Damping Wing-Like Features in the Spectra of High Redshift Quasars: a Challenge for Fully-Coupled Simulations

Abstract

Recently, several observational detections of damping-wing-like features at the edges of ``dark gaps" in the spectra of distant quasars (the ``Malloy-Lidz effect") have been reported, rendering strong support for the existence of ``neutral islands" in the universe at redshifts as low as $z<5.5$. We apply the procedure from one of these works, Zhu et al (2024), to the outputs of fully coupled cosmological simulations from two recent large projects, ``Cosmic Reionization On Computers" (CROC) and ``Thesan". Synthetic spectra in both simulations have statistics of dark gaps similar to observations, but do not exhibit the damping wing features. Moreover, a toy model with neutral islands added ``by hand" only reproduces the observational results when the fraction of neutral islands among all dark gaps approaches 90%. I.e., simulations and observations appear to produce two distinct ``populations" of dark gaps. In addition, in the simulations, the neutral islands at $z=5.9$ should be short-lived and should not extend to $z<5.5$. A possible explanation for this discrepancy is that both simulations underestimate the fluctuations in the photoionization rate and, hence, miss a population of long-lived neutral islands, located in the large downward fluctuations of the photoionization rate.

Published in the Open Journal of Astrophysics