A Color Analysis of the NICMOS Parallel Image Archive

We present a photometric analysis of all high Galactic lattitude (|b| > 20d)broad-band parallel images taken by the Near Infrared Camera and Multi-ObjectSpectrometer (NICMOS) instrument during its initial lifetime in HST Cycle 7.These images, taken through the F110W and F160W filters, reach a mean 3-sigmalimiting magnitude of approximately 22 in both bands, and cover a total area ofapproximately 92 square arcminutes. The reddest of the 358 galaxies detectedhave F110W-F160W colors and F160W magnitudes consistent with being acombination 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 thepopulation of Extremely Red Objects (EROs) discovered in ground-based surveys(~ 100 deg-2), suggesting that EROs also represent a combination of both galaxytypes in this redshift range. Roughly 10% of the detected galaxies appear to beblue compact dwarf galaxies at z < 1, a result consistent with studies of theHST Medium Deep Survey Fields. The surface density of these objects down tomagnitude of 22 in F160W is ~ 300 deg-2. None of the 681 point sources detectedhave F110W-F160W colors matching those expected for QSOs whose continua havebeen significantly reddened by internal dust. Our data limit the surfacedensity of such QSOs to be < 50 deg-2 down the mean limiting magnitudes of thesample images. Since the surface density of QSOs detected on the basis ofultraviolet excess in optical surveys to comparable depth is ~ 100 deg-2, thisargues against the suggestion that dust-reddened QSOs comprise the undetectedmajority of the QSO population. The F110W-F160W color can also be used toidentify unreddened QSOs at z > 8, but we find no such candidates. This isconsistent with the evidence that QSO space density declines sharply at z > 5.Comment: To appear in the September 2000 Astronomical Journa