Galaxy number counts--IV. Surveying the Herschel Deep Field in the near-infrared

Abstract

We present results from two new near-infrared imaging surveys. One survey covers 47.2 arcmin2 to K(3σ)=20 mag whilst a second, deeper catalogue covers a subarea of 1.8 arcmin2 to K(3σ)=22.75 mag. Over the entire area we have extremely deep optical photometry in four bandpasses (UBRI), allowing us to track the colour evolution of galaxies to very faint magnitude limits. Our K-band number counts are consistent with the predictions of non-evolving models with 0q00.5. The K-selected (B−K) galaxy colour distributions from our surveys move sharply bluewards fainter than K∼20. At brighter magnitudes (K 4 and 19 4 galaxies at K∼20 are lower than the predictions of passively evolving models or PLE models with a low level of continuing star formation, suggesting that at least part of the larger deficiency observed in (B−K) at K∼20 may be caused by star formation rather than dynamical merging. As we and others have noted, the number redshift distribution at 18 1. Dynamical merging is one possible solution to reduce the numbers of these galaxies but (as we have suggested previously) a dwarf-dominated initial mass function for early-type galaxies could offer an alternative explanation; we show here that such a model reproduces well the optical–infrared colour distributions and K-band galaxy counts.