Hubble Boldly Goes: The Frontier Fields Program

The HFF program

The HDFI SWG recommended that the Institute director should pursue a joint strategy of deep imaging on six strongly lensing galaxy clusters, together with parallel observations of adjacent “blank” fields (Figure 1). Quoting from the report,

[The HFF program] combines proven techniques for studying high-redshift galaxies in blank fields with the potentially revolutionary use of natural gravitational telescopes to exploit their magnification of the faintest galaxies in the distant universe. The blank fields will increase three-fold the area covered at comparable depth by the HUDF09 and its parallel fields, tracing the history of star formation and the growth of stellar mass with improved statistics and reduced cosmic variance. In the cluster fields, HST can reach high redshift galaxies as faint intrinsically as those that JWST can detect in blank fields, even down to the dwarf galaxies thought to be the progenitors of typical L_* galaxies in the modern universe.

The proposed observations reach magnitude limits comparable to the HUDF09 parallel fields, matching the second deepest images that we currently have for the high-redshift universe. The blank-field observations will therefore provide important constraints on cosmic variance at redshifts z ~ 6–8. Observations in the field have pushed to galaxies at redshifts z ~ 8–10. Nevertheless, it is important to recognize that those galaxies represent the tip of the luminosity function at those redshifts. The cluster fields imaged by the HFF program will not necessarily reveal more high-redshift galaxies, but they offer the potential to detect the intrinsically fainter galaxies that are the progenitors of galaxies like the Milky Way in the local universe. Moreover, intrinsically luminous galaxies might be amplified to the point where spectroscopic observations with Hubble or Webb become possible.

The HDFI WG made a number of recommendations regarding the development of the observing program. In particular, they identified the following criteria for selecting the target clusters:

  • The clusters must be massive and among the strongest lenses known, with high magnification caustics for objects at z ~ 4–10 that fit within the WFC3-IR field of view. This requirement is most easily met by clusters at z > 0.37.
  • The clusters must be observable with Hubble, Spitzer, and Webb.
  • The cluster fields should have low zodiacal background and low Galactic extinction.
  • To the extent possible, the blank fields should avoid bright stars and outlying structure due to the galaxy cluster.
  • Every effort should be made to choose clusters that are observable by the Atacama Large Millimeter/submillimeter Array and the observatories on Mauna Kea.
  • If possible, ancillary data should be available from Hubble, Spitzer-MIPS, Herschel, Chandra, and ground-based telescopes.

The HDFI WG recognized that satisfying every criterion for every cluster may not be possible.

They noted that analyzing the cluster data depends critically on understanding the cluster magnification maps, and how those translate to the volume probed at high redshift. This is a specialized area of research, and they urged the Institute to provide the appropriate reference data and analysis tools to level the playing field for all potential participants in this community program.