Equation of state of the photoionized intergalactic medium

Abstract

We develop an efficient method to study the effects of reionization history on the temperature-density relation of the intergalactic medium in the low density limit (overdensity δ < ∼ 5). It is applied to the study of photo-reionization models in which the amplitude, spectrum and onset epoch of the ionizing flux, as well as the cosmology, are systematically varied. We find that the mean temperature-density relation at z = 2 − 4 is well approximated by a powerlaw equation of state for uniform reionization models. We derive analytical expressions for its evolution and exhibit its asymptotic behavior: it is found that for sufficiently early reionization, imprints of reionization history prior to z ∼ 10 on the temperature-density relation are washed out. In this limit the temperature at cosmic mean density is proportional toWhile the amplitude of the radiation flux at the ionizing frequency of H i is found to have a negligible effect on the temperature-density relation as long as the universe reionizes before z ∼ 5, the spectrum can change the overall temperature by about 20%, through variations in the abundances of helium species. However the slope of the mean equation of state is found to lie within a narrow range for all reionization models we study, where reionization takes place before z ∼ 5. We discuss the implications of these findings for the observational properties of the Lyα forest. In particular, uncertainties in the temperature of the intergalactic medium, due to the uncertain reionization history of our universe, introduces a 30% scaling in the amplitude of the column density distribution while the the slope of the distribution is only affected by about 5%. Finally, we discuss how a fluctuating ionizing field affects the above results. We argue that under certain conditions, the loss of memory c 0000 RAS 2 Hui and Gnedin of reionization history implies that at late times, the temperature-density relation of a gas in a fluctuating ionizing background can be approximated by one that results from a uniform radiation field, provided the universe reionizes sufficiently early.

Warm gas in and around simulated galaxy clusters as probed by absorption lines

“…As expected, the HI absorbers arise primarily from the first phase, with most absorbers below about NHI < 10 14 cm −2 arising from gas which shows a clear density-temperature relation (Hui & Gnedin, 1997). For column densities above about 10 17 cm −2 , the absorbers arise almost entirely from dense, cold gas which we expect to be in or near galaxies.…”

Observational probes of cosmic acceleration

“…The Lyα forest is described to surprisingly good accuracy by the Fluctuating Gunn-Peterson Approximation (FGPA, Weinberg et al 1998; see also Gunn and Peterson 1965;Rauch et al 1997;Croft et al 1998), which relates the transmitted flux F = exp(−τ Lyα ) to the dark matter overdensity ρ/ρ, with the latter smoothed on approximately the Jeans scale of the diffuse intergalactic medium (IGM) where gas pressure supports the gas against gravity (Schaye, 2001). Most gas in the low density IGM follows a power-law relation between temperature and density, T = T 0 (ρ/ρ) α with α 0.6, which arises from the competition between photo-ionization heating and adiabatic cooling (Katz et al, 1996;Hui and Gnedin, 1997). The Lyα optical depth is proportional to the hydrogen recombination rate, which scales as ρ 2 T −0.7 in the relevant temperature range near 10 4 K. This line of argument leads to the relation…”

On the intergalactic temperature–density relation

“…We first specialize to the simplified case of an IGM with only photoheating and adiabatic processes, the limit considered in Hui & Gnedin (1997). While we saw in §2 that this limit is not a good approximation for the evolution of T0 at high redshifts owing to Compton cooling off of the CMB, this limit is a better approximation for the evolution of γ − 1 (Fig.…”

“…While the mathematics of what sets the asymptotic temperature and its power-law slope in density were worked out in the seminal study of Hui & Gnedin (1997), their linearized approach does not explain why the same powerlaw holds over a range of ∼ 100 in density. Subsequently, ⋆ mcquinn@uw.edu 1 Most studies that use numerical simulations implicitly adopt such a parametrization, as standard cosmological simulations employ uniform radiation backgrounds -an approximation which guarantees a near power-law relation.…”

“…When necessary, we assume a flat ΛCDM Universe with h = 0.7, Ωm = 0.3, Ω b = 0.045, and YHe = 0.25. Following Hui & Gnedin (1997), we use the Zeldovich approximation to follow the density (necessary for computing the temperature) of fluid elements for certain calculations. Our Zeldovich approximation calculations draw matter elements randomly from the probability distribution of collapse eigenvalues, assuming that the gas traces the dark matter.…”