HST/ACS colour–magnitude diagrams of M 31 globular clusters

Abstract

Aims. With the aim of increasing the sample of M 31 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 M 31 globular clusters . Methods. Sixty-three such objects were found. We used the ACS images to confirm or revise their classification and were able to obtain 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. Results. For the old clusters, we found metallicities in the range −0.4 ≤ [Fe/H] ≤ −1.9. The individual estimates generally agree with existing spectroscopic estimates. At least four of them display a clear blue horizontal branch, indicating ages > ∼ 10 Gyr. All six candidate young clusters are found to have ages <1 Gyr. The photometry of the clusters is made publicly available through a dedicated web page. Conclusions. With the present work the total number of M 31 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 M 31 (R p = 19.8 kpc) and from the major axis of the galaxy (Y = 11.3 kpc). Metal-rich globulars at large galactocentric distances are rare both in M 31 and in the Milky Way. B407, in addition, has a velocity in stark contrast with the rotation pattern shared by the bulk of M 31 clusters of similar metallicity. This, along with other empirical evidence, supports the hypothesis that the cluster (together with B403) is physically associated with a substructure in the halo of M 31 that has been interpreted as the relic of a merging event.

HST/ACS color-magnitude diagrams of candidate intermediate-age M 31 globular clusters

“…For the fields with multiple objects, orientation requirements were set to guarantee the best coverage of all targets, and additional orientational constraints were added to the B292 pointing to avoid a bright star in the field of view. The field of B058 also contains B049 (already studied in Perina et al 2009b); the field of B337 also contains B336 and in the field of B292 we discovered a new cluster that was never detected before (dubbed B531, according to the RBC nomenclature). For these clusters it was possible to obtain meaningful CMD that are presented and discussed below.…”

“…In general, typical CMDs of M 31 GCs obtained by HST data barely reach the base of the red giant branch (RGB), including most horizontal branch (HB) stars, except the bluest/faintest ones, when present (Ajhar et al 1996;Rich et al 1996;Fusi Pecci et al 1996;Holland et al 1997;Jablonka et al 2000;Meylan et al 2001;Rich et al 2005;Perina et al 2009b;Huxor et al 2004Huxor et al , 2005Huxor et al , 2008Galleti et al 2006;Mackey et al 2006Mackey et al , 2007. In this magnitude range the HB morphology can be used as a rough age indicator, because the presence of RR Lyrae variables and/or blue HB stars implies low-mass progenitors, hence old ages.…”

The horizontal branch morphology of M 31 globular clusters

“…In the Galaxy, slightly younger GCs are generally found to be associated with halo substructures and/or relics of disrupting dwarf galaxies (Marin-Franch et al 2009). Also most ARSA clusters can be associated to similar substructures in the M 31 halo (Perina et al 2009;Mackey et al 2010), and hence younger ages would fit as a general characteristics of recently accreted clusters from dwarf satellites 9 . The fact that Sample A clusters are even younger than the MW OH clusters may be connected to the more complex and extended recent accretion activity that took place in Andromeda (w.r.t.…”

“…Moreover, ≈20 per cent of Sample A clusters have R p > 40 kpc, a range that contains less than 5 per cent of the whole sample. Finally, many of the outermost clusters of Sample A have been found to be likely associated with known existing substructures in the outer halo of M 31 (Perina et al 2009;Mackey et al 2010). Therefore, Sample A is more likely representative of the outer halo population of the GC system of M 31, and/or of the population of GCs more recently accreted via the disruption of their parent dwarf galaxies.…”

“…It is possible to reach the MSTO luminosity of M 31 clusters, but this requires such a large amount of HST time as to make a systematic study unpractical (Brown et al 2004). -Global metallicity estimates from integrated spectra and/or from the color of the RGB are also available (Galleti et al 2009;Caldwell et al 2011;Perina et al 2009), analogously to the MW GC compilation by Zinn & West (1984).…”