Compact Star Clusters in the M 31 Disk

Based on Suprime-Cam high-resolution images, about 17.'5 x 28.'5 field of the southwest part of the disk of M31 was surveyed for compact clusters of an apparent size less than similar to 3". UBVRI photometry of 285 compact cluster candidates was performed using Local Group Galaxy Survey (LGCS) archive images. Cluster colors indicate a wide range of ages, 1 Myr less than or similar to t less than or similar to 10 Gyr. About 50 bright (17.0 less than or similar to V less than or similar to 19.0) clusters were studied in detail, to reveal that their structure is similar to classic globular clusters, except. for some irregularities, resembling the massive clusters in LMC and SMC. According to the results of single stellar population (SSP) model fitting to the observed photometric data, main part of them are of intermediate mass, between that of Milky Way open clusters and that of Milky Way globular clusters (3.0 less than or similar to log (m/m(circle dot)) less than or similar to 4.5), These intermediate-mass clusters have ages of 30 Myr less than or similar to t less than or similar to 3 Gyr and an average metallicity of [M/H] similar to -0.4. These findings suggest that they are disk clusters of intermediate population, which are not well known in the Milky Way, and may serve as useful tools to understand the evolution of the galaxy disk.