The IRAC Ultra Deep Fields 2010: Using IRAC to Characterize Ultrafaint z~7-10 Galaxies

Abstract

Recent extremely-deep, near-IR imaging (to 29.3 mag 5sigma) with HST WFC3/IR in the HUDF from our 192 orbit HUDF09 program have enabled a huge leap in our knowledge of the galaxies populating the early universe at z>7. These early galaxies are compelling targets for many reasons: as beacons of the first sites of star formation, as testbeds to constrain galaxy formation models, and as probes of reionization. Existing deep IRAC observations of the newly-found z~7 and z~8 galaxies in the HUDF demonstrate that Spitzer/IRAC plays a vital role in probing their rest-frame optical light and constraining their stellar masses and star formation histories at just 600-800 million years from the Big Bang. Now the only constraint to immediate and enormous progress in this area lies in the limited depth and incomplete IRAC coverage of the three ultra-deep WFC3 HUDF fields. These three fields are by far the best places of the sky for z>7 searches: the ACS and WFC3 supporting data are unique, and large samples of z~7-8.5 galaxies have already been identified (but are awaiting deep IRAC imaging). Here we propose immediate progress by pushing deeper on the HUDF and completing the very deep IRAC coverage of the HUDF09-1 and HUDF09-02 fields. This will result in (a) 2X reduction in field-to-field variance, (b) up to 2-4X reduced uncertainties in the stellar mass density at z~7 and z~8, and (c) up to 5X increases in IRAC detection rates of existing z~7 and z~8 galaxies. An ultra-deep IRAC data set will yield a peerless Legacy dataset for a dazzling array of science questions for galaxy formation during reionization out to z~10. These public IRAC data will be a crucial Legacy complement to the thousands of hours of existing or approved IRAC wide-field (shallow) surveys and the lensed clusters studies. They will also position us to exploit the upcoming unique capabilities of ALMA and JWST, as well as providing an observational framework for increasingly sophisticated galaxy formation and evolution modeling.