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So what are the most important scientific results from HDF so far? I only have sufficient time to sketch what I consider to be the five most significant results whilst giving due credit to earlier workers who established the foundations of what has emerged. I have to be a bit selective so I present this as a personal account rather than a comprehensive review.

3.1. The flattening of the count slope N(m)

The quest to take deeper images of the sky motivated the HDF at its most basic level but this quest has a long and distinguished history. Sandage (1995) discusses the classical work, whereas Koo & Kron (1992) and Ellis (1997) review the more recent observations. The modern era begins with the commissioning of the wide-field prime focii on our national 4-m telescopes in the 1970's. Combining fine-grain emulsions and automatic measuring machines, Kron, Kibblewhite and Tyson laid the foundations of image processing of faint galaxy images (Kron 1978, Peterson et al. 1979, Tyson & Jarvis 1979). In an era when the photographic plate is so often disregarded, it is salutary to note how much of our observational achievement was established from photographic plates. Only now, after 20 years, are giant CCD cameras rivaling the combined depth and field of view. Of particular note for this meeting is Koo's thesis (1981) where, in an early version of the HDF, four-colour photographic photometry was analyzed in the context of photometric redshifts to demonstrate enhanced star formation as a function of look-back time.

The importance of pushing deeper was obviously recognized. Tyson (1988) was the first to attempt ambitious long CCD exposures developing, with Jarvis, Valdes, Seitzer and others, the relevant observing and processing technologies. The steep blue count slope first found by Kron (1978) seemed to continue and many of us imagined we might soon hit the confusion limit. The suggestion that the count slope flattened below the Olber's limit, dlogN / dm = 0.4, came tentatively from Lilly et al. (1991) and later, with greater confidence, from the very deep exposures taken by Metcalfe et al. (1995) including the Herschel Deep Field, a friendly ground-based rival of the HDF. (1)

Figure 1

Figure 1. Differential number magnitude counts in the B-band (including those derived from the HDF) and K-band (including recent Keck determinations using the Keck telescope). The K counts have been offset by +1 dex for clarity. The two power law slopes (dashed lines) indicate the point beyond which the integrated night sky brightness begins to converge. Solid lines refer to no evolution predictions in the Einstein-de Sitter case (see Ellis 1997 for further details).

The flattening was dramatically confirmed in the HDF counts and overcounting multi-component galaxies as separate units (Colley et al. 1996) would make the faint slopes even flatter. The bulk of the received extragalactic light must therefore come from the point of inflexion - an apparent magnitude (B appeq 25) within spectroscopic reach where the mean redshift is modest (z appeq 1). Importantly, the same effect has been seen in the infrared at K appeq 18 (Gardner et al. 1993, Moustakas et al. 1997). Very few of the faint K-selected galaxies are not visible in the optical suggesting the result is a fundamental feature of galactic history.

1I should add, somewhat topically, that Tom Shanks looked like doing marvelously well out of the recent British election as his Herschel Deep Field blue galaxies were adopted as a slogan by the Conservative party whose emblem was projected on them in a national newspaper. However, the colour of the British sky has since switched dramatically from blue to red! Back.

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