Galaxy Build-up in the First 2 Billion Years: New Insights from Deep HST and Spitzer Observations

The installation of the WFC3/IR camera on the HST has revolutionized high-redshift galaxy science and has pushed our observational frontier well into the reionization epoch. Based on our HUDF09 survey and the CANDELS program we have now identified more than 200 galaxies at z~7-8, and we even detected one robust galaxy candidate at z~10, only 500 Myr from the Big Bang. The inferred cosmic star-formation rate density increases by more than an order of magnitude in only 170 Myr from z~10 to z~8, indicating that the galaxy population was growing very rapidly at these early times. Together with ultra-deep Spitzer IRAC observations from our IUDF10 program, these WFC3/IR samples allowed for the first robust rest-frame optical detections and mass measurements of individual z~8 galaxies. The combination of deep HST and Spitzer data therefore enables us to directly track the star-formation and mass build-up of galaxies across the reionization epoch, and it provides detailed new insights into the stellar population properties of galaxies down to z~4. In this talk I will highlight some of our recent progress in understanding the growth of galaxies in the first two billion years and what this implies for the contribution of galaxies to cosmic reionization.

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