Involved people at OAB: Bellazzini, Cacciari, Federici, Fusi Pecci, Marano,
Parmeggiani.
The study of the overall properties of the cluster systems in the galaxies of the Local Group and beyond, and of the stellar populations in nearby galaxies, is important for a complete understanding of the cluster genesis and its connection with the host galaxy formation and evolution.
The OAB M31 team, in collaboration with scientists of the DA and 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 this program is to utilize 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. Several studies are currently being carried on, that are described below.
The M31 cluster system is the largest sample of globular clusters found in the Local Group, sufficiently close to allow individual stellar observations, and negligible affected by reddening, at least for a large sub-set in the outer halo. Moreover, since its intrinsic depth is small compared to the distance (about 0.1 mag in distance modulus), all the degeneracies and problems due to uncertain knowledge of the individual distances can be removed. Since the typical cluster size at the M31 distance is comparable with the seeing disk (10 pc corresponds to 3.3 arcsec), the available existing samples are not yet complete and uncontaminated because the selections made via morphological-visual inspection of candidates fails to select the most compact clusters in the inner areas (mistaken for stars), and the most extended ones in the outer halo (mistaken for background galaxies); moreover, the halo of M31 outside 25 kpc has never been surveyed with deep images taken in sufficiently good seeing conditions and with large-scale plates or CCD-frames. The search and study of globular clusters in the outermost halo are necessary to get a direct estimate of the total mass of M31 (via the Virial Theorem and the Projected Mass Method), hence setting more stringent constraints and yielding fundamental information on its radial distribution.
In this framework (in collaboration
with Meusinger (Tautenburg) and Testa & Corsi (OAR), we are extending the
search for globular clusters in M31 up to a distance of more than 50 Kpc from
the nucleus; this is done i) by inspecting the UBVR deep plates obtained at
the 2m Schmidt telescope in Tautenburg (59 arcs/mm) on a 
field
centered on M31 with the new projector specifically built to inspect four
plates in different colors of the same field simultaneously, in order to get
hints on the image structure and color; and ii) by further analyzing the UBVI
CCD images of the 9x9 sq.deg. field that will be taken with the 2K-EEV CCD
Camera (1 sq. deg. field, 1.5 arcs/pix) mounted on the 91cm Schmidt telescope
of Campo Imperatore, in order to obtain magnitudes and colors of all the
objects down to U=21, B,V,I=22 or deeper (the peak of the GC LF in M31 being
at V=18).
The sample of GC candidates obtained by combining the color selection on the
CCD data with the morphological analysis on the Tautenburg plates will be
finally decontaminated using subsequent spectroscopic observations. This
survey will thus allow to detect a significant fraction of faint candidates in
the outer regions, that have never been detected so far.
The dependence on metallicity of the HB luminosity is a crucial parameter when deriving relative distances of stellar systems at various metallicities, and can be derived with good accuracy from the analysis of the CM diagrams of globular clusters in external nearby galaxies. Our team is carrying out a long-term project based on observations with HST (FOC+ WFPC + WFPC2) of a wide sample of confirmed GCs in M31 to obtain color-magnitude diagrams from photometry of individual stars. The main goals of the program are: a) to improve the accuracy of globular cluster distance and age determinations, via the accurate determination of the Horizontal Branch luminosity as a function of metallicity for a number of clusters in M31; b) to improve our knowledge of the M31 clusters' stellar populations, basic input for the study of the early evolutionary stages of M31 itself and for the possible use of M31 clusters as templates in population synthesis models; this can be achieved by studying the characteristics of the various parts of the HR diagrams (i.e. HB, RGB and AGB, post-AGB, BSS); and c) to study the cluster surface brightness profiles, in order to get information on the frequency of post-core-collapse vs. King models, and its relation with a number of galactic parameters. CMDs for 10 GCs in M31, reaching at least one magnitude fainter than the Horizontal Branch level, have been already obtained and published; cycle 6 WFPC2 observations (in collaboration with Rich from UCLA and collaborators) for 10 additional clusters have been recently obtained, providing a statistically significant sample of clusters for a meaningful comparison with our Galaxy. The reduction of the data is in progress, and preliminary results have been presented in an international conference.
It is also under construction a comprehensive, homogeneous revised catalog of all morphological, spectrophotometric, astrometric and kinematic data for the clusters and the clusters candidates in M31.