M31 Globular Clusters in the [ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] Archive. I. Cluster Detection and Completeness

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Abstract
Globular clusters at the distance of M31 have apparent angular sizes of a few arcseconds. While many M31 globular clusters have been detected and studied from ground-based images, the high spatial resolution of the Hubble Space Telescope (HST) allows much more robust detection and characterization of star cluster properties. We present the results of a search of 157 HST Wide Field Planetary Camera 2 images of M31. We found 82 previously cataloged globular cluster candidates, as well as 32 new globular cluster candidates and 20 open cluster candidates. We present images of the new candidates and photometry for all clusters. We assess existing cluster catalogs' completeness and use the results to estimate the total number of globular clusters in M31 as 460 ± 70. The specific frequency is SN = 1.2 ± 0.2 and the mass specific frequency T = 2.4 ± 0.4; these values are at the upper end of the range seen for spiral galaxies.
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