Galaxy number counts – VI. An H-band survey of the Herschel Deep Field

Abstract

We present H-band infrared (IR) galaxy data to a 3σ limit of H ~ 22.9 and optical–IR colours of galaxies on the William Herschel Deep Field (WHDF). These data were taken from a 7 × 7 arcmin^2 area observed for 14 h with the Ω Prime camera on the 3.5-m Calar Alto telescope. We also present counts derived from the Hubble Deep Field-South (HDF-S) NICMOS camera to the limit of H ~ 29 mag over a 0.95 × 0.95 arcmin^2 area. Following previous papers, we derive H-band number counts, colour–magnitude diagrams and colour histograms for the whole H-selected sample. We review our Pure Luminosity Evolution (PLE) galaxy count models based on the spectral synthesis models of Bruzual & Charlot. We find that our previously assumed forms for the luminosity function (LF) agree well with those recently derived from 2dF Galaxy Redshift Survey (2dFGRS)/Two-Micron All-Sky Survey (2MASS) at B and K, except that the 2dFGRS K LF has an unexpectedly flat slope which, if correct, could affect our interpretation of the faintest H and K counts. We find that these PLE models give an excellent fit to the WHDF H-band count data to H < 22.5 and HDF count data to H < 28. However, if we use the flat 2dFGRS/2MASS near-infrared (NIR) LF, then the predicted count is too flat at H > 21. We confirm that PLE models that assume a Salpeter initial mass function (IMF) for early-type galaxies overestimate the average galaxy redshift in K < 20 galaxy redshift surveys. Models that assume a steep x = 3 IMF continue to give better agreement with the N(z) data than even models based on a Scalo IMF, 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. But this feature may simply reflect a larger scatter in optical–IR colours than in the optical 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 ongoing star formation in an intermediate subpopulation 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 subclass which is simultaneously redder in IR bands and bluer in the bluer bands than the classical, passive early-type galaxy; this subclass appears at relatively low redshifts and may constitute an intermediate-age, early-type population. Finally, we have also detected a candidate z > 1 galaxy cluster using our panoramic H-band observations of the WHDF.

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