Cosmic Reionization and Early Star-Forming Galaxies: A Joint Analysis of New Constraints from Planck and Hubble Space Telescope

Abstract: We discuss new constraints on the epoch of cosmic reionization and test the assumption that most of the ionizing photons responsible arose from high redshift star-forming galaxies. Good progress has been made in charting the end of reionization through spectroscopic studies of z~6-8 QSOs, gamma-ray bursts and galaxies expected to host Lyman-alpha emission. However, the most stringent constraints on its duration have come from the integrated optical depth, tau, of Thomson scattering to the cosmic microwave background. Using the latest data on the abundance and luminosity distribution of distant galaxies from Hubble Space Telescope imaging, we simultaneously match the reduced value tau=0.066 +/- 0.012 recently reported by the Planck collaboration and the evolving neutrality of the intergalactic medium with a reionization history within 6 <~ z <~ 10, thereby reducing the requirement for a significant population of very high redshift (z>>10) galaxies. Our analysis strengthens the conclusion that star-forming galaxies dominated the reionization process and has important implications for upcoming 21cm experiments and searches for early galaxies with James Webb Space Telescope.

• The paper refines the reionization timeline by reconciling Planck's lower τ value with HST data, reducing the inferred need for extreme high-z galaxy populations.
• It employs spectroscopic studies and multi-wavelength observations of quasars and gamma-ray bursts to constrain the end of reionization around z ≈ 6.
• The study underscores the dominant role of z ~6–10 star-forming galaxies, guiding future observational strategies with JWST and 21cm experiments.

Analyzing Cosmic Reionization through Planck and Hubble Space Telescope Data

The paper "Cosmic Reionization and Early Star-Forming Galaxies: A Joint Analysis of New Constraints from Planck and Hubble Space Telescope" by Robertson et al. provides a comprehensive examination of the epoch of cosmic reionization, focusing on the contributions of high redshift star-forming galaxies. Utilizing recent data from the Planck satellite and Hubble Space Telescope (HST), the authors seek to refine constraints on the timeline and contributors of reionization, emphasizing the role of star-forming galaxies at high redshifts.

Key Findings and Methodology

The authors use spectroscopic studies of quasars (QSOs), gamma-ray bursts, and galaxies expected to host Lyman α emission to constrain the end of cosmic reionization around redshift $z \simeq 6$. Nevertheless, the onset and duration are primarily informed by measurements of the Thomson scattering optical depth, $\tau$, to the cosmic microwave background (CMB). The Planck collaboration reported a lower $\tau$ value of $0.066 \pm 0.012$, compared to the earlier Wilkinson Microwave Anisotropy Probe (WMAP) result, which suggested a value of $0.088\pm0.014$. This reduction implies a more compressed timeline for reionization than previously thought, with significant implications for the role and necessity of very high redshift galaxies ($z > 10$).

Robertson et al. employ the latest data on galaxy abundance and luminosity distribution from HST to model the reionization history within redshifts $6 \lesssim z \lesssim 10$. They find that this model can satisfactorily explain the Planck $\tau$ without requiring extensive star-formation activity beyond $z > 10$. The study reinforces the dominant role of star-forming galaxies during reionization but reduces the urgency to posit a significant, yet undetected, population of galaxies beyond $z \gg 10$.

Implications and Future Prospects

The reduction in the needed contribution from extremely high redshift galaxies shifts focus towards better understanding the populations between redshifts $6$ and $10$. The implications posit a clearer observational strategy for upcoming 21cm experiments and the JWST. These instruments aim to directly observe the faint galaxies and infer the ionization state of the intergalactic medium (IGM) during these critical epochs, which can further validate the assumptions about escape fractions and photon efficiency made in the authors' model.

The constrained timeline of reionization (spanning approximately $400$ million years) aligns with constraints from the Planck $\tau$ and extends favorably towards estimates of star formation rates derived from HST data. This shortened duration is consistent with current upper limits on the kinetic Sunyaev-Zel'dovich effect, which restrict the duration of the reionization era.

Conclusion

Robertson et al.'s work advances the quantitative understanding of cosmic reionization by integrating multifaceted observational data and theoretical models. It underscores the centrality of star-forming galaxies in the reionization process while adjusting the necessity for contributions from high-redshift periods (i.e., $z \gg 10$). Such insights refine the timeline and physical processes involved in the epoch of reionization, pointing to promising avenues for future empirical investigations, particularly utilizing the JWST and next-generation radio arrays sensitive to redshifted 21cm signals.