The HDF-S dataset includes deep near-IR NICMOS images and the deepest observation ever made with the STIS 50CCD filterless imaging mode. The galaxy sample used here was extracted from version 1 of the HDF-S catalog on ftp://archive.stsci.edu/pub/hdf_south/version1/. At near-IR wavelengths (in the F110W, F160W, and F222M bandpasses, corresponding to the J, H, and K filters), it consists of 425 objects detected in the J + H image, over a field of 50" x 50". The 50CCD (corresponding roughly to a V + I filter) STIS catalog consists of 674 objects detected again over a field of the same size.
Figure 1. Left: Differential UBVIJHK galaxy counts as a function of AB magnitudes. The sources of the data points are given in the text. Note the decrease of the logarithmic slope d log N / dm at faint magnitudes. Right: Extragalactic background light per magnitude bin, i = 10-0.4(mAB+48.6) N(m), as a function of U (filled circles), B (open circles), V (filled pentagons), I (open squares), J (filled triangles), H (open triangles), and K (filled squares) magnitudes. For clarity, the BVIJHK measurements have been multiplied by a factor of 2, 6, 15, 50, 150, and 600, respectively .
Figure 1 shows the HDF-N and -S galaxy counts compiled directly from the catalogs, as a function of AB isophotal magnitudes in the UBVIJHK bandpasses for all galaxies with signal-to-noise ratio S/N > 3 within the band. No correction for detection completeness have been made. A compilation of existing HST and ground-based data is also shown , . All magnitudes have been corrected to the AB system, while the second order colour corrections for the differences in the filter effective wavelengths have not been applied to the ground-based data (for the typical colours of galaxies in the HDF these corrections are less than 0.1 mag). The HDF optical counts agree well with previous surveys, to within 20% in the magnitude range 22 < mAB < 26. One should note, however, that different algorithms used for ``growing'' the photometry beyond the outer isophotes of galaxies can significantly change the magnitude of faint galaxies. According to , roughly 50% of the flux from resolved galaxies with V > 23 mag lie outside the standard-sized apertures used by photometric packages. An extragalactic sky pedestal created by the overlapping wings of resolved galaxies may also contribute significantly to the sky level, and would be undetectable except by absolute surface photometry . Also, at faint magnitude levels, distant objects which are brighter than the nominal depth of the catalog may be missed due to the (1 + z)4 dimming factor. All these systematic errors are inherent in HST faint-galaxy photometry; as a result, our estimate of the integrated flux from resolved galaxies will typically be too low, and must be strictly considered as a lower limit.