Colors andK‐Band Counts of Extremely Faint Field Galaxies,

We combine deep K -band (W. M. Keck Telescope) with V - and I -band (New Technology Telescope) observations of two "blank" high Galactic latitude fields, surveying a total of ~2 arcmin 2 . The K -band number-magnitude counts continue to rise above K ≈ 22 mag, reaching surface densities of few × 10 5 deg -2 . The slope for the galaxy counts is approximately d log ( N )/ d mag deg -2 = 0.23 ± 0.02 over the range 18-23 mag. While this slope is consistent with other recent deep K -band surveys, there is a definite scatter in the normalizations by about a factor of 2. In particular, our normalization is ~2 × greater than the galaxy counts reported by Djorgovski et al. in 1995. Optical-near-infrared color-magnitude and color-color diagrams for all objects detected in the V + I + K image are plotted and discussed in the context of grids of Bruzual-Charlot isochrone synthesis galaxy evolutionary models. The colors of most of the observed galaxies are consistent with a population drawn from a broad redshift distribution. A few galaxies at K ≈ 19-20 are red in both colors ( V - I ≳ 3; I - K ≳ 2, consistent with being early-type galaxies having undergone a burst of star formation at z ≳ 5 and viewed at z ~ 1. At K ≳ 20, we find several (approximately eight) "red outlier" galaxies with I - K ≳ 4 and V - I ≲ 2.5, whose colors are difficult to mimic by a single evolving or nonevolving stellar population at any redshift unless they either have quite low metallicity or are highly reddened. We compare the data against the evolutionary tracks of second-burst ellipticals and against a grid of models that does not constrain galaxy ages to a particular formation redshift. The red outliers' surface density is several per square arcminute, which is so high that they are probably common objects of low luminosity L < L * . Whether these are low-metallicity, dusty dwarf galaxies, or old galaxies at high redshift, they are curious and merit spectroscopic follow-up.