Beyond Ultra-deep Frontier Fields and Legacy Observations ("BUFFALO")

The Beyond Ultra-deep Frontier Fields and Legacy Observations program (BUFFALO, Steinhardt et al. 2020, HST Program ID 15117) is a 101-orbit Cycle 25 HST program of prime ACS/WFC + parallel WFC3/IR imaging around all 6 of the clusters in the Hubble Frontier Fields program (HFF, Lotz et al. 2017). The BUFFALO program significantly expands the area imaged around each HFF cluster by a factor of about 3-4 times, in a total of 5 filters (ACS/WFC F606W and F814W, and WFC3/IR F105W, F125W, and F160W). This additional area had not previously been extensively observed by HST but was already covered by deep multi-wavelength data sets, including Spitzer and Chandra. As with the original HFF program, BUFFALO is designed to take advantage of gravitational lensing from massive clusters to simultaneously find high-redshift galaxies that would otherwise lie below HST detection limits and model foreground clusters to study the properties of dark matter and galaxy assembly. The expanded area is providing the first opportunity to study both cosmic variance at high redshift and galaxy assembly in the outskirts of the large HFF clusters.