Session M9: Invited Session: Windows on the Epoch of Reionization

Progress Towards Measuring the Cosmic 21cm Background from the Epoch of Reionization

Measuring 21cm hyperfine emission from neutral hydrogen at cosmological distances is one of the most promising techniques for probing our early universe. During cosmic reionization, this signal is sensitive to myriad cosmological and astrophysical processes as the first stars and galaxies heat and ionize the intergalactic medium. Recently, the Precision Array to Probe the Epoch of Reionization (PAPER) has overcome the key technical hurdles facing 21cm reionization experiments to place physically constraining upper limits on the cosmological signal. These limits imply a level of heating of the intergalactic medium inconsistent with a rapid decrease in star formation rate density at high redshifts and inconsistent with lower prescriptions relating X-ray luminosity to star formation rate. Building on these successes, the US community has coalesced around a next-generation experiment for exploring cosmic reionization via 21cm emission. The Hydrogen Epoch of Reionization Array (HERA) will be a large array of zenith-pointing parabolic dishes optimized for power spectral measurements. HERA's considerable collecting area enables it to precisely measure ionization fraction versus redshift, to directly image larger ionization bubbles, and to probe heating in pre-reionization epochs. Phase I of HERA was recently funded and construction has already begun.   

The Cosmic Microwave and Infrared Background Windows on the Epoch of Reionization

Recent observations of the sky at submm wavelengths offers new insights on the Epoch or Reionization. I will review in particular how the multi-frequency observations of the Planck satellite instruments contain multiple probes of the reionization history of the Universe, either through the scattering of CMB photons or from the unresolved emission of star-forming galaxies. I will discuss the status of these measurements and their interpretation before describing promising novel methods to directly map the Epoch or Reionization in 3D using emission lines such as CO, CII or Lya.   

Observing the Earliest Galaxies: Looking for the Sources of Reionization

Systematic searches for the earliest galaxies in the reionization epoch finally became possible in 2009 when the Hubble Space Telescope was updated with a powerful new infrared camera during the final Shuttle servicing mission SM4 to Hubble. The reionization epoch represents the last major phase transition of the universe and was a major event in cosmic history. The intense ultraviolet radiation from young star-forming galaxies is increasingly considered to be the source of the photons that reionized intergalactic hydrogen in the period between the ``dark ages'' (the time before the first stars and galaxies at about 100-200 million years after the Big Bang) and the end of reionization around 800-900 million years. Yet finding and measuring the earliest galaxies in this era of cosmic dawn has proven to a challenging task, even with Hubble's new infrared camera. I will discuss the deep imaging undertaken by Hubble and the remarkable insights that have accrued from the imaging datasets taken over the last decade on the Hubble Ultra-Deep Field (HUDF, HUDF09/12) and other regions. The HUDF datasets are central to the story and have been assembled into the eXtreme Deep Field (XDF), the deepest image ever from Hubble data. The XDF, when combined with results from shallower wide-area imaging surveys (e.g., GOODS, CANDELS) and with detections of galaxies from the Frontier Fields, has provided significant insights into the role of galaxies in reionization. Yet many questions remain. The puzzle is far from being fully solved and, while much will done over the next few years, the solution likely awaits the launch of JWST.