The impact of varying inhomogeneous reionization histories on metrics of Ly$\alpha$ opacity
arxiv(2023)
摘要
The epoch of hydrogen reionization is complete by $z=5$, but its progression
at higher redshifts is uncertain. Measurements of Ly$\alpha$ forest opacity
show large scatter at $z<6$, suggestive of spatial fluctuations in neutral
fraction ($x_\mathrm{HI}$), temperature, or ionizing background, either
individually or in combination. However, these effects are degenerate,
necessitating modeling these physics in tandem in order to properly interpret
the observations. We begin this process by developing a framework for modeling
the reionization history and associated temperature fluctuations, with the
intention of incorporating ionizing background fluctuations at a later time. To
do this, we generate several reionization histories using semi-numerical code
AMBER, selecting histories with volume-weighted neutral fractions that adhere
to the observed CMB optical depth and dark pixel fractions. Implementing these
histories in the \texttt{Nyx} cosmological hydrodynamics code, we examine the
evolution of gas within the simulation, and the associated metrics of the
Ly$\alpha$ forest opacity. We find that the pressure smoothing scale within the
IGM is strongly correlated with the adiabatic index of the temperature-density
relation. We find that while models with 20,000 K photoheating at reionization
are better able to reproduce the shape of the observed $z=5$ 1D flux power
spectrum than those with 10,000 K, they fail to match the highest wavenumbers.
The simulated autocorrelation function and optical depth distributions are
systematically low and narrow, respectively, compared to the observed values,
but are in better agreement when the reionization history is longer in
duration, more symmetric in its distribution of reionization redshifts, or if
there are remaining neutral regions at $z<6$. The systematically low variance
likely requires the addition of a fluctuating UVB.
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