3D-DASH: A Wide Field WFC3/IR Survey of COSMOS

The Hubble Space Telescope has enabled us to establish a broad picture of how present day galaxies came to be. We propose to extend this legacy into unprecedented new territory - degree-scale infrared imaging and spectroscopy - that will allow us for the first time to study the physical processes that shape the behemoths of the universe: the most massive, highly-star-forming, rare galaxies up to z=3.5. Using the "drift and shift" (DASH) technique, developed in Cycle 23, we propose to survey the entire 1.7 sq. deg. COSMOS field in WFC3/H(F160W) and WFC3/G141 in order to map the build up of stellar structure via >1000 resolved H-alpha maps, carry out a statistically robust census of ~450 M>10^11 M_sun galaxies, identify up to 100 massive major mergers to constrain theoretical models, determine emission line redshifts for ~37,000 z>0.7 galaxies and provide the most accurate predictions on the bias and number density of emission line galaxies prior to Euclid and WFIRST. 3D-DASH will create a wide shallow tier in the infrared "wedding cake" of extragalactic surveys, increasing the area observed with infrared grism spectroscopy by almost an order of magnitude and adding HST-resolution infrared imaging to COSMOS, one of the most actively studied fields. This program will fill in an important gap in HST extragalactic surveys in the pre-JWST era and is unlikely to be surpassed in sensitivity, area and resolution until the launch of WFIRST.