New Constraints on Cosmic Reionization from the 2012 Hubble Ultra Deep Field Campaign

TL;DR

This study uses deep infrared imaging from the 2012 Hubble UDF campaign to constrain early galaxy populations and their role in cosmic reionization, integrating these findings with CMB data to refine reionization history models.

Contribution

It provides the first systematic analysis combining deep galaxy observations with CMB constraints to infer the properties of early star-forming galaxies during reionization.

Findings

Star-forming galaxies at z~7-9 must extend below M_UV~-13 to fully reionize the universe.

Low-level star formation at z~15-25 is likely necessary to match CMB optical depth.

Reionization likely involved faint, early galaxies with sustained star formation over a broad redshift range.

Abstract

Understanding cosmic reionization requires the identification and characterization of early sources of hydrogen-ionizing photons. The 2012 Hubble Ultra Deep Field (UDF12) campaign has acquired the deepest infrared images with the Wide Field Camera 3 aboard Hubble Space Telescope and, for the first time, systematically explored the galaxy population deep into the era when cosmic microwave background (CMB) data indicates reionization was underway. The UDF12 campaign thus provides the best constraints to date on the abundance, luminosity distribution, and spectral properties of early star-forming galaxies. We synthesize the new UDF12 results with the most recent constraints from CMB observations to infer redshift-dependent ultraviolet (UV) luminosity densities, reionization histories, and electron scattering optical depth evolution consistent with the available data. Under reasonable assumptions about the escape fraction of hydrogen ionizing photons and the intergalactic medium clumping factor, we find that to fully reionize the universe by redshift z~6 the population of star-forming galaxies at redshifts z~7-9 likely must extend in luminosity below the UDF12 limits to absolute UV magnitudes of M_UV\sim -13 or fainter. Moreover, low levels of star formation extending to redshifts z~15-25, as suggested by the normal UV colors of z\simeq7-8 galaxies and the smooth decline in abundance with redshift observed by UDF12 to z\simeq10, are additionally likely required to reproduce the optical depth to electron scattering inferred from CMB observations.