HST/ACS color-magnitude diagrams of candidate intermediate-age M 31 globular clusters. The role of blue horizontal branches

TL;DR

This study uses deep HST/ACS photometry to analyze the color-magnitude diagrams of candidate intermediate-age M31 globular clusters, revealing that horizontal branch morphology can significantly affect age diagnostics from integrated spectra.

Contribution

It provides detailed CMDs for M31 globular clusters and demonstrates the influence of horizontal branch morphology on spectral age indicators, challenging previous age estimates based solely on integrated spectra.

Findings

B292 and B350 are as old as Galactic globulars based on blue horizontal branches.

Spectral indices can be misleading due to HB morphology effects.

CMDs reveal diverse properties, including RR-Lyrae populations and red giant branches.

Abstract

We present deep (V ~ 28.0) BV photometry obtained with the wide field channel of the Advanced Camera for Surveys on board HST for four M31 globular clusters that were identified as candidate intermediate-age (age ~ 1-9 Gyr) by various authors, based on their integrated spectra and/or broad/intermediate-band colors. Two of them (B292 and B350) display an obvious blue horizontal branch, indicating that they are as old as the oldest Galactic globulars. On the other hand, for the other two (B058 and B337), which display red horizontal branches, it was not possible either to confirm or disconfirm the age estimate from integrated spectra. The analysis of the distribution in the spectral indices Mg2 and H_beta of the M31 and Milky Way clusters whose horizontal branch can be classified as red or blue based on existing CMDs, strongly suggests that classical age diagnostics from integrated spectra may be significantly influenced by the HB morphology of the clusters and can lead to erroneous age-classifications. We also provide the CMD for another two clusters that fall into the field of the main targets, B336, an old and metal-poor globular with a significant population of RR-Lyrae variables, and the newly discovered B531, a cluster with a very red red giant branch.