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Astron. Astrophys. 364, 517-531 (2000)

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1. Introduction

Cosmogonic models, in particular those based on the hierarchical clustering of cold dark matter halos, now including detailed physical descriptions of gas cooling, star-formation and feed-back processes in the baryonic component, make specific predictions about the evolutionary history of galaxy populations as a function of their morphology.

Basically, in the hierarchical scheme, forming galaxies acquire angular momentum from tidal interactions with the surrounding structure and then dissipate and collapse preferentially along the rotation vector and tend to form flattened rotational-supported structures (disk galaxies). There are indeed indications of a substantial population of large structures of this kind up to the highest redshifts from absorption-line studies in the distant quasar spectra (e.g. Wolfe A.M. 1999). The pressure-supported stellar bulges dominating E/S0 galaxies, in this scheme, would originate from the violent relaxation and dynamical evolution following strong interactions and mergers of primordial disk galaxies. At the zero-th order, these models predict that spheroidal galaxies are assembled somewhat later than spirals, although their stellar populations might not differ much if the merger occurs among gas-poor systems. One such extreme case has been discussed by Kauffmann & Charlot (1998) based on the [FORMULA] standard CDM cosmology, predicting a substantial dearth of spheroidal galaxies already by z=1, most of them being formed at lower z.

More recently it has been pointed out that, within this scheme, the morphological appearance of a galaxy may repeatedly change with cosmic time not only from a late- to an early-type following a merging event, but also from a spheroidal to a disk configuration following the acquisition of new infalling gas from the environment (e.g. Ellis R.S. 1997). This may reflect in long-wavelength (V to K) colour distributions un-distinguishable between early- and late-types, while shorter-wavelength (U to V) colours would be dominated by the on-going SF in disks.

An opposite pattern is contemplated by the "traditional" models of galaxy formation, assuming that massive galaxies, in particular elliptical and S0's, originated first at high redshifts as single entities from rapid homologous collapse of primordial gas. Gas-rich systems, in this view, form instead more quiescently from progressive inflow of gas into the dark matter halos during most of the Hubble time. This formation scenario then predicts a marked differentiation in colours and ages for the stellar populations of the two classes of galaxies, late-type galaxies containing much younger stellar populations on average than the early-types. Also a substantial population of massive spheroids would be expected to be visible at [FORMULA] in this case.

The ultra-deep integrations at various wavelengths performed by the Hubble Space Telescope in the Hubble Deep Field (Williams et al. 1996; we consider here only the survey in the North area) offer an extremely valuable dataset to study morphological properties of high-redshift galaxies. Furthermore, the very accurate photometry achievable in such deep images allows accurate estimates of the photometric redshifts for vast numbers of faint galaxies in the field.

We have recently exploited these data to study the colours, masses, age distributions, and the star-formation history of a sample of elliptical-S0 galaxies (Franceschini et al. 1998, FA98 hereafter). The basic result was to find colours indicative of wide ranges of ages for the stellar populations and a remarkable absence of objects at [FORMULA], both facts telling against the predictions of the "traditional" monolithic formation scenario.

As a natural complement, we present in this paper an analysis of late-type and irregular galaxies in the HDF. Similarly to what we did there, our primary selection is in the K-band, obtained from a deep KPNO image, to minimize the biases in the sample due to the effects of K- and evolutionary corrections. The completion of our previous analysis of E/S0 to account for the complementary set of late-type systems is also needed for a global evaluation of the star-formation history as a function of redshift. The advantage of our approach over previous attempts (Lilly et al. 1996; Cowie et al. 1999) is in our careful treatment of dust extinction from a detailed fitting of the UV-optical-NIR spectral energy distributions (SED). In addition, the detailed knowledge of the near-IR (NIR) spectrum for sources at the relevant redshifts is informative on the baryonic mass function in stars, which provides an essential constraint on the cumulative star-formation rate as a function of time.

In Sect. 2 we discuss the selection scheme and photometric corrections used to construct a complete K-band flux limited sample of late-type galaxies. In Sect. 3 we describe the population synthesis code that we used to model the optical-NIR SEDs of our sample objects, taking into full account the effects of a dusty interstellar medium in the galaxy spectra. Our main results are then reported in Sect. 4, where we perform detailed analyses of the space distributions, colours and ages of the stellar populations of field spiral and irregular galaxies, compared with ellipticals and S0. We discuss the difficulties inherent in the spectral modelling of gas-rich systems affected by dust extinction. We finally attempt to construct the global star formation histories of field galaxies (E/S0+spiral/irregulars). In Sect. 5 we summarize our main conclusions.

We anticipate that the results of the present analysis, based on a survey over a very small sky field, are to be considered as only tentative, until larger areas will be surveyed to similarly deep limits.

We adopt [FORMULA] throughout the paper. For consistency with FA98 the analysis is made assuming [FORMULA], and zero cosmological constant [FORMULA].

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001