An HST/WFPC2 Survey of Bright Young Clusters in M31 III. Structural Parameters

TL;DR

This study measures the structural parameters of 23 young, massive star clusters in M31 using HST images, comparing their properties to clusters in other galaxies and exploring their potential evolution.

Contribution

It provides detailed structural measurements of M31 clusters and compares their properties to those in the Magellanic Clouds and the Milky Way, highlighting universal relations.

Findings

Clusters are young (~10^8 yr) and massive (~10^4.5 solar masses).

Clusters follow the same age-size relation and fundamental plane as older clusters.

Most clusters are expected to dissolve within a few Gyr.

Abstract

Surface brightness profiles for 23 M31 star clusters were measured using images from the Wide Field Planetary Camera 2 on the Hubble Space Telescope, and fit to two types of models to determine the clusters' structural properties. The clusters are primarily young (~10^8 yr) and massive (~10^4.5 solar masses), with median half-light radius 7 pc and dissolution times of a few Gyr. The properties of the M31 clusters are comparable to those of clusters of similar age in the Magellanic Clouds. Simulated star clusters are used to derive a conversion from statistical measures of cluster size to half-light radius so that the extragalactic clusters can be compared to young massive clusters in the Milky Way. All three sets of star clusters fall approximately on the same age-size relation. The young M31 clusters are expected to dissolve within a few Gyr and will not survive to become old, globular clusters. However, they do appear to follow the same fundamental plane relations as old clusters; if confirmed with velocity dispersion measurements, this would be a strong indication that the star cluster fundamental plane reflects universal cluster formation conditions.