Spectral Energy Distributions and Masses of 304 M31 Old Star Clusters

TL;DR

This study provides multicolor photometry and mass estimates for 304 old star clusters in M31, including 55 clusters observed for the first time, revealing their mass distribution and comparison with young clusters.

Contribution

First multicolor CCD photometry for 55 old star clusters in M31, with mass estimates based on stellar population models and comparison to previous data.

Findings

Cluster masses range from 3×10^4 to 10^7 solar masses.

Mass distribution peaks at approximately 4×10^5 solar masses.

Old clusters are ten times more massive on average than young clusters.

Abstract

This paper presents CCD multicolor photometry for 304 old star clusters in the nearby spiral galaxy M31. Of which photometry of 55 star clusters is first obtained. The observations were carried out as a part of the Beijing--Arizona--Taiwan--Connecticut (BATC) Multicolor Sky Survey from 1995 February to 2008 March, using 15 intermediate-band filters covering 3000--10000 \AA. Detailed comparisons show that our photometry is in agreement with previous measurements. Based on the ages and metallicities from Caldwell et al. and the photometric measurements here, we estimated the clusters' masses by comparing their multicolor photometry with stellar population synthesis models. The results show that the sample clusters have masses between $\sim 3\times10^4 M_\odot$ and $\sim 10^7 M_\odot$ with the peak of $\sim 4\times10^5 M_\odot$. The masses here are in good agreement with those in previous studies. Combined with the masses of young star clusters of M31 from Wang et al., we find that the peak of mass of old clusters is ten times that of young clusters.