Number Counts of GALEX Sources in Far-Ultraviolet (1530 Å) and Near-Ultraviolet (2310 Å) Bands

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Abstract
Number counts of galaxies in two Galaxy Evolution Explorer (GALEX) bands [far-UV (FUV: 1530 Å) and near-UV (NUV: 2310 Å); both in AB magnitudes] are reported. They provide for the first time in the literature homogeneously calibrated number counts of UV galaxies continuously covering a very wide range in UV magnitude (14-23.8). Both the FUV and NUV counts are inconsistent with a nonevolution model, whereas they are in good agreement with evolution models (essentially luminosity evolution) derived from the high-z UV luminosity functions of Arnouts et al. We find that the contribution from GALEX-detected galaxies to the UV background is 0.68 ± 0.10 nW m-2 sr-1 at 1530 Å and 0.99 ± 0.15 nW m-2 sr-1 at 2310 Å. These are 66% ± 9% and 44% ± 6% of the total contributions of galaxies to the UV background at 1530 Å (1.03 ± 0.15 nW m-2 sr-1) and at 2310 Å (2.25 ± 0.32 nW m-2 sr-1), respectively, as estimated using the evolution models. Galaxy counts and star counts in seven regions, each containing a few square degrees of GALEX coverage in an area of ~4-13 deg2, are compared with each other to study the region-by-region variance. This shows that for the galaxy counts, the cosmic variance is comparable to the net error due to other uncertainties. The star counts increase with decreasing absolute Galactic latitude |b|.
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