The Andromeda Project. I. Deep HST-WFPC2 V,I photometry of 16 fields toward the disk and the halo of the M31 galaxy. Probing the stellar content and metallicity distribution

TL;DR

This study uses deep HST photometry of 16 fields in M31 to analyze stellar populations, metallicity distributions, and star formation history across the galaxy's disk and halo, revealing complex, spatially variable stellar content.

Contribution

First comprehensive HST-based analysis of M31's disk and halo stellar populations, metallicity, and star formation history across multiple fields.

Findings

Presence of diverse stellar populations including young, intermediate, and old stars.

Metallicity distributions show a dominant component at [Fe/H] ~ -0.6 with some fields having very metal-rich stars.

Star formation rate varies dramatically across different regions of M31.

Abstract

HST-WFPC2 F555W and F814W photometry were obtained for 16 fields of the luminous nearby spiral galaxy M31, 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) 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 M31 generally show a blue plume of stars which we identify with main sequence members. Accordingly, the star formation rate over the last 0.5 Gyr appears to have varied dramatically with location in the disk. All the CMDs show 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 detected in all of the fields, and a weak blue horizontal branch is frequently present. The metallicity distributions (MDs), obtained by comparison of the RGB stars with globular cluster templates, are basically similar in all the sampled fields: they all show a long, albeit scantly populated metal-poor tail and a main component at [Fe/H] ~ -0.6. However, some differences also exist, e.g. in some fields a very metal-rich ([Fe/H] >= -0.2) component is present. Whereas the fraction of metal-poor stars seems to be approximately constant in all fields, the fraction of 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. This might indicate and possibly trace interaction effects with some companion, e.g. M32.