Abstract
Recent years have brought more precise temperature measurements of the
low-density intergalactic medium (IGM). These new measurements constrain the
processes that heated the IGM, such as the reionization of HI and of HeII. We
present a semi-analytical model for the thermal history of the IGM that follows
the photoheating history of primordial gas. We compare this model with recent
temperature measurements spanning z= 1.6-4.8, finding that these measurements
are consistent with scenarios in which the HeII was reionized at z= 3-4 by
quasars. Significantly longer duration or higher redshift HeII reionization
scenarios are ruled out by the measurements. For hydrogen reionization, we find
that only low redshift and high temperature scenarios are excluded. For
example, a model in which the IGM was heated to 30,000K when an ionization
front passed, and with hydrogen reionization occurring over 6<z<9, is ruled
out. Finally, we place constraints on how much heating could owe to TeV
blazars, cosmic rays, and other nonstandard mechanisms. We find that by z= 2 a
maximum of 1~eV of additional heat could be injected per baryon over standard
photoheating-only models, with this limit becoming ~< 0.5 eV at z>3.
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