Near-Infrared Galaxy Counts to J and K ∼ 24 as a Function of Image Size

We have used the Keck 10 m telescope to count objects as a function of image size in two high Galactic latitude Ðelds covering 1.5 arcmin2 and reaching 50% completeness depths of K \ 24 and J \ 24.5 for stellar sources. Our counts extend D1 mag deeper in K than those of surveys with other telescopes; complement other Keck surveys in the K-band that provide counts at comparable or shallower depths but that have not utilized image structure; and extend by several magnitudes the J-band counts from brighter surveys using smaller telescopes that cover larger areas. We Ðnd the surface density of objects at K \ 23 to be higher than previously found (D500,000 mag~1 deg~2), but at K \ 22 to be consistent with other surveys. The slope of the K-band counts (d log A/dm\ 0.36) is similar to others near this depth as well as to our own J-band counts (0.35). Counts in the J- and K-bands are both in excess of our empirical no-evolution models for an open universe, with the largest excess observed in J. The counts are a factor of 2 higher than mild-evolution models at J and K D 23. The slope of the model counts is insensitive to the assumed geometry even in the near-infrared primarily because the model counts are dominated by low-luminosity (\0.1L *) objects at modest redshift (z\ 1) with small apparent sizes (half-light i.e., radii ¹ 0A.4, \4 kpc). The numbers of observed counts rise most steeply for these h 50~1 smaller objects, which dominate the counts fainter than K \ 22.3 and J \ 23.3. However, the greatest excess relative to no-evolution models occurs for the apparently larger objects, which have a median J[K of D1.5. At these depths, the size and colors of such objects correspond equally well to luminous (º0.1L *) blue galaxies at 1 \ z\ 4, or progressively more di†use, blue, low-luminosity (0.001È0.1L *) galaxies at z\ 1. The majority of these sources are too faint for spectroscopic measurement. Based on optical colors, we can rule out the possibility that the excess is caused by very low luminosity (\0.0001L *) red galaxies at z\ 0.25. We also Ðnd a deÐcit of galaxies with red J[K colors corresponding to nonevolving, luminous, early-type (i.e., ““red envelopeÏÏ) galaxies at 1\ z\ 3. Even assuming that the deÐcit is caused by their appearance as blue galaxies, they could account for only 10%È30% of the excess of large, blue galaxies. The nature and redshift distribution of excess large and small galaxy populations at K \ 24 and J \ 24.5 remain indeterminate from these data alone