Astronomy Colloquium

Deep exposures with the Hubble Space Telescope (HST) have provided the primary evidence that star-forming galaxies were present in the first billion years of cosmic history. Sometime during this early period the intergalactic medium transitioned from a neutral gas to one that is fully ionized. How did this `cosmic reionization' occur and were star-forming galaxies responsible? The electron scattering optical depth inferred from cosmic microwave background observations suggest that reionization occurred sometime in the redshift interval z=20 to z=6 so probing the abundance, luminosity distribution and spectral properties of galaxies during this uncharted period holds the key to progress in addressing these fundamental questions. The Ultra Deep Field 2012 campaign represents a new series of infrared images totaling 128 orbits taken with HST's Wide Field Camera 3. Through its added depth and appropriate use of additional filters compared to earlier work, new insight has been gained into our understanding of when reionization occurred and the role of early galaxies in the process.