are summarized in the following:

1. Our continuously updated on-line catalogue (Revised Bologna Catalogue of M31 globular clusters-RBC, Galleti et al. 2004) has become the most complete and widely used reference in the field. The RBC web site scores more than 600 contacts per year from all over the world. A thorough analysis to add spectroscopic metallicities to as many RBC clusters as possible is currently ongoing (Galleti et al. 2009).

2. The study of a sample of 63 M31 GC candidates using HST/ACS archive data has been completed. We derived sufficiently accurate CMDs for 17 such objects, namely 11 old GCs and 6 luminous young clusters. For the 11 old GCs, estimates of metallicity, reddening and distance could be obtained by comparison with template Galactic GCs. The age of the young clusters was estimated by fitting with theoretical isochrones. This work brings to 44+11 the total number of M31 old+young GCs with usable CMD. A paper is in press (Perina et al. 2009b).

3. The presence in M31 of stellar systems similar to the MW globulars in luminosity and shape, but with integrated colours significantly bluer than the bluest MW counterparts, is well known and documented. As a follow-up of our recent study (Fusi Pecci et al. 2005) on these young and bright clusters, a large HST survey in collaboration with scientists of foreign institutions (HST cycle 15, PI Cohen) is in progress, to check the real nature of these objects that seem to have no counterpart in the Milky Way. Twenty candidates were imaged with WFPC2. The data reduction is now complete (see Fig. 8) and four papers will be published in 2009 (Huchra et al. 2009, Barmby et al. 2009, Perina et al. 2009a,c).

This work is in collaboration with J. Cohen (Caltech, USA), P. Hodge (Univ. Washington, USA), P. Barmby (Univ. Ontario, Canada), M. Rich (Caltech, USA), C. Corsi (INAF-OAR) and many others. The PhD research project of Sibilla Perina is aimed at the study of star clusters in M31, particularly focussed on the class of young and luminous clusters identified by Fusi Pecci et al. (2005).