Effect of non-equilibrium ionization on derived physical conditions of the high-$z$ intergalactic medium. (arXiv:1812.01016v1 [astro-ph.CO])
<a href="http://arxiv.org/find/astro-ph/1/au:+Gaikwad_P/0/1/0/all/0/1">Prakash Gaikwad</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Srianand_R/0/1/0/all/0/1">Raghunathan Srianand</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Khaire_V/0/1/0/all/0/1">Vikram Khaire</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Choudhury_T/0/1/0/all/0/1">Tirthankar Roy Choudhury</a>
Non-equilibrium ionization effects are important in cosmological
hydrodynamical simulations but are computationally expensive. We study the
effect of non-equilibrium ionization evolution and UV ionizing background (UVB)
generated with different quasar spectral energy distribution (SED) on the
derived physical conditions of the intergalactic medium (IGM) at $2leq z leq
6$ using our post-processing tool ‘Code for Ionization and Temperature
Evolution’ (CITE). CITE produces results matching well with self-consistent
simulations more efficiently. The HeII reionization progresses more rapidly in
non-equilibrium model as compared to equilibrium models. The redshift of HeII
reionization strongly depends on the quasar SED and occurs earlier for UVB
models with flatter quasar SEDs. During this epoch the temperature of the IGM
at mean density, $T_0(z)$, has a maximum while the slope of the effective
equation of state, $gamma(z)$, has a minimum, but occurring at different
redshifts. The $T_0$ is higher in non-equilibrium models using UVB obtained
with flatter quasar SEDs. The observed median HeII effective optical depth
evolution and its scatter are well reproduced in equilibrium and
non-equilibrium models even with the uniform UVB assumption. For a given
thermal history the redshift dependence of HI photo-ionization rate derived
from observed HI effective optical depth ($tau_{rm eff,HI}$) are different
for equilibrium or non-equilibrium models. This may lead to different
requirements on the evolution of ionizing emissivities of sources. We show
that, in the absence of strong differential pressure smoothing effects, it is
possible to recover the $T_0$ and $gamma$ in non-equilibrium model from the
equilibrium models generated by rescaling photo-heating rates while producing
the same $tau_{rm eff,HI}$.
Non-equilibrium ionization effects are important in cosmological
hydrodynamical simulations but are computationally expensive. We study the
effect of non-equilibrium ionization evolution and UV ionizing background (UVB)
generated with different quasar spectral energy distribution (SED) on the
derived physical conditions of the intergalactic medium (IGM) at $2leq z leq
6$ using our post-processing tool ‘Code for Ionization and Temperature
Evolution’ (CITE). CITE produces results matching well with self-consistent
simulations more efficiently. The HeII reionization progresses more rapidly in
non-equilibrium model as compared to equilibrium models. The redshift of HeII
reionization strongly depends on the quasar SED and occurs earlier for UVB
models with flatter quasar SEDs. During this epoch the temperature of the IGM
at mean density, $T_0(z)$, has a maximum while the slope of the effective
equation of state, $gamma(z)$, has a minimum, but occurring at different
redshifts. The $T_0$ is higher in non-equilibrium models using UVB obtained
with flatter quasar SEDs. The observed median HeII effective optical depth
evolution and its scatter are well reproduced in equilibrium and
non-equilibrium models even with the uniform UVB assumption. For a given
thermal history the redshift dependence of HI photo-ionization rate derived
from observed HI effective optical depth ($tau_{rm eff,HI}$) are different
for equilibrium or non-equilibrium models. This may lead to different
requirements on the evolution of ionizing emissivities of sources. We show
that, in the absence of strong differential pressure smoothing effects, it is
possible to recover the $T_0$ and $gamma$ in non-equilibrium model from the
equilibrium models generated by rescaling photo-heating rates while producing
the same $tau_{rm eff,HI}$.
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