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

G. Rodighiero et al., Spiral and irregular galaxies in the Hubble Deep Field North - Comparison with early types and implications for the global SFR density, ASTRON ASTR, 364(2), 2000, pp. 517-531
Citations number
Categorie Soggetti
Space Sciences
Journal title
ISSN journal
0004-6361 → ACNP
Year of publication
517 - 531
SICI code
We analyze a morphologically-selected complete sample of 52 late-type (spir al and irregular) galaxies in the Hubble Deep Field North with total K-magn itudes brighter than K=20.47 and typical redshifts z similar to 0.5 to 1.4. This sample exploits in particular the ultimate imaging quality achieved b y HST in this field, allowing us to clearly disentangle the early- from lat e-type galaxy morphologies, based on accurate profiles of the surface brigh tness distributions. Our purpose was to investigate systematic differences between the two classes, as for colours, redshift distributions and ages of the dominant stellar populations. Our analysis makes also use of an exhaus tive set of modellistic spectra accounting for a variety of physical and ge ometrical situations for the stellar populations, the dusty Interstellar Me dium (ISM), and relative assemblies. The high photometric quality and wide spectral coverage allow to estimate accurate photometric redshifts for 16 o bjects lacking a spectroscopic measurement, and allow a careful evaluation of all systematics of the selection [e.g. that due to the surface-brightnes s limit]. 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 fu nction 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 com plication in spectro-photometric modelling introduced by dust-extinction in the gas-rich systems prevents us to reach conclusive results on the single sources (only future long-wavelength IR observations will allow to break t he age/extinction degeneracy). However, we find that an integrated quantity like the comoving star-formation rate density as a function of redshift Ps i (z) is much less affected by these uncertainties: by combining this with the previously studied early-type galaxy sample, we find a shallower depend ence of Psi (z) on z between z = 0.2 and z = 1.5 than found by Lilly et al. (1995). Our present results, based on a careful modelling of the UV-optica l-NIR SED of a complete galaxy sample - exploiting the observed time-depend ent baryonic mass function in stars as a constraint and attempting a first- order correction for dust extinction - support a revision of the Lilly-Mada u plot at low-redshifts for both UV- and K-band selected samples, as sugges ted by independent authors (Cowie et al. 1999).