Thermal state of the intergalactic medium at $z\sim 2-4$

Abstract

We present a new method to infer parameters of the temperature-density relation in the intergalactic medium in the post-reionization epoch at $z\sim 2-4$. This method is based on the analysis of the Ly$\alpha$ absorbers distribution over column densities and Doppler parameters by the model joint probability density function. This approach allows us to measure the power-law index $\gamma$ of the temperature-density relation and a certain combination of the temperature at the mean density $T_0$ and hydrogen photoionization rate $\Gamma$. To estimate $T_0$ and $\Gamma$ separately, we employ measurements of the Ly$\alpha$ forest effective opacity and the model gas probability density function. We show that $\gamma$ tends to be lower than 1.6 and reaches 1.3 at redshift $\sim 3$. The inferred temperatures at the mean density are $\sim (2\pm 0.5)\times 10^4$ K in the studied redshift range. Both these estimates favour HeII reionization at $z\gtrsim 3$. We find that the hydrogen photoionization rate is $\sim 0.6\times 10^{-12}$ s${}^{-1}$, which is consistent with previous measurements.

arXiv comment20 pages, 13 figures, 2 tables. Accepted to ApJ

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