A Color Analysis of the NICMOS Parallel Image Archive

Abstract

We present a photometric analysis of all high Galactic latitude (|b|>20^deg) broadband parallel images taken by the Near-Infrared Camera and Multi-Object Spectrometer instrument of the Hubble Space Telescope during its initial lifetime in HST Cycle 7. These images, taken through the F110W and F160W filters, reach a mean 3 σ limiting magnitude of approximately 22 in both bands, and cover a total area of approximately 92 arcmin². The reddest of the 358 galaxies detected have F110W-F160W colors and F160W magnitudes consistent with being a combination of both dusty star-forming and evolved early-type galaxies at 1<~z<~2. The surface density of these galaxies is comparable to that of the population of extremely red objects (EROs) discovered in ground-based surveys (~100 deg^-2), suggesting that EROs also represent a combination of both galaxy types in this redshift range. Roughly 10% of the detected galaxies appear to be blue compact dwarf galaxies at z<~1, a result consistent with studies of the HST Medium Deep Survey fields. The surface density of these objects down to a magnitude of 22 in F160W is ~300 deg^-2. None of the 631 point sources detected have F110W-F160W colors matching those expected for quasi-stellar objects (QSOs) whose continua have been significantly reddened by internal dust. Our data limit the surface density of such QSOs to be <~50 deg^-2 down to the mean limiting magnitudes of the sample images. Since the surface density of QSOs detected on the basis of ultraviolet excess in optical surveys to comparable depth is ~100 deg^-2, this argues against the suggestion that dust-reddened QSOs compose the undetected majority of the QSO population. The F110W-F160W color can also be used to identify unreddened QSOs at z>~8, but we find no such candidates. This is consistent with the evidence that QSO space density declines sharply at z>~5.