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 8: Stamp images ($8 \times 8$ arcsec$^2$) of eighteen candidate young clusters observed within our HST-WFPC2 survey. The pictures are the central region of the PC images in F450W.

The main ongoing programmes are summarized in the following:

i) 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.

ii) The clusters located at large projected distances from the centre of the galaxy are particularly interesting because they provide the maximum ``leverage'' to constrain the galaxy mass profile at large radii, where kinematical information is missing even from the H I surveys. In this framework, we have started a programme to search for new clusters at projected galactocentric distances larger than 40 kpc. Candidates are selected from the 2MASS Extended Source Catalog (XSC) and then followed up with low resolution spectroscopy with Dolores@TNG and BFOSC@Loiano, as described in Galleti et al. (2005, 2007). At present we have identified five new remote and bright clusters (Galleti et al. 2005, 2007), one of which has been studied in great detail with HST (Galleti et al 2006a, Federici et al. 2007; see also Sect. 1.5.5). Observing time was obtained at the TNG and at the Loiano telescope to complete the survey.

iii) 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 one candidates were imaged with WFPC2. The data reduction is now complete and the analysis of the CMDs is ongoing (see Fig. 8); a first paper has been submitted, presenting the data reduction strategy to be applied to the whole survey and the analysis.

iv) In a previous study of the CMDs obtained from WFPC2/HST observations for 19 GCs in M31 we showed that the M31 globular clusters are similar to the MW globulars, with an indication of the presence of an intermediate age cluster population and of the occurrence of the second-parameter effect (Rich et al. 2005). To verify and enforce that result, in 2007 we performed the same analysis on a further sample of M31 GCs, using HST/ACS archive data. We derived the CMDs of 24 objects classified as GC in the Revised Bologna Catalogue. Of these, 2 turned out to be stars and 11 had very noisy data (due to field contamination) and no reliable CMD could be derived. For the remaining 11 GCs the CMDs were good enough to obtain estimates of metallicity, reddening and distance for the individual objects. This brings to 30 the total number of M31 GCs with usable CMD. A paper is in preparation.

This work is in collaboration with M. Rich (Caltech) and C. Corsi (INAF-OAR). 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).