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

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 a parts per thousand(2) z a parts per thousand(2) 4, though the evidence for the subsequent fall-off below z similar to 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 (Ly alpha) forest at z a parts per thousand integral 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 Ly alpha forest [T((Delta) over bar)] increases for increasing overdensity from T((Delta) over bar) similar to 22670 to 33740 K in the redshift range z similar to 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 a parts per thousand(2) 2.8. In the case of gamma similar to 1.5, our results are consistent with previous ones which indicate a falling T-0 for redshifts z a parts per thousand(2) 2.8. Finally, our T((Delta) over bar) values show reasonable agreement with moderate blazar heating models.