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