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 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.

• The paper integrates 2012 HST UDF and CMB data to quantify the UV luminosity density driving cosmic reionization.
• It employs robust Bayesian inference to model star-forming galaxy properties at z ~ 7–9, indicating significant activity crucial for reionization.
• The study underscores the role of faint galaxies below MUV ~ -13 in sustaining an extended era of star formation necessary for full reionization by z ~6.

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

The paper by Robertson et al. offers significant insights into cosmic reionization processes, leveraging data from the 2012 Hubble Ultra Deep Field (UDF12) campaign. The study is pivotal in framing cosmic reionization in terms of high-redshift galaxy observations using the deepest infrared images available from the Hubble Space Telescope (HST) at that time. This paper synthesizes UDF12 findings with constraints from cosmic microwave background (CMB) observations, providing fresh perspectives on reionization histories and ultraviolet (UV) luminosity density across varying redshifts.

Key results are derived from updated measurements of the abundance and spectral properties of star-forming galaxies during the era of cosmic reionization. The authors integrate spectroscopic data from the Hubble campaign with CMB analyses to derive the UV luminosity density necessary for sustaining reionization. Particularly, the paper infers substantial star formation activity at redshifts $z\sim7-9$, which is crucial to achieving full reionization by $z\sim6$. The study identifies the need for significant contributions from faint galaxies, with absolute UV magnitudes extending below UDF12's detection limit ($\sim-13$), emphasizing the importance of low-luminosity galaxies to the reionization process.

The use of robust Bayesian inference methods in the study allows for detailed modeling of the UV luminosity density, showing the necessity of an extended period of star formation up to $z\sim15-25$. This approach matches the smooth decline in galaxy abundance with redshift observed up to $z\sim10$. The inferred evolutionary paths of ionizing photon escape fractions and the intergalactic medium’s (IGM) clumping factors are consistent with observational constraints, reinforcing the dual reliance on existing CMB data and forward-modeling based on synthesized HST observations.

From a theoretical standpoint, this paper advances our understanding of reionization dynamics by postulating a comprehensive empirical model that reconciles data-driven constraints with known astrophysical processes. Practically, the incorporation of stellar mass density constraints underscores the nuanced balance between empirical consistency and the need for theoretical models that accommodate potential variations in reionization histories.

Considering the implications of this work, future directions in astrophysical research should aim at reducing uncertainties in escape fractions and photon production efficiencies. Moreover, the noted reliance on faint galaxy populations anticipates observational strategies by next-generation telescopes, like the James Webb Space Telescope (JWST), which are better equipped to probe these regimes. This would not only validate the paper’s assumptions but also refine models of IGM ionization histories.

In conclusion, Robertson et al.’s exploration of the UDF12 data marks an important contribution to the discourse on cosmic reionization. By offering an integrative assessment that confidently straddles observational astronomy and theoretical modeling, this research contributes a foundational layer for future studies investigating the early Universe's ionizing mechanics.