Spitzer UltRa Faint SUrvey Program (SURFS UP) I: An Overview
  SURFSUP is a joint Spitzer and HST Exploration Science program using 10
galaxy clusters as cosmic telescopes to study z >~ 7 galaxies at intrinsically
lower luminosities, enabled by gravitational lensing, than blank field surveys
of the same exposure time. Our main goal is to measure stellar masses and ages
of these galaxies, which are the most likely sources of the ionizing photons
that drive reionization. Accurate knowledge of the star formation density and
star formation history at this epoch is necessary to determine whether these
galaxies indeed reionized the universe. Determination of the stellar masses and
ages requires measuring rest frame optical light, which only Spitzer can probe
for sources at z >~ 7, for a large enough sample of typical galaxies. Our
program consists of 550 hours of Spitzer/IRAC imaging covering 10 galaxy
clusters with very well-known mass distributions, making them extremely precise
cosmic telescopes. We combine our data with archival observations to obtain
mosaics with ~30 hours exposure time in both 3.6$\mu$m and 4.5$\mu$m in the
central 4 arcmin x 4 arcmin field and ~15 hours in the flanking fields. This
results in 3-$\sigma$ sensitivity limits of ~26.6 and ~26.2AB magnitudes for
the central field in the IRAC 3.6 and 4.5$\mu$m bands, respectively. To
illustrate the survey strategy and characteristics we introduce the sample,
present the details of the data reduction and demonstrate that these data are
sufficient for in-depth studies of z >~ 7 sources (using a z=9.5 galaxy behind
MACSJ1149.5+2223 as an example). For the first cluster of the survey (the
Bullet Cluster) we have released all high-level data mosaics and IRAC empirical
PSF models. In the future we plan to release these data products for the entire
survey.
