Ultraviolet Galaxy Counts from Space Telescope Imaging Spectrograph Observations of the Hubble Deep Fields

Abstract

We present galaxy counts in the near-ultraviolet (NUV; 2365 Å) and far-ultraviolet (FUV; 1595 Å) obtained from Space Telescope Imaging Spectrograph observations of portions of the Hubble Deep Field-North (HDF-N), the Hubble Deep Field-South (HDF-S), and a parallel field near the HDF-N. All three fields have deep (AB > 29) optical imaging, and we determine magnitudes by taking the ultraviolet flux detected within the limiting optical isophote. An analysis of the UV-optical colors of detected objects, combined with a visual inspection of the UV images, indicates that there are no detectable objects in the UV images that are not also detected in the optical. We determine our detection area and completeness as a function of magnitude with a simulated distribution of galaxies based on the HDF-N Wide Field Planetary Camera 2 (WFPC2) V + I image. The galaxy counts reach to NUV_AB = 29 and FUV_AB = 30, which is 1 mag fainter than the HDF-N WFPC2/F300W counts and 7 mag fainter than balloon-based counts. We integrate our measurements in order to present the extragalactic background radiation coming from resolved objects. The NUV galaxy counts show a flattening or turnover beginning at about NUV_AB = 26.0, which is not predicted either by no-evolution models based on a local luminosity function with a steep faint-end slope or by a semianalytic model in which starbursts are caused by major mergers. The FUV counts also show a flat slope. We argue that the flat slopes could be caused by a short duty cycle for star formation, additional starbursts triggered by minor mergers, and an extended quiescent phase between starburst episodes.