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

Abstract

Understanding cosmic reionization requires the identification and characterization of early sources of hydrogenionizing 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 indicate 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 ∼ −13 or fainter. Moreover, low levels of star formation extending to redshifts z ∼ 15-25, as suggested by the normal UV colors of z 7-8 galaxies and the smooth decline in abundance with redshift observed by UDF12 to z 10, are additionally likely required to reproduce the optical depth to electron scattering inferred from CMB observations.

Spatially adaptive radiation-hydrodynamical simulations of galaxy formation during cosmological reionization

“…In our simulations, reionization completes at higher redshifts than suggested by constraints on the neutral fraction from observations of Lyman-α emitting galaxies and high-redshift quasars (e.g., Figure 5 in Robertson et al 2013). However, interpretations of these observations are dominated by large systematic uncertainties (for a discussion see, e.g., Robertson et al 2013;Jensen et al 2013;Dijkstra 2014;Mesinger et al 2014;Taylor & Lidz 2014).…”

Constraints on the star formation efficiency of galaxies during the epoch of reionization

“…There has been some controversy in the literature about the shape of the SFRD at high redshifts (e.g., Robertson et al 2013;Oesch et al 2015), reflected by the wide range in estimated star formation rates at higher redshifts shown by the data points in Fig. 7.…”

“…At the same time, the first stars and galaxies were forming from primordial gas clouds in the Universe. Although there is a clear connection between these two events (Barkana & Loeb 2001;Bromm & Yoshida 2011;Loeb & Furlanetto 2013;Robertson et al 2013), their detailed relation is unknown, thanks to uncertainties in the properties of the galaxy populations. Fortunately, understanding the epoch of reionization itself will also shed light on the formation and evolution of early galaxies.…”

The First Billion Years project: the escape fraction of ionizing photons in the epoch of reionization

“…The escape fraction is a key parameter in studies of the contribution of the observed galaxy population to reionization (e.g. Bouwens et al 2012;Robertson et al 2013), semi-analytic modelling of reionization (e.g. Choudhury et al 2009;Pritchard et al 2010;Santos et al 2010;Mesinger et al 2011;Raskutti et al 2012;Mitra et al 2013;Shull et al 2012) and numerical simulations of reionization (e.g.…”