AN UPDATED CATALOG OF M31 GLOBULAR-LIKE CLUSTERS:UBVRIPHOTOMETRY, AGES, AND MASSES

Abstract

We present an updated U BV RI photometric catalog containing 970 objects in the field of M31, selected from the Revised Bologna Catalog (RBC v.4.0), including 965, 967, 965, 953, and 827 sources in the individual U BV RI bands, respectively, of which 205, 123, 14, 126, and 109 objects do not have previously published photometry. Photometry is performed using archival images from the Local Group Galaxies Survey, which covers 2.2 deg 2 along the major axis of M31. Detailed comparisons show that our photometry is fully consistent with previous measurements in all filters. We focus on 445 confirmed 'globular-like' clusters and candidates, comprising typical globular and young massive clusters. The ages and masses of these objects are derived by comparison of their observed spectralenergy distributions with simple stellar population synthesis. Approximately half of the clusters are younger than 2 Gyr, suggesting that there has been significant recent active star formation in M31, which is consistent with previous results. We note that clusters in the halo (r projected > 30 kpc) are composed of two different components, older clusters with ages > 10 Gyr and younger clusters with ages around 1 Gyr. The spatial distributions show that the young clusters (< 2 Gyr) are spatially coincident with the galaxy's disk, including the '10 kpc ring,' the 'outer ring,' and the halo of M31, while the old clusters (> 2 Gyr) are spatially correlated with the bulge and halo. We also estimate the masses of the 445 confirmed clusters and candidates in M31 and find that our estimates agree well with previously published values. We find that none of the young disk clusters can survive the inevitable encounters with giant molecular clouds in the galaxy's disk and that they will eventually disrupt on timescales of a few Gyr. Specifically, young disk clusters with a mass of 10 4 M ⊙ are expected to dissolve within 3.0 Gyr and will, thus, not evolve to become globular clusters.

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

“…7). Fan et al (2010) estimates from integrated photometry an age of 1.015 ± 0.167 Gyr, a value incompatible with the observed CMD. The authors obtained a metallicity of [Fe/H] − 2.221 ± 0.085 in marginal disagreement with the value estimated here, and a reddening E(B − V) = 0.420 ± 0.040.…”

“…Another widely used method to age-date extragalactic star clusters relies on multi-band photometry: the age can be derived from the spectral energy distributions (SED) measured in broad-band photometric systems, by comparing them with simple stellar population (SSP 3 ) synthesis models (e.g. Jiang et al 2003;Fan et al 2006Fan et al , 2008Fan et al , 2010Ma et al 2007Ma et al , 2009. Clearly, photometrybased technique are prone to errors owing to the uncertainty in the interstellar extinction, while spectral indices are virtually free from this effect.…”

The effects of the WISE/GALEX photometry for the SED-fitting with M31 star clusters and candidates

“…The slope of the linear fit is −0.02, suggesting that there is almost no correlation between results of the two works, which may be due to the systematic offsets between literature values gathered in their final catalogue. In addition, Fan et al (2010) updated the UBV RI photometry and in order to determine the ages and masses of 445 confirmed globular-like and candidate clusters of M31, the spectroscopic metallicities are collected from Perrett et al (2002), Barmby et al (2000) and Huchra et al (1991), which are shown on the bottom right panel. It suggests that the metallicities from the compiled catalogue of Fan et al (2010) is systematically higher than the value of our work, with the systematic offset of [Fe/H] Fan+2010 − [Fe/H] our work = 0.35 ± 0.75 dex.…”

“…Actually in the updated version of RBC v5, a series of high precision photometry have been included. In our sample, most of the photometry of RBC are from Barmby et al (2000); Barmby & Huchra (2001) and Fan et al (2010) and we adopted the photometric errors from these literature works if available. For those photometric errors which can not be found in the literature works, we adopted a mean photometric errors depending on the brightness of the sources.…”

New 2mass Near-Infrared Photometry for Globular Clusters in M31

“…Because the reddening values from Fan et al (2008) comprise a homogeneous data set and the number of GCs included is greater than that of Barmby et al (2000), we preferentially adopted Fan et al (2008) reddening values, followed by those of Barmby et al (2000). Finally, the results from Fan et al (2010), which derived reddening values from spectral-energy distribution fitting, were adopted.…”