Early star-forming galaxies and the reionization of the Universe

Abstract: Star forming galaxies represent a valuable tracer of cosmic history. Recent observational progress with Hubble Space Telescope has led to the discovery and study of the earliest-known galaxies corresponding to a period when the Universe was only ~800 million years old. Intense ultraviolet radiation from these early galaxies probably induced a major event in cosmic history: the reionization of intergalactic hydrogen. New techniques are being developed to understand the properties of these most distant galaxies and determine their influence on the evolution of the universe.

• The paper shows that early star-forming galaxies emitted abundant ionizing UV photons capable of reionizing the intergalactic medium by redshift 7.
• Robust Hubble observations reveal a steep faint-end luminosity function at z ≈ 7–8, indicating numerous low-luminosity galaxies were key drivers of reionization.
• The study quantifies variable escape fractions and ionizing photon production rates, underpinning the role of early galaxies in transforming the cosmic landscape.

Early Star-Forming Galaxies and Their Role in Cosmic Reionization

In the paper by Robertson et al., an examination of early star-forming galaxies highlights their pivotal role during the epoch of cosmic reionization. Using robust observational datasets from the Hubble Space Telescope, the research provides a compelling analysis of galaxies formed approximately 800 million years post-Big Bang. It discusses how ionizing ultraviolet (UV) photons emitted by these primitive systems contribute to the reionization of intergalactic hydrogen, ultimately changing the cosmic landscape from a neutral to an ionized medium.

Central to the study is the pursuit to confirm the connection between early galaxy formation and cosmic reionization. The authors argue that the abundance and distribution of high-redshift galaxies provide insights into the mechanisms and sources of the energetic photons responsible for reionization. This work underscores the need for accurate quantification of the rest-frame UV luminosity density, which serves as an observable proxy for star formation rate density, from these early cosmic structures.

Key findings point towards a fundamental connection between the UV luminosity of these galaxies and their ability to sustain an ionized intergalactic medium (IGM) till redshift $z \approx 7$. With over 50 sources from early HST Wide Field Camera 3 (WFC3/IR) observations, the study details the luminosity function at $z \approx 7-8$, revealing a steep faint-end slope. The observations indicate that the bulk of UV luminosity at this epoch originates from numerous faint galaxies, suggesting a possible significant contribution from these galaxies to the reionization process.

The paper meticulously evaluates the escape fraction and the production rate of ionizing photons, showing that these parameters are highly variable among galaxies yet critical for determining their collective impact on reionization. From the luminosity function and associated star formation rates, the authors suggest that early galaxies could produce sufficient ionizing photons to account for the reionization observed by redshift $z \approx 7$.

The research also explores the use of the Lyman α emission line as a diagnostic for discerning the ionization state of the universe. The observed decline in the abundance of Lyman α emitters beyond redshift $z \approx 6.5$ potentially indicates increasing neutral hydrogen fractions, serving as a concrete signpost for the end stages of the reionization epoch.

This paper implies significant practical and theoretical implications for our understanding of cosmic reionization. It establishes a strong observational basis to infer that early star-forming galaxies were viable sources of ionizing photons necessary for reionizing the universe's hydrogen. The upcoming improvements in observational technology, such as those expected from the James Webb Space Telescope (JWST) and future ground-based telescopes, are likely to refine these constraints and potentially unveil more about the high-redshift universe.

Overall, this work offers profound insights into cosmic history, emphasizing the need for continued observational campaigns to elucidate the intricacies of the reionization process while paving the way for future investigations that could reshape our understanding of the early universe.