The Lyman α forest in a blazar-heated Universe

Abstract

It has been realised only recently that TeV emission from blazars can\nsignificantly heat the intergalactic medium (IGM) by pair-producing high-energy\nelectrons and positrons, which in turn excite vigorous plasma instabilities,\nleading to a local dissipation of the pairs' kinetic energy. In this work, we\nuse cosmological hydrodynamical simulations to model the impact of this blazar\nheating on the Lyman-alpha forest at redshifts z~2-3. We find that blazar\nheating produces an inverted temperature-density relation in the IGM and\nnaturally resolves many of the problems present in previous simulations of the\nforest that included photoheating alone. In particular, our simulations with\nblazar heating simultaneously reproduce the observed effective optical depth\nand temperature as a function of redshift, the observed probability\ndistribution functions of the transmitted flux, and the observed flux power\nspectra, over the full redshift range 2<z<3 analysed here. Additionally, by\ndeblending the Lyman-alpha forest into a sum of thermally broadened individual\nlines, we find superb agreement with the observed lower cutoff of the\nline-width distribution and abundances of neutral hydrogen column densities.\nUsing the most recent constraints on the cosmic ultraviolet (UV) background,\nthis excellent agreement with observations does not require rescaling the\namplitude of the UV background; a procedure that was routinely used in the past\nto match the observed level of transmitted flux. We also show that our\nblazar-heated model matches the data better than standard simulations even when\nsuch a rescaling is allowed. This concordance between Lyman-alpha data and\nsimulations, which are based on the most recent cosmological parameters,\nsuggests that the inclusion of blazar heating alleviates previous tensions on\nconstraints for sigma_8 derived from Lyman-alpha measurements and other\ncosmological data. [abridged]\n