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

Abstract

We have used the Keck 10m telescope to count objects as a function of image size in 2 high galactic latitude fields covering 1.5 arcmin^2 and reaching 50% completeness at K=24 and J=24.5 for stellar sources. Counts extend ~1 mag deeper in K than surveys with other telescopes; complement Keck surveys providing counts at comparable or shallower depths but not utilizing image structure; and extend by several magnitudes the J band counts from other surveys. We find the surface-density of objects at K=23 to be higher than previously found (~500,000/mag/deg^2), but at K<22 to be consistent with most other surveys in amplitude and slope (~0.36). J band counts have similar slope. J and K counts are in excess of our empirical no-evolution models for an open universe, and a factor of 2 higher than mild-evolution models at J and K ~ 23. The slope of the model counts is insensitive to geometry even in the near-infrared because the counts are dominated by low-luminosity (<0.1L*) objects at modest redshift (z<1) with small apparent sizes (r05<0.4", i.e. <4 kpc). The observed counts rise most steeply for these smaller objects, which dominate fainter than K=22.3 and J=23.3. However, the greatest excess relative to no-evolution models occurs for the apparently larger objects (median J-K~1.5). The size and colors of such objects correspond equally well to luminous (>0.1L*), galaxies at 1<z<4, or progressively more diffuse, low-luminosity (0.001-0.1L*) galaxies at z<1. We rule out the excess is from very low luminosity (<0.0001L*) red galaxies at z<0.25. There is a deficit of galaxies with red J-K colors corresponding to luminous, early-type galaxies at 1<z<3. Assuming the deficit is due to their appearance as blue galaxies, they account only for 10-30% of the excess of large, blue galaxies. [abridged]

accepted for publication in ApJ; 34 pages text, 9 tables, 10 figures (embedded); full resolution figures available at http://www.astro.wisc.edu/~mab/publications/pub.html