Spitzer Ultra Faint SUrvey Program (SURFS UP). I. An Overview

SURFS UP 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 each 3.6 µm and 4.5 µm in the central 4 ′ ×4 ′ field and ∼ 15 hours in the flanking fields. This results in 3-� sensitivity limits of ∼ 26.6 and ∼ 26.2AB magnitudes for SURFS UP in the IRAC 3.6 and 4.5 µm 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 ofz & 7 sources (using a z = 9.5 galaxy behind MACS J1149.5+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. Subject headings: galaxies: high-redshift — gravitational lensing: strong — galaxies: clusters: individual — dark ages, reionization, first stars