Overview and Motivation for the UDF12 Program
Dramatic progress has been made in the past year in charting the assembly history of the earliest galaxies seen at redshifts z > 6, less than a billion years after the Big Bang. The study of such primitive systems is driven by our desire to understand how and when the Universe was reionized, as well as to determine the physical processes which shaped the mass and luminosity distributions of the emerging generation of star-forming galaxies. These studies represent the final frontier in our quest for a coherent picture of galaxy evolution, and motivate the substantial investments in future facilities such as JWST and the next generation of ground-based near-infrared telescopes.
As the community awaits the next generation facilities, it has become very clear that HST with its new infrared imager, WFC3/IR, can meanwhile play a vital role in exploring this frontier (see Robertson et al. 2010 for a recent review). Through publicly-available images in the Hubble Ultra Deep Field (HUDF) and related deep surveys, important results have emerged (Bouwens et al. 2010a,b; McLure et al. 2010,2011; Bunker et al. 2010; Finkelstein et al. 2010). Rapid progress with HST has been complemented with ambitious ground-based surveys, including panoramic narrow-band imaging of Lyman-α emitting galaxies (LAEs) with Subaru (Ouchi et al. 2010), and ultra-deep spectroscopic surveys of Lyman-break galaxies (LBGs) with Keck and VLT (Stark et al. 2010, 2011; Vanzella et al. 2009, 2010; Lehnert et al. 2010, Schenker et al. 2012).
The questions motivating these studies are fundamental. Sometime in the 600 Myr between a redshift of z = 20 and z = 7, the Universe underwent a phase transition whereby neutral hydrogen created at recombination was reionized. This ‘cosmic reionization’ renders the Universe transparent to ultraviolet (UV) photons and is as significant a milestone in cos- mic history as the decoupling of matter and radiation revealed by the microwave background. Reionization required a sustained source of ionizing photons and the popularly-assumed ex- planation is that it arises from energetic photons that escaped from the first generation of star-forming galaxies (Robertson et al. 2010). The UDF12 program is motivated by the conviction that carefully-planned WFC3/IR observations, building on the past investment of HST in the HUDF, can provide an important test of this hypothesis.
Outstanding Questions and Controversies UDF12 Will Address
As reviewed by Robertson et al. (2010), early star-forming galaxies can maintain reionization over the redshift range 6 < z < 10 provided there is a long-lived population with a dominant contribution from low-luminosity sources and a modest fraction of ionizing photons escape into the intergalactic medium. Our proposal is carefully designed to build on earlier HST investment to address three issues arising from these requirements which collectively hold the key to understanding the contribution of early galaxies to cosmic reionization.