Understanding the processes governing the formation and the evolution of galaxies from the early stages is one of the major goals of the present day cosmology. Comprehensive numerical codes, linked with powerful CPU capabilities, make now possible to follow the evolution of primordial density fluctuations down to the stages preceding the formation of real galaxies (see Jenkins et al. 1997 for a recent review). White (1997) has also shown that the scaling relations expected after evolution of proto-galaxies of different morphologies seem to be in good agreement with those observed today, i.e. the Tully-Fisher (1977) and the Fundamental Plane (Dressler et al. 1987; Djorgovski & Davis 1987) relations.
Still, a number of key questions remain to be answered. We mention (among the others) the following ones: which is the parameter driving galaxies towards different morphologies? Are elliptical galaxies originated from gravitational collapse of primordial fluctuations (single burst of star formation whose duration depends on the onset of galactic winds) or are they the result of multiple merging (infall and recursive bursts)? Which is the influence of the internal and intergalactic absoption in determining the observed brightness and color profiles of high redshift galaxies?
Besides the global approach to the star formation history (Madau et al. 1997), answering these questions also requires detailed morphological and dynamical studies of galaxies in the early evolutionary stages. In particular, luminosity and geometrical profiles in different bands are needed to obtain unambiguous morphological classifications as well as reliable estimates of the galaxy sizes at intermediate and high redshifts.
Recent advances in the sensitivity and resolution of the observations both in imaging and spectroscopy with the Hubble Space Telescope (HST) and from the ground have greatly enlarged the horizon of morphological and dynamical studies for high redshift galaxies (Giavalisco et al. 1996; van Dokkum & Franx 1996; Shade et al. 1996; Shade et al. 1997; Oemler et al. 1997; Lowenthal et al. 1997; Pettini et al. 1997 and references therein). The Hubble Deep Field (HDF) is perhaps the most impressive example of these progresses (Ellis 1997).
The HDF project (Williams et al. 1996) has been realized using
observational procedures (dithering) and data handling
techniques (drizzling) aimed to improve not only the cosmetic of
the final images, but also the resolution performances. In particular,
after "drizzling'', the pixel size in arcseconds of the three WF
cameras turns out to be even better than that of the PC
(
vs. 
). Moreover,
the very long total exposure times of the final images allow to
overcome the main limitation of the WFPC data, that is the relatively
high surface brightness level usually reached for extended
sources. This makes the HDF frames particularly suited in order to
perform the detailed surface photometry of objects whose surface
luminosity slowly decreases outwards down to very faint levels, as in
the case of elliptical galaxies. We can rightfully assert that the HDF
represents the best opportunity we had since now to study the
morphology of elliptical galaxies at very high redshifts.
We have undertaken a long-term project aimed to produce the detailed surface photometry, in different bands, for a sample of early-type galaxies in the HDF. The main scientific goal of this project is to settle strong observational constraints to the theories modelling the evolution of elliptical galaxies from the early stages (e.g. Tantalo et al. 1996; Kauffmann & Charlot 1998; Chiosi et al. 1997 and references therein).
In this paper we present the surface photometry in the V606 band. In the forthcoming papers (Filippi & Fasano 1998; Fasano et al. 1998) we will present the surface photometry in the remaining 3 bands and discuss the improvements that the morphological information, together with the photometry in different optical and infrared bands, can produce in understanding the processes of galaxy formation and evolution.
In Sect. 2 we discuss the sample selection. Section 3 illustrates the techniques we used to extract the morphological information from the HDF frames. In Sect. 4 we present the results of the detailed surface photometry and discuss the global morphological properties of the sample.