Compact Star Clusters in the M31 Disk

Abstract

We have carried out a survey of compact star clusters (apparent size lsim3'') in the southwest part of the M31 galaxy, based on the high-resolution Suprime-Cam images (17farcm5 × 28farcm5), covering ~15% of the deprojected galaxy disk area. The UBVRI photometry of 285 cluster candidates (V lsim 20.5 mag) was performed using frames of the Local Group Galaxies Survey. The final sample, containing 238 high probability star cluster candidates (typical half-light radius r_h ~ 1.5 pc), was selected by specifying a lower limit of r_h gsim 0farcs15 (gsim0.6 pc). We derived cluster parameters based on the photometric data and multiband images by employing simple stellar population models. The clusters have a wide range of ages from ~5 Myr (young objects associated with 24 μm and/or Hα emission) to ~10 Gyr (globular cluster candidates), and possess mass in a range of 3.0 lsim log(m/m _sun) lsim 4.3 peaking at m ~ 4000 m _sun. Typical age of these intermediate-mass clusters is in the range of 30 Myr lsim t lsim 3 Gyr, with a prominent peak at ~70 Myr. These findings suggest a rich intermediate-mass star cluster population in M31, which appears to be scarce in the Milky Way galaxy.