STAR CLUSTERS IN M31. I. A CATALOG AND A STUDY OF THE YOUNG CLUSTERS

We present an updated catalog of 1300 objects in the field of M31, including 670 likely star clusters of various types, the rest being stars or background galaxies once thought to be clusters. The coordinates in the catalog are accurate to 0.″2, and are based on images from the Local Group Survey (LGS) or from the Digitized Sky Survey (DSS). Archival Hubble Space Telescope (HST) images and the LGS were inspected to confirm cluster classifications where possible, but most of the classifications are based on spectra taken of ∼1000 objects with the Hectospec fiber positioner and spectrograph on the 6.5 m MMT. The spectra and images of young clusters are analyzed in detail in this paper; analysis of older clusters will appear in a later paper. Ages and reddenings of 140 young clusters are derived by comparing the observed spectra with model spectra. Seven of these clusters also have ages derived from HST color–magnitude diagrams (two of which we present here); these agree well with the spectroscopically determined ages. Combining new V-band photometry with the M/L values that correspond to the derived cluster ages, we derive masses for the young clusters, finding them to have masses as great as 105 with a median of 104 M☉, and a median age of 0.25 Gyr. In comparison, therefore, Milky Way (MW) open clusters have the lowest median mass, the MW and M31 globulars the highest, and the LMC young massive clusters and the M31 young clusters are in between. The young clusters in M31 show a range of structures. Most have the low concentration typical of MW open clusters, but there are a few which have high concentrations. We expect that most of these young clusters will be disrupted in the next Gyr or so; however, some of the more massive and concentrated of the young clusters will likely survive for longer. The spatial distribution of the young clusters is well correlated with the star-forming regions as mapped out by mid-IR emission. A kinematic analysis likewise confirms the spatial association of the young clusters with the star-forming young disk in M31.