Deep GALEX Observations of the Coma Cluster: Source Catalog and Galaxy
  Counts
  We present a source catalog from deep 26 ks GALEX observations of the Coma
cluster in the far-UV (FUV; 1530 A) and near-UV (NUV; 2310 A) wavebands. The
observed field is centered 0.9 deg (1.6 Mpc) south-west of the Coma core, and
has full optical photometric coverage with SDSS. The catalog consists of 9700
galaxies with GALEX and SDSS photometry, including 242
spectroscopically-confirmed Coma member galaxies that range from giant spirals
and elliptical galaxies to dwarf irregular and early-type galaxies. The full
multi-wavelength catalog (cluster plus background galaxies) is ~80% complete to
NUV=23 and FUV=23.5, and has a limiting depth at NUV=24.5 and FUV=25.0 which
corresponds to a star formation rate of ~0.001 Msun/yr at the distance of Coma.
Our deep GALEX observations required a two-fold approach to generating a source
catalog: we used a Bayesian deblending algorithm to measure faint and compact
sources (using SDSS coordinates as a position prior), and relied on the GALEX
pipeline catalog for bright/extended objects. We performed simulations to
assess the influence that systematic effects (e.g. object blends, source
confusion, Eddington Bias) have on source detection and photometry when using
both methods. The Bayesian deblending method roughly doubles the number of
source detections and provides reliable photometry to a few magnitudes deeper
than the GALEX pipeline catalog. This method is also free from source confusion
over the UV magnitude range studied here; conversely, we estimate that the
GALEX pipeline catalogs are confusion limited at magnitudes fainter than NUV~23
and FUV~24. We have measured the total UV galaxy counts using our catalog and
report a ~50% excess of counts across FUV=22-23.5 and NUV=21.5-23 relative to
previous GALEX measurements, which is not attributed to cluster member
galaxies. Our galaxy counts are a better match to deeper UV counts measured
with HST.
