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Register a new userMulticolour Photometric Study of M31 Globular Clusters
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Z. Fan
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We present the photometry of 30 globular clusters (GCs) and GC candidates in 15 intermediate-band filters covering from ~3000 to ~10000 AA using the archival CCD images of M31 observed as part of the Beijing - Arizona - Taiwan - Connecticut (BATC) Multicolour Sky Survey. We transform these intermediate-band photometric data to the photometry in the standard UBVRI broad-bands. These M31 GC candidates are selected from the Revised Bologna Catalogue (RBC V.3.5), and most of these candidates do not have any photometric data. Therefore the present photometric data are supplement to RBC V.3.5. We find that 4 out of 61 GCs and GC candidates in RBC V.3.5 do not show any signal on the BATC images at their locations. By linear fit of the distribution in colour-magnitude diagram of blue GCs and GC candidates using the data from RBC V.3.5, and in this study we find the ``blue-tilt of blue M31 GCs with a high confidence at 99.95% or 3.47 sigma for the confirmed GCs, and >99.99% or 4.87 sigma for GCs and GC candidates.
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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.
We present photometry of 104 M31 globular clusters (GCs) and GC candidates in 15 intermediate-band filters of the Beijing-Arizona-Taiwan-Connecticut (BATC) photometric system. The GCs and GC candidates were selected from the Revised Bologna Catalog (v.3.5). We obtain the cluster ages by comparing the photometric data with up-to-date theoretical synthesis models. The photometric data used are {sl GALEX} far- and near-ultraviolet and 2MASS near-infrared $JHK_{rm s}$ magnitudes, combined with optical photometry. The ages of our sample clusters cover a large range, although most clusters are younger than 10 Gyr. Combined with the ages obtained in our series of previous papers focusing on the M31 GC system, we present the full M31 GC age distribution. The M31 GC system contains populations of young and intermediate-age GCs, as well as the `usual complement of well-known old GCs, i.e., GCs of similar age as the majority of the Galactic GCs. In addition, young GCs (and GC candidates) are distributed nearly uniformly in radial distance from the center of M31, while most old GCs (and GC candidates) are more strongly concentrated.
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