Hubble Boldly Goes: The Frontier Fields Program

Implementing the HFF program

The responsibility for executing the HFF program rests with a core implementation team of Institute staff led by one of the authors (JL). That team is receiving advice and guidance from a number of external science advisors. From the outset, Spitzer researchers have been closely involved in the HDFI. The Spitzer Director, Tom Soifer, has committed up to 1,000 hours for observations by the InfraRed Array Camera of the target fields, and Spitzer staff members are working closely with the Hubble team to coordinate and implement the observations. In addition, several key members of the community are serving as external science advisors to the program, providing advice throughout.

The Institute has taken steps to involve the community in every aspect of the HFF implementation process. Immediately following the program’s announcement and the release of the HDFI WG report, the community was solicited for additional input on the observing strategy, filter selection, and cluster selection. In addition, members of the community with appropriate expertise were contacted individually and invited to comment on various aspects of the program. Finally, a website and blog are being maintained to keep the community informed of developments as they happen.

The HDFI WG report outlines a potential observing scheme coupling 70 orbits of red/far-red optical observations using the Advanced Camera for Surveys (ACS) with 70 orbits of imaging with the WFC3-IR camera. This scenario achieves magnitude limits matching those in the HUDF09 parallel fields.  Following discussion with the community, those recommendations were modified slightly—adding F140W observations in the blank field—to give the filter selection illustrated in Figure 2. The resulting sensitivities are summarized in Table 1, where the AB magnitude limits represent 5σ detections of a point source, as measured in an aperture 0.4 arcsecond in diameter and corrected to total magnitude. The observations will be taken at two epochs approximately six months apart, with the telescope rotated by 180o between epochs to switch cameras between cluster and blank field. Orientation constraints typically limit the scheduling window at each epoch to 30–50 days. Further details on the observations, including the dithering strategy that will be adopted, can be found at the Frontier Fields website.

The HDF WG provided an initial set of 16 candidate strong-lensing clusters. The implementation team supplemented that list with other candidates suggested by the community and matched their individual properties against the selection criteria defined by the Working Group. Particular attention had to be paid to the availability of suitable offset fields, since even moderately bright stars can cause significant problems in Spitzer IRAC imaging.