Annu. Rev. Astron. Astrophys. 2000. 38: 667-715 Copyright © 2000 by Annual Reviews. All rights reserved

2. PART 1: OBSERVATIONS, MEASUREMENTS, AND PHENOMENOLOGY

The HDFs were carefully selected to be free of bright stars, radio sources, nearby galaxies, etc., and to have low Galactic extinction. The HDF-S selection criteria included finding a quasi-stellar object (QSO) that would be suitable for studies of absorption lines along the line of sight. Field selection was limited to the "continuous viewing zone" around = ± 62°, because these declinations allow HST to observe, at suitable orbit phases, without interference by earth occultations. Apart from these criteria, the HDFs are typical high-galactic-latitude fields; the statistics of field galaxies or faint Galactic stars should be free from a priori biases. The HDF-N observations were taken in December 1995 and the HDF-S in October 1998. Both were reduced and released for study within 6 weeks of the observations. Many groups followed suit and made data from follow-up observations publicly available through the world wide web.

Details of the HST observations are set out in [Williams et al. 1996], for HDF-N and in a series of papers for the southern field [Williams et al. 2000, Ferguson et al. 2000, Gardner et al. 2000, Casertano et al. 2000, Fruchter et al. 2000, Lucas et al. 2000]. The HDF-N observations primarily used the WFPC2 camera, whereas the southern observations also took parallel observations with the new instruments installed in 1997: the near-infrared camera and multi-object spectrograph (NICMOS) and the space telescope imaging spectrograph (STIS). The area of sky covered by the observations is small: 5.3 arcmin² in the case of WFPC2 and 0.7 arcmin² in the case of STIS and NICMOS for HDF-S. The WFPC2 field subtends about 4.6 Mpc at z ~ 3 (comoving, for _M, , _tot = 0.3, 0.7, 1.0). This angular size is small relative to scales relevant for large-scale structure.

The WFPC2 observing strategy was driven partly by the desire to identify high-redshift galaxies via the Lyman-break technique [Guhathakurta et al. 1990, Steidel et al. 1996], and partly by considerations involving scattered light within HST [Williams et al. 1996]. The images were taken in four very broad bandpasses (F300W, F450W, F606W, and F814W), spanning wavelengths from 2500 to 9000 Å. Although filter bandpasses and zeropoints are well calibrated, ¹ no standard photometric system has emerged for the HDF. In this review, we use the notation U₃₀₀, B₄₅₀, V₆₀₆ and I₈₁₄ to denote magnitudes in the HST passbands on the AB system [Oke1974]. On this system m(AB) = - 2.5 log f(nJy) + 31.4. Where we drop the subscript, magnitudes are typically on the Johnson-Cousins system, as defined by [Landolt 1973, Landolt 1983, Landolt 1992a, Landolt 1992b] and as calibrated for WFPC2 by [Holtzman et al. 1995]; however we have not attempted to homogenize the different color corrections and photometric zeropoints adopted by different authors.

During the observations, the telescope pointing direction was shifted ("dithered") frequently, so that the images fell on different detector pixels. The final images were thus nearly completely free of detector blemishes, and were sampled at significantly higher resolution than the original pixel sizes of the detectors. The technique of variable pixel linear reconstruction ("drizzling") [Fruchter & Hook 1997] was developed for the HDF and is now in widespread use.

Both HDF campaigns included a series of shallow "flanking field" observations surrounding the central WFPC2 field. These have been used extensively to support ground-based spectroscopic follow-up surveys. Since 1995 HDF-N has also been the target of additional HST observations. Very deep NICMOS imaging and spectroscopy were carried out on a small portion of the field by [Thompson et al. 1999]. [Dickinson et al. 2000b] took shallower NICMOS exposures to make a complete map of the WFPC2 field. A second epoch set of WFPC2 observations were obtained in 1997, 2 years after the initial HDF-N campaign [Gilliland et al. 1999]. STIS ultraviolet (UV) imaging of the field is in progress, and a third epoch with WFPC2 is scheduled.

Table 1 gives a rough indication of the different types of sources found in the HDF-N. Since 1995, the field has been imaged at wavelengths ranging from 10^-3 µm (Chandra) to 2 × 10⁵ µm (MERLIN and VLA), with varying sensitivities, angular resolutions, and fields of view. Not all of the data have been published or made public, but a large fraction have, and they form the basis for much of the work reviewed here. Links to the growing database of observations for HDF-S and HDF-N are maintained on the HDF world wide web sites at STScI.

¹ None of the analyses to date have corrected for the WFPC2 charge-transfer inefficiency (CTE; [Whitmore et al. 1999]). This is likely to be a small correction 5 % for the F450W, F606W, and F814W bands. However, because of the low background in the F300W band, the correction could be significantly larger. Back.