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One of the major goals of observational cosmology is to understand the processes involved in galaxy formation and evolution. A full theoretical treatment of this problem requires an understanding of the primordial density fluctuations, of the formation of dark matter halos, of the dissipation that occurs in the cooling gas, of the processes involved in star formation, of the feedback between stellar explosions and the interstellar medium, of galaxy mergers, and of the interactions of galaxies with intergalactic gas.

Most current models for the formation of structure in the universe assume that dark matter halos build up hierarchically, with the assembly being controled by the cosmological parameters, the power spectrum of the density fluctuations, and the nature of the dark matter itself. The build-up of the stellar mass within galaxies is a secondary process, in which gaseous dissipation, the more intricate physics of star formation, and feedback, all play a role.

Since galaxy formation involves non-linear processes on many scales, the problem has been addressed theoretically, mainly using large-scale N-body hydrodynamic simulations (e.g., Pearce et al. 2001). On the observational side, HST, the Chandra Observatory and the Keck and VLT telescopes have allowed for an unprecedented view of the high-redshift universe, probing galaxies from infancy to old age.

Short of the awsome pillars of dust and molecular gas revealed by the HST images of the "Eagle Nebula," the best known images that Hubble has produced are those of the "Hubble Deep Fields" (HDFs). In fact, the first HDF and the follow-up observations of the same field by other observatories probably represent the most concentrated research effort ever in astronomy into what was previously a blank piece of the sky!

Observations taken shortly after the first servicing mission, which restored the HST optics capabilities, demonstrated that HST could resolve distant galaxies spectacularly well. In particular, observations of the relatively distant clusters CL 0939+4713 (Dressler et al. 1994) and the cluster around the radio galaxy 3C324 (Dickinson 1995) revealed faint galaxies of small angular sizes, the morphologies of which were entirely inaccessible from the ground.

The idea of the Hubble Deep Field North - ten days of one continuous observation of a field 2.6 arcminutes on the side in December 1995 - was conceived by the then Director of the Space Telescope Science Institute, Bob Williams. Williams decided to use his Director's Discretionary Time on HST to produce the deepest image of the universe in optical/UV wavelengths.

The northern field itself (Fig. 23) was carefully selected so as to be in HST's northern continuous viewing zone (CVZ; without interference by Earth occultations), to be free of bright stars, nearby galaxies and radio sources, and to have relatively low Galactic extinction. The southern field (Fig. 24) was selected so as to include a quasi-stellar object (QSO) that could be used to study absorption systems along the line of sight. The HDF-N observations were taken in December 1995 and the HDF-S in October 1998.

Figure 23

Figure 23. Hubble Deep Field North, HST/WFPC2, December 1995. Credit: NASA, R. Williams (STScI) and the HDF Team.

Figure 24

Figure 24. Hubble Deep Field South, HST/WFPC2, October 1998. Credit: NASA, R. Williams (STScI) and the HDF Team.

The filter selection for the observations was driven partly by the desire for depth and color information (to identify high-redshift galaxies by their Lyman-break), and partly by practical considerations involving scattered light within the telescope. Accordingly, images were taken in four broad-band passes, spanning a wavelength range from about 2500 Å to 9000 Å.

In order to achieve a higher resolution than the detectors' pixel sizes, the pointing of the telescope was dithered (shifted slightly) during the observation, thus ensuring that images were recorded on different pixels. A "drizzling" (subpixel linear reconstruction) technique was developed by Fruchter and Hook (2002). Details on the observational techniques and a review of some of the results can be found in the excellent reviews by Ferguson (1998) and Ferguson, Dickinson, and Williams (2000).

As soon as the HDF-N image was obtained, it was obvious that a new era in astronomy has begun. The image revealed some 3000 galaxies of different colors, shapes, and sizes. A broad summary of key results can be found in Livio, Fall, and Madau (1998). Here I will concentrate only on two main topics, to which HST's contribution has been crucial: (i) galaxy sizes and morphologies (and their implications for galaxy formation and evolution), and (ii) the global, cosmic star-formation history. The dramatic discovery of a particular supernova in the HDF-N will be discussed in Section VIII.

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