People involved at OAB: Bellazzini, Buzzoni, Cacciari, Federici, Fusi Pecci,
Parmeggiani, 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.
M31 offers the unique opportunity of studying the GC system of a spiral galaxy that is similar to the Milky Way in very good detail and without some of the limitations that affect the Galactic GC system. The OAB M31 team, in collaboration with scientists of other Italian and foreign institutions, is studying the globular clusters in M31 and other galaxies of the Local Group using both photometry (from the UV to the IR bands) and spectroscopy. The scope of the program is to use the globular cluster systems to improve our knowledge on the mass, dynamics and chemical evolution of the parent galaxies, and as secondary distance indicators and stellar population templates.
In this framework, we have revised the catalog of candidate GCs of M31
(Galleti et al. 2004), providing a
comprehensive list of 1164 candidates - the revised Bologna catalogue -
with full
and updated photometric information, including also JHK photometry for 693 of
them that has become recently available from the 2MASS database.
The final revised Bologna catalog includes 337 confirmed GCs, 688 GC candidates,
10 objects with controversial classification, 70 confirmed galaxies, 55 confirmed
stars and 4 HII regions lying within 
deg from the center of
M31. It is the most
comprehensive list presently available of confirmed and candidate M31 globular
clusters, and is available in electronic form at the CDS (Strasbourg Data
Center). Using this newly
assembled database we have found that the 
color provides a powerful tool
to discriminate between M31 clusters and background galaxies, and we have
identified a subset of 83 blue GC candidates that have a high probability of
being true M31 clusters and that will be observed spectroscopically to
ascertain their nature. The confirmed clusters will be important additional
kinematical probes for a better determination of the mass profile and the
total mass of M31.
A large sample of confirmed GCs in M31 has been observed with HST with the aim of comparing the characteristics of these stellar populations with those of the Milky Way, and to measure the dependence of the HB luminosity on metallicity for a sample of clusters at the same distance. The CMDs obtained from WFPC2/HST observations for a total sample of 19 GCs in M31 [in collaboration with Rich (UCLA)] reach at least one magnitude fainter than the Horizontal Branch level. Our analysis shows that M31 globular clusters are very similar to the Milky Way globular clusters, and that there is no strong indication of an intemediate age cluster population analogous to those found (for example) in the SMC.