Abstract
According to the photoheating 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 ≲ z ≲ 4, though the evidence for the subsequent fall-off below z ∼ 2.8 is weaker and depends on the slope of the temperature–density relation, γ. 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-α (Lyα) forest at z ≃ 1.5. Applying the curvature method on a sample of 60 Ultraviolet and Visual Echelle Spectrograph (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 Lyα forest [|$T(\bar{\Delta })]$| increases for increasing overdensity from |$T(\bar{\Delta })\sim 22670$| to 33740 K in the redshift range z ∼ 2.8–1.6. Under the assumption of two reasonable values for γ, the temperature at the mean density (T0) shows a tendency to flatten at z ≲ 2.8. In the case of γ ∼ 1.5, our results are consistent with previous ones which indicate a falling T0 for redshifts z ≲ 2.8. Finally, our |$T(\bar{\Delta })$| values show reasonable agreement with moderate blazar heating models.
Highlights
Starting from a very hot plasma made of electrons and protons after the big bang, to the gas that fills the space between galaxies, the intergalactic medium (IGM) has been one of the main ‘recorders’ of the different phases of evolution of the Universe
According to many models of the former, the He II reionization should have left a footprint in the thermal history of the IGM: during this event, considerable additional heat is expected to increase the temperature at the mean density of the cosmic gas (T0) at z 4 (Hui & Gnedin 1997)
Even if some evidence has been found for an increase in the temperature at the mean density from z ∼ 4 down to z ∼ 2.1 (e.g. Becker et al 2011), the subsequent change in the evolution of T0 expected after the end of the He II reionization has not been clearly characterized yet and remains strongly degenerate with the imprecisely constrained slope of the temperature–density relation γ
Summary
Starting from a very hot plasma made of electrons and protons after the big bang, to the gas that fills the space between galaxies, the intergalactic medium (IGM) has been one of the main ‘recorders’ of the different phases of evolution of the Universe. The IGM thermal history can be an important source of information about reionizing processes that injected vast amounts of energy into this gas on relatively short time-scales. For this reason considerable efforts have been made to find any ‘footprint’ of either H I (z < 6) or He II (z < 3) reionization. Because the ionization potential of He II (from He II to He III) is 54.4 eV and fully ionized helium recombines more than five times faster than ionized hydrogen, this second reionization event should have begun later, after the reionization of hydrogen and He I (11 z 6; Becker et al 2001; Fan, Carilli & Keating 2006; Larson et al 2011) when quasars (QSOs) started to dominate the ultraviolet background (UVB; Miralda-Escude, Haehnelt & Rees 2000). The much harder photons from QSOs would have been able to fully ionize He II around redshifts 3 z 4.5 but these
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