Model-independent evidence in favor of an end to reionization by z~6

Abstract: We present new upper limits on the volume-weighted neutral hydrogen fraction, <xHI>, at z~5-6 derived from spectroscopy of bright quasars. The fraction of the Lyman-alpha and Lyman-beta forests that is "dark" (with zero flux) provides the only model-independent upper limit on <xHI>, requiring no assumptions about the physical conditions in the intergalactic medium or the quasar's unabsorbed UV continuum. In this work we update our previous results using a larger sample (22 objects) of medium-depth (~ few hours) spectra of high-redshift quasars obtained with the Magellan, MMT, and VLT. This significantly improves the upper bound on <xHI> derived from dark pixel analysis to <xHI> <= 0.06 + 0.05 (1{\sigma}) at z=5.9, and <xHI> <= 0.04 + 0.05 (1{\sigma}) at z=5.6. These results provide robust constraints for theoretical models of reionization, and provide the strongest available evidence that reionization has completed (or is very nearly complete) by z~6.

• The paper uses dark pixel analysis in quasar spectra to establish model-independent constraints on the neutral hydrogen fraction.
• It finds that the neutral fraction is less than 0.11 at z=5.9 and 0.09 at z=5.6, suggesting reionization was nearly complete by z~6.
• The methodology avoids assumptions about the IGM’s state, aligning its findings with independent measurements from CMB and ionizing background data.

A Model-Independent Approach to Understanding Cosmic Reionization

The paper by McGreer, Mesinger, and D'Odorico presents an intriguing and methodologically robust study aimed at restricting the volume-weighted neutral hydrogen fraction, denoted as $x$, in the universe during the redshift range 5 to 6. This period of cosmic history marks the end of reionization, a pivotal phase transition in the early universe when the predominantly neutral intergalactic medium (IGM) became ionized due to the emergence of the first luminous sources. Accurately constraining $x$ during this epoch provides valuable insights into the timing and duration of reionization, thereby informing models of the first structures.

Their research focuses on a detailed analysis of quasar absorption spectra, specifically those drawn from observations using some of the most prominent telescopes, including Magellan, MMT, and the Very Large Telescope (VLT). The authors revisit and expand upon previous work, incorporating a broader dataset of 22 quasar spectra, which significantly diminishes statistical uncertainties compared to past studies.

One of the novel aspects of their methodology is the use of “dark pixel analysis.” Dark pixels are sections of quasar spectra with zero observed flux, presumed to be locations of significant hydrogen absorption. By assessing the fraction of these dark pixels in the Lyman-alpha (Ly$\alpha$) and Lyman-beta (Ly$\beta$) forests of the quasar spectra, the authors derive a model-independent upper limit on $x$. This approach does not rely on assumptions about the IGM’s physical state or the unabsorbed quasar UV continuum, thereby avoiding degeneracies and uncertainties associated with astrophysical modeling. Their findings result in robust constraints where the neutral hydrogen fraction is shown to be less than 0.11 at $z=5.9$ and less than 0.09 at $z=5.6$.

These strong upper limits suggest that reionization was nearing completion by $z\approx6$. This timeline is crucial as it aligns well with other indirect measurements, like the CMB (Cosmic Microwave Background) observations of Thomson scattering optical depth and measurements of ionizing backgrounds from lower redshift. Their results are notable for not assuming the complete ionization of the IGM and maintaining agnosticism about the physical details and sources driving reionization.

Additionally, the presence of a quasar with a purported complete Gunn-Peterson trough demonstrates the large-scale ionizing background fluctuations within the IGM at these redshifts. While the derived $x$ fractions themselves may not indicate a fully completed reionization before $z=6$, the method's constraints indeed provide convincing evidence against the persistence of substantial neutral regions at lower redshifts, thereby suggesting that reionization had predominantly finished by this epoch.

In terms of future directions, while observational advancements in constraining $x$ using dark pixel analysis remain limited, improvements in theoretical modeling and simulations of IGM dynamics and reionization processes are expected to refine the interpretation of such constraints. As such, they would incorporate and appropriately account for the substantial fluctuations and inhomogeneities introduced by the patchy nature of reionization.

In summary, McGreer et al.'s work delivers significant model-independent constraints on one of cosmology's unanswered questions—the exact completion timeframe of cosmic reionization. By leveraging the advanced spectroscopic techniques and high signal-to-noise observations afforded by modern astronomical infrastructure, this paper provides a valuable reference point for future theoretical and observational studies aimed at unraveling the mysteries of the early universe.