Hubble Boldly Goes: The Frontier Fields Program
The Hubble Frontier Fields (HFF) program is a Director’s Discretionary Time (DDT) campaign to observe 4–6 strongly lensing galaxy clusters and, in parallel, adjacent “blank” fields with the aim of detecting and characterizing high-redshift galaxies. The program will capitalize on the lensing capabilities of the clusters to probe redshifts beyond z ~ 10, reaching galaxies with intrinsic luminosities thirty times fainter than those detected in the HUDF12. The associated blank fields will have sensitivity comparable to the parallel fields of the HUDF09, and will expand the areal coverage by a factor of three. Together, these measurements will enable Hubble to provide a preliminary peek at the distant universe accessible to the James Webb Space Telescope.
Deep-field imaging designed to probe galaxy formation and evolution in the early universe is now well established as a key theme of the Hubble science program. Such was not always the case. Support for the original HDF DDT program was far from unanimous within the astronomical community. Some felt that the observations would offer little insight into galaxy formation, which many contemporary theoretical models predicted to be a sedate, gradualist process. Others worried that, so soon after the correction of Hubble’s vision at considerable tax-payer expense, there might be Congressional repercussions from investing ten days of Hubble time on a single project with a dubious prospect of returns. Those fears proved unfounded; the HDF revealed galaxy assembly to be an active, dynamic process—and provided iconic images that have now permeated the public consciousness. The original HDF together with Keck 10-m spectroscopy of the brightest 125 galaxies in the field validated the concept of photometric redshifts that have now succeeded in opening distant objects to analysis via multi-bandpass imaging.
The HDF lies at northern declination. Following its success, Hubble compiled matching observations of a field in the southern sky, the HDF-S. Since then, as each new servicing mission enhanced its scientific capabilities, Hubble has devoted considerable time and resources to deep-imaging programs that span a range of depth and areal coverage. Those programs include GOODS (the southern field within the CDF-S), the HUDF (within the CDF-S), COSMOS, HUDF09, CANDELS, UDF12, and 3D-HST. As with the HDF, each Hubble deep-imaging program has been supported by extensive complementary observations from other space observatories, notably Spitzer, Chandra and Herschel, together with substantial photometric and spectroscopic contributions from ground-based facilities. All told, over 3,000 orbits of Hubble observations (approximately one cycle) have been invested in major deep-field survey programs, with more than 800 orbits devoted to the 15-square-arcminute UDF alone. This wide panoply of multi-wavelength observation has revolutionized our view of the universe: z ~ 1 galaxies are demoted to “low-redshift” systems, the z ~ 2–3 peak in star formation now lies at “moderate redshifts,” and detection limits have been pushed through the era of reionization to the brink of cosmic dawn at z ~ 10–12.