Kavli Affiliate: Nickolay Y. Gnedin
| First 5 Authors: , , , ,
| Summary:
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.
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