The Hubble Deep Field South: Project Description

In December, 1995 the Hubble Space Telescope pointed at a undistinguished high-galactic latitude patch of sky and observed for 10 straight days. The result was the deepest optical image of the sky yet obtained, allowing sources as faint as V = 30 in four bandpasses spanning the near-UV to the near-IR (Williams et al. 1996). The data were released to the community within one month of the observations and have been used in a wide variety of projects and publications, ranging from studies of the star-formation rate as a function of redshift, to studies of faint M dwarfs in the Galactic halo.

A second Hubble Deep Field campaign will be carried out with HST in September 28 to October 10 1998 (give or take a day or two). The data are slated for release on or about Nov. 23, 1998. The observations will be much the same as the original HDF, with several important differences:

• The field is located in the southern Continuous Viewing Zone (J2000 coordinates 22:32:56.2, -60:33:02.7)
• A moderate redshift quasar of z ~ 2.24, identified by Boyle, Hewett, Weymann and colleagues, will be placed in the STIS field for both imaging and spectroscopy so that correlations between quasar absorption redshifts and the redshifts of galaxies in the fields may be determined. Information on the QSO can be found in papers by Sealey et al. (1998) and by Savaglio (1998).
• Simultaneous, parallel observations will be made with the three HST instruments WFPC2, NICMOS, and STIS of separate, neighboring fields. The STIS and NICMOS data will be a significant enhancement over what was possible for HDF-N.
• Many of the "followup" observations will be carried out in advance. Information on these observations can be found on the HDF-S Clearinghouse web page.

The rationale for undertaking a second deep field follows from the wealth of information that has come out of HDF-N, and from the desire to provide a point of focus for similar studies of the distant universe from southern-hemisphere facilities. Choosing a field in the CVZ maximizes the efficiency of HST for such projects. The wide public access to the HDF-N data stimulated extensive followup observations across the electromagntic spectrum, both from major ground-based observatories and from other satellites. A similar level of effort is anticipated for HDF-S. We will maintain a ``clearinghouse'' for supporting observations on the HDF-S web page.

Figure 1 shows the position of the HDF-S overlayed on a deep ground-based image. As was the case for HDF-N, approximately 150 consecutive orbits will be devoted to a single telescope pointing, with additional flanking field images to be obtained surrounding the deep WFPC2, STIS, and NICMOS fields. Both raw and reduced data will be made publicly available in the HST Data Archive approximately 4-6 weeks after the end of the observing campaign.

Fig 1. Ground-based view of the HDF-S area. The image is a 3000s R-band exposure from the CTIO 4m telescope (courtesy of Alistair Walker). The WFPC-2 region is to the West, the STIS region is to the East and NICMOS region is to the South. North is up and East is left. The WFPC2 field is centered at J2000 coordinates 22:32:56.2 -60:33:02.7. The positions of the fields are subject to change at the level of a few arcsec. The NICMOS positions on this figure are only approximate at the ~ 10 arcsec level.

The details of the observing strategy for WFPC2, NICMOS, and STIS are being finalized as of July 1, 1998. The current plans are discussed below; we welcome comments and suggestions on any aspect of the observations.

In CVZ observations, scattered earth light increases the sky background in certain bandpasses on the day side of the orbit. Observations for all the instruments should be tailored to make optimal use of ``bright'' and ``dark'' time. This drives much of the strategy outlined below.

WFPC-2 Strategy

Table 1. WFPC-2 Filters, exposure times and limiting AB magnitudes

NICMOS Strategy

Nearly all of the dark-time orbits will be used for broad band imaging with the F110W and F160W filters (J and H--bands, approximately), giving approximately 48 hours of observing in each band. Limiting magnitudes are given in Table 2. It appears that scattered earthlight during the ``bright'' portions of CVZ orbits affects NICMOS imaging, and various options (including K or narrow band imaging) are being considered to take advantage of these periods. Slitless grism spectroscopy is another option during dark time, but its utility may be compromised by the fact that we cannot easily obtain data at multiple position angles during the HDF-S campaign.

Table 2. NICMOS Filters, exposure times and limiting AB magnitudes

STIS Strategy

The flowchart (Fig. 2) shows the STIS observing plan schematically. This plan puts most of the MAMA observing time into high-resolution spectroscopy. This will provide high resolution (10 km/s) QSO absorption-line data in the region from 2650-3200 Angstroms, and low resolution (250 km/s) spectra from 1600 to 1200 Angstroms. The region from 1600-2650 is suppressed by a Lyman continuum absorption-line system at z ~ 1.9.

MAMA imaging will provide UV morphologies of galaxies near the QSO, and a measurement of the Lyman break for galaxies as faint as B_AB = 27 at redshifts z ~ 1.7 and 0.5.

The STIS CCD will images will provide a deep view of galaxies immediately surrounding the QSO. The images will be significantly deeper than those with the WFPC-2, and will have a higher spatial resolution. Color information will be a bit cruder than for the WFPC-2 images, but in the portion of the field with MAMA UV imaging and long-pass filter imaging, there will be four bandpasses available for photometric redshifts, and the inclusion of the UV will provide greater accuracy for galaxies in the redshift range 0.5-2.5.

Bright time for the CCD will be used to obtain a QSO spectrum at a resolution of 30 km/s and with S/N ~50 per resolution element.

Table 3. STIS Modes and exposure times

Flanking Fields

Tentative HDF-S flanking field arrangement. The red is the main HDF-S WFPC2 field. The green are flanking fields to be taken with the F814W filter. The blue are the STIS CCD fields associated with the flanking fields, and the yellow are the NIC3 flanking-field parallels, which will be taken with the F160W filter. The main NIC3 field will be observed for 9 orbits with the STIS CCD (no filter). The right-most WFPC2 flanking field is the parallel frame associated with that exposure.

Test Observations

The Space Telescope Science Institute home page.

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