An HST/WFPC2 survey of bright young clusters in M31. IV. Ages and mass estimates

TL;DR

This study uses HST imaging to identify and analyze young massive clusters in M31, estimating their ages, metallicities, and masses, revealing that such clusters are common in large star-forming galaxies.

Contribution

First comprehensive HST-based survey of young massive clusters in M31, providing detailed age, metallicity, and mass estimates, and confirming their prevalence in the Local Group.

Findings

19 of 20 candidates confirmed as clusters

14 clusters aged 25-280 Myr, 2 older than 500 Myr

Masses range from 0.6 to 6 x 10^4 solar masses

Abstract

{Aims.} We present the main results of an imaging survey of possible young massive clusters (YMC) in M31 performed with the Wide Field and Planetary Camera2 (WFPC2) on the Hubble Space Telescope (HST). We present the images and color magnitude diagrams (CMDs) of all of our targets. {Methods.} The reddening, age and, metallicity of the clusters were estimated by comparing the observed CMDs and luminosity functions with theoretical models. Stellar masses were estimated by comparison with theoretical models in the log(Age) vs. absolute integrated magnitude plane. {Results.} Nineteen of the twenty surveyed candidates were confirmed to be real star clusters. Three of the clusters were found not to be good YMC candidates from newly available integrated spectroscopy and were in fact found to be old from their CMD. Of the remaining sixteen clusters, fourteen have ages between 25 Myr and 280 Myr, two have older ages than 500 Myr (lower limits). By including ten other YMC with HST photometry from the literature we have assembled a sample of 25 clusters younger than 1 Gyr, with mass ranging from 0.6 x 10^4 M_sun to 6 x 10^4 M_sun, with an average of ~ 3 x 10^4 M_sun. {Conclusions.} The clusters considered here are confirmed to have masses significantly higher than Galactic open clusters in the same age range. Our analysis indicates that YMCs are relatively common in all the largest star-forming galaxies of the Local Group.