Spiral and irregular galaxies in the Hubble Deep Field North - Comparison with early types and implications for the global SFR density

Abstract

We analyze a morphologically-selected complete sample of 52 late-type (spiral and irregular) galaxies in the Hubble Deep Field North with total K-magnitudes brighter than K=20.47. This sample exploits in particular the ultimate imaging quality achieved by HST in this field, allowing us to clearly disentangle the galaxy morphologies, based on accurate profiles of the surface brightness distributions. Our purpose was to investigate systematic differences between the two classes, as for colours, redshift distributions and ages of the dominant stellar populations. This sample appears to miss significantly galaxies above z=1.4 (in a similar way as an early-type galaxy sample previously studied by us), a fact which may be explained as a global decline of the underlying mass function for galaxies at these high redshifts. Differences between early and late-types are apparent -particularly in the colour distributions and the evolutionary star-formation (SF) rates per unit volume-, although the complication in spectro-photometric modelling introduced by dust-extinction in the gas-rich systems prevents us to reach conclusive results on the single sources. However, we find that an integrated quantity like the comoving star-formation rate density (SFR(t)) as a function of redshift is much less affected by these uncertainties: by combining this with the previously studied early-type galaxy sample, we find a shallower dependence of SFR(t) on z between z=0.2 and z=1.5 than found by Lilly et al. (1995).