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Revised GALEX Ultraviolet Catalog of Globular Clusters in M31Kyungsook Lee(1), Soo-Chang Rey(1), Sangmo Tony Sohn(2), and GALEX Science Team(1) Department of Astronomy and Space Science, Chungnam National University(2) Center for Space Astrophysics, Yonsei University and California Institute of Technology, USA


We present near-ultraviolet (NUV) and far-ultraviolet (FUV) photometry for the globular clusters(GCs) of M31 from mosaic images of Galaxy Evolution Explorer (GALEX). The data consist of images of 23 different 1.25 degree circular fields. Our entire survey overage is ~ 17 square degree, which includes most of disk regions currently active in forming massive stars and halo of M31. We construct UV-optical merged catalog of GCs ( and GC candidates) in M31 by cross-matching between UV photometry and optical and near-IR photometry from Revised Bolobna Catalog. The UV catalog of M31 GCs includes 510GCs and 1050 GC candidates. We explored the general UV properties and age distribution of GCs comparing with the stellar population models. We suggest UV color-color diagram has advantage of separating GCs very effectively from background galaxies and foreground stars than the case of optical color-color diagram.


GCs Separating from Galaxies

We show that the UV colors provide a useful discriminate to separate good candidate clusters from background galaxies using the large baseline provided by UV-optical colors.

Spatial centermost 3o × 3o region of the GALEX M31 mosaic. The image, a composite representation of our FUV and NUV data, has been created so as to make sources brightest in FUV appear blue and those features brightest in NUV appear orange.

(Left) 14 GALEX fields observed in 2003 and (Right) 9 additional GALEX fields observed in 2004 projected onto a 6o × 6o DSS image. Exposure times are 650 - 5100 sec. North is to the top and East is to the left.

(upper) Optical color-color diagram and (lower) UV-Optical color-color diagram for the confirmed globular clusters (open circles), confirmed star in M31 (cyan stars) and confirmed galaxies (yellow triangle).

UV-Optical Value Added Catalog

• Previous UV catalog- Rey et al. (2007)

• Updated UV catalog- UV-Optical Value Added Catalog

Age distribution of GCs in M31

Detection Rate



(B-V) vs. (UV-V) diagrams for confirmed globular cluster (open circles), cluster candidates (cross), confirmed star in M31 (cyan stars) and confirmed galaxies (yellow triangle). We superpose our model isochrones (0.1 Gyr ~14Gyr). Not a few of cluster candidates are out of model isochrones range. They occupied area such as confirmed galaxies.

  • We use the B and V magnitude to examine the detection rate of clusters in GALEX fields. The CMD show that the majority of the detected objects belong to an optically blue subgroup with B-V < 1. The blue clusters with B-V < 0.5 are younger than 2Gyr, the others are metal-poor old cluster.
  • Most confirmed clusters are old cluster (red box), cluster candidates are young cluster (green box) compared with population synthesis model.
  • Some peculiar objects with red optical color and blue UV color( majority of cluster candidates) are likely background galaxies.
  • Caldwell et al. (2008) reclassify as stars and background galaxies by spectroscopy.

Color Distribution of M31 Cluster and Cluster Candidates


GC candidates


GC candidates


GC candidates

Summary & Future Works

  • UV catalog update; 510 GCs and 1049 GC candidates
  • M31 includes young globular clusters that do not exist on Milky Way
  • UV color-color diagram has advantage of separating GCs from background galaxies and stars than Optical color-color diagram
  • Some of GC candidates are young GCs and some may be background galaxies or stars
  • Construction of multi-wavelength catalog of star clusters in nearby galaxies
  • Study of the age, metallicitiy, and spatial distributions of star clusters
  • Understanding the correlations between properties of star clusters and host galaxy.
  • (FUV-V) color distribution seem to be separated with globular cluster and cluster candidates. If blue cluster candidates are bona fide clusters, they are young clusters.