Compact Star Clusters in the M31 Disk

TL;DR

This study surveys compact star clusters in M31's disk, revealing a diverse age range and a rich population of intermediate-mass clusters, highlighting differences from the Milky Way.

Contribution

First high-resolution survey of compact star clusters in M31 covering ~15% of the galaxy's disk, providing detailed photometric and age data.

Findings

Clusters range from 5 Myr to 10 Gyr in age.

Masses peak around 4000 solar masses.

Intermediate-mass clusters are abundant in M31.

Abstract

We have carried out a survey of compact star clusters (apparent size <3 arcsec) in the southwest part of the M31 galaxy, based on the high-resolution Suprime-Cam images (17.5 arcmin x 28.5 arcmin), covering ~15% of the deprojected galaxy disk area. The UBVRI photometry of 285 cluster candidates (V < 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 > 0.15 arcsec (>0.6 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 um and/or Ha emission) to ~10 Gyr (globular cluster candidates), and possess mass in a range of 3.0 < log(m/M_sol) < 4.3 peaking at m ~ 4000 M_sol. Typical age of these intermediate-mass clusters is in the range of 30 Myr < t < 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.