HST/ACS colour-magnitude diagrams of M31 globular clusters

TL;DR

This study uses HST/ACS images to analyze the colour-magnitude diagrams of M31 globular clusters, confirming classifications, estimating their properties, and expanding the sample of clusters with reliable optical CMDs, revealing insights into their ages, metallicities, and origins.

Contribution

The paper provides new CMDs for 17 M31 clusters, refines their classifications, and offers detailed estimates of their physical properties, enhancing the understanding of M31's globular cluster system.

Findings

Old clusters have metallicities between -0.4 and -1.9.

At least four old clusters have ages >10 Gyr with blue HB.

All six young clusters are younger than 1 Gyr.

Abstract

With the aim of increasing the sample of M31 clusters for which a colour magnitude diagram is available, we searched the HST archive for ACS images containing objects included in the Revised Bologna Catalogue of M31 globular clusters. Sixty-three such objects were found. We used the ACS images to confirm or revise their classification and we obtained useful CMDs for 11 old globular clusters and 6 luminous young clusters. We obtained simultaneous estimates of the distance, reddening, and metallicity of old clusters by comparing their observed field-decontaminated CMDs with a grid of template clusters of the Milky Way. We estimated the age of the young clusters by fitting with theoretical isochrones. For the old clusters, we found metallicities in the range -0.4<=[Fe/H]<=-1.9, that generally agree with existing spectroscopic extimates. At least four of them display a clear blue HB, indicating ages >10 Gyr. All six candidate young clusters are found to have ages <1Gyr. With the present work the total number of M31 GCs with reliable optical CMD increases from 35 to 44 for the old clusters, and from 7 to 11 for the young ones. The old clusters show similar characteristics to those of the MW. We discuss the case of the cluster B407, with a metallicity [Fe/H] ~-0.6 and located at a large projected distance from the centre of M31 and from the galaxy major axis. Metal-rich globulars at large galactocentric distances are rare both in M31 and in the MW. B407, in addition, has a velocity in stark contrast with the rotation pattern shared by the bulk of M31 clusters of similar metallicity. This, along with other empirical evidence, supports the hypothesis that the cluster is physically associated with a substructure in the M31 halo that has been interpreted as the relic of a merging event.