Kavli Affiliate: Nickolay Y. Gnedin
| First 5 Authors: Alexandra Wells, David Robinson, Camille Avestruz, Nickolay Y. Gnedin,
| Summary:
We examine the evolution of the phase diagram of the low-density
intergalactic medium during the Epoch of Reionization in simulation boxes with
varying reionization histories from the Cosmic Reionization on Computers
project. The PDF of gas temperature at fixed density exhibits two clear modes:
a warm and cold temperature mode, corresponding to the gas inside and outside
of ionized bubbles. We find that the transition between the two modes is
"universal" in the sense that its timing is accurately parameterized by the
value of the volume-weighted neutral fraction for any reionization history.
This "universality" is more complex than just a reflection of the fact that
ionized gas is warm and neutral gas is cold: it holds for the transition at a
fixed value of gas density, and gas at different densities transitions from the
cold to the warm mode at different values of the neutral fraction, reflecting a
non-trivial relationship between the ionization history and the evolving gas
density PDF. Furthermore, the "emergence" of the tight temperature-density
relation in the warm mode is also approximately "universally" controlled by the
volume-weighted neutral fraction for any reionization history. In particular,
the "emergence" of the temperature-density relation (as quantified by the rapid
decrease in its width) occurs when the neutral fraction is $10^{-4}lesssim
X_mathrm{HI} lesssim10^{-3}$ for any reionization history. Our results
indicate that the neutral fraction is a primary quantity controlling the
various properties of the temperature-density relation, regardless of
reionization history.
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