The Andromeda project - I. Deep HST-WFPC2 V, I photometry of 16 fields toward the disk and the halo of the M 31 galaxy. Probing the stellar content and metallicity distribution

We have obtained HST-WFPC2 F555W and F814W photometry for 16 fields in the vicinity of the luminous nearby spiral galaxy M 31, sampling the stellar content of the disk and the halo at different distances from the center, from ~20 to ~150 arcmin (i.e. ~4.5 to 35 kpc), down to limiting V and I magnitudes of ~27. The Color–Magnitude Diagrams (CMD) obtained for each field show the presence of complex stellar populations, including an intermediate age/young population and older populations with a wide range of metallicity. Those fields superposed on the disk of M 31 generally show a blue plume of stars which we identify with main sequence members. According to this interpretation, we find that the star formation rate over the last 0.5 Gyr has varied dramatically with location in the disk. The most evident feature of all the CMDs is a prominent Red Giant Branch (RGB) with a descending tip in the V band, characteristic of metallicity higher than 1/10 Solar. A red clump is clearly detected in all of the fields, and a weak blue horizontal branch is frequently present. The metallicity distributions, obtained by comparison of the RGB stars with globular cluster templates, all show a long, albeit scantly populated, metal-poor tail and a main component peaking at $\rm [Fe/H] \sim - \,0.6$. The most noteworthy characteristic of the abundance distributions is their overall similarity in all the sampled fields, covering a wide range of environments and galactocentric distances. Nevertheless, a few interesting differences and trends emerge from the general uniformity of the metallicity distributions. For example, the median [Fe/H] shows a slight decrease with distance along the minor axis ( Y ) up to $Y\simeq 20 \arcmin$, but the metallicity gradient completely disappears beyond this limit. Also, in some fields a very metal-rich ($\rm [Fe/H] \ge -0.2$) component is clearly present. Whereas the fraction of metal-poor stars seems to be approximately constant (within few percent) in all fields, the fraction of metal-rich and, especially, very-metal-rich stars varies with position and seems to be more prominent in those fields superposed on the disk and/or with the presence of streams or substructures (e.g. Ibata et al. 2001). This might indicate and possibly trace interaction effects with some companion, e.g. M 32.