Abstract
We present a post-processing tool for GADGET-2 simulations to model various
observed properties of the Ly$\alpha$ forest at $2 z 4$ that enables
an efficient parameter estimation. In particular, we model the thermal and
ionization histories that are not computed self-consistently by default in
GADGET-2. We capture the effect of pressure smoothing by running GADGET-2 at an
elevated temperature floor and using an appropriate smoothing kernel. We
validate our procedure by comparing different statistics derived from our
method with those derived using self-consistent simulations with GADGET-3.
These statistics are: line of sight density field power spectrum (PS), flux
probability distribution function (PDF), flux PS, wavelet statistics, curvature
statistics, HI column density ($N_HI$) distribution function, linewidth
($b$) distribution and $b$ versus $N_HI$ scatter. For the
temperature floor of $10^4$ K and typical signal-to-noise of 25, the results
agree well within $1\sigma$ level. Moreover for a given cosmology, we gain a
factor of $N$ in computing time for modelling the intergalactic medium
under $N 1$ different thermal histories. In addition, our method allows us
to simulate the non-equilibrium evolution of thermal and ionization state of
the gas and include heating due to non-standard sources like cosmic rays and
high energy $\gamma$-rays from Blazars.
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