An H-band survey of the Herschel Deep Field

We present H-band infra-red galaxy data to a 3 sigma limit of H=22.9 and optical-infra-red colours of galaxies on the William Herschel Deep Field. These data were taken from a 7'x 7' area observed for 14 hours with the Omega Prime camera on the 3.5-m Calar Alto telescope. We derive H-band number counts, colour-magnitude diagrams and colour histograms. We review our Pure Luminosity Evolution galaxy count models based on the spectral synthesis models of Bruzual & Charlot. These PLE models give an excellent fit to the our H band count data to H<22.5 and HDF count data to H<28. However, PLE models with a Salpeter IMF for early-type galaxies overestimate the average galaxy redshift in K<20 galaxy redshift surveys. Models that assume a steep x=3 IMF give better agreement data, although they do show an unobserved peak in B-H and I-H colour distributions at faint H magnitudes corresponding to z>1 early-type galaxies. This feature may simply reflect a larger scatter in optical-infra-red colours than in the B-R colour of early-type galaxies at this redshift. This scatter is obvious in optical-IR colour-colour diagrams and may be explained by on-going star-formation in an intermediate sub-population of early-type galaxies. The numbers of EROs detected are a factor of 2-3 lower than predicted by the early-type models that assume the Salpeter IMF and in better agreement with those that assume the x=3 IMF. The tight sequence of early-type galaxies also shows a sub-class which is simultaneously redder in infrared bands and bluer in the bluer bands than the classical, passive early-type galaxy; this sub-class appears at relatively low redshifts and may constitute an intermediate age, early-type population.