The thermal history of the intergalactic medium down to redshift z=1.5: a new curvature measurement

Abstract

According to the photo-heating model of the intergalactic medium (IGM), He II reionization is expected to affect its thermal evolution. Evidence for additional energy injection into the IGM has been found at $3\lesssim z\lesssim 4$, though the evidence for the subsequent fall-off below $z\sim 2.8$ is weaker and depends on the slope of the temperature--density relation, $\gamma$. Here we present, for the first time, an extension of the IGM temperature measurements down to the atmospheric cut-off of the H I Lyman-$\alpha$ forest at $z\simeq 1.5$. Applying the curvature method on a sample of 60 UVES spectra we investigated the thermal history of the IGM at $z<3$ with precision comparable to the higher redshift results. We find that the temperature of the cosmic gas traced by the Ly-$\alpha$ forest [$T(\bar{\Delta })]$ increases for increasing overdensity from $T(\bar{\Delta })\sim 22670$ K to 33740 K in the redshift range $z\sim 2.8-1.6$. Under the assumption of two reasonable values for $\gamma$, the temperature at the mean density ($T_0$) shows a tendency to flatten at $z\lesssim 2.8$. In the case of $\gamma \sim 1.5$, our results are consistent with previous ones which indicate a falling $T_0$ for redshifts $z\lesssim 2.8$. Finally, our $T(\bar{\Delta })$ values show reasonable agreement with moderate blazar heating models.

19 pages, 21 figures, accepted for publication in MNRAS

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