HST/ACS color-magnitude diagrams of candidate intermediate-age M 31 globular clusters.
The role of blue horizontal branches
S. Perina, S. Galleti, F.
Fusi Pecci, M.
Federici, A. Buzzoni
We present deep (V ~ 28)
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.