The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey
(CANDELS) is a new and powerful multicycle HST treasury imaging survey.
It is the next major step forward in the study of galaxy evolution in
the early universe (z = 2 to 8). Using WFC3/IR and ACS, the program
incorporates a two-tiered strategy: a 'Wide' component (~0.2 sq.
degrees) and a 'Deep' component (~0.04 sq. degrees). Combining these
fields with the ultra-deep imaging from the HST Cycle 17 HUDF09 program
(~0.004 sq degrees) yields a three-tiered strategy for efficient
sampling of both rare/bright and faint/common objects. This approach
mitigates cosmic variance and yields statistically robust samples of
galaxies out to z = 8. These early galaxies are important because they
serve as beacons of the first sites of star formation, as a constraint
on galaxy formation models, and as probes of reionization. A major
limitation on further progress in exploring the early universe using the
new CANDELS data is the relatively modest depths of the current
Spitzer/IRAC mid-infrared data in these fields. IRAC adds the unique and
important capability of tracing early galaxy stellar mass growth,
stellar ages, and star formation rates back to z ~ 7 to 10. This Cycle-8
Spitzer Exploration Science program proposes significantly increasing
the IRAC depth in the CANDELS fields to 26.8 AB mag (5 sigma, 3.6
micron; 50 hrs/pointing), thus providing deep IRAC coverage in all the
CANDELS fields. With this sensitivity Spitzer/IRAC will detect every
source detected in the CANDELS Wide survey, probe galaxies to z ~ 8 at
~109 Msun, and permit measurement of their spectral energy
distributions. The proposed program will increase the total area mapped
jointly by WFC3/IR and IRAC to 26.8 mag AB depth by a factor of ~35.
This probe of the early Universe, over a relatively large area and to
such a depth by HST and Spitzer/IRAC, will have a major impact on our
knowledge of the galaxy evolution and leave an important legacy for
years to come.