Large-Scale Structure Shocks at Low and High Redshifts

Cosmological simulations show that, at the present time, a substantial fraction of the gas in the intergalactic medium (IGM) has been shock-heated to T>10^5 K. Here we develop an analytic model to describe the fraction of shocked, moderately overdense gas in the IGM. The model is an extension of the Press & Schechter (1974) description for the mass function of halos: we assume that large-scale structure shocks occur at a fixed overdensity during nonlinear collapse. This in turn allows us to compute the fraction of gas at a given redshift that has been shock-heated to a specified temperature. We show that, if strong shocks occur at turnaround, our model provides a reasonable description of the temperature distribution seen in cosmological simulations at z~0, although it does overestimate the importance of weak shocks. We then apply our model to shocks at high redshifts. We show that, before reionization, the thermal energy of the IGM is dominated by large-scale structure shocks (rather than virialized objects). These shocks can have a variety of effects, including stripping ~10% of the gas from dark matter minihalos, accelerating cosmic rays, and creating a diffuse radiation background from inverse Compton and cooling radiation. This radiation background develops before the first stars form and could have measurable effects on molecular hydrogen formation and the spin temperature of the 21 cm transition of neutral hydrogen. Finally, we show that shock-heating will also be directly detectable by redshifted 21 cm measurements of the neutral IGM in the young universe.Comment: 12 pages, 8 figures, submitted to Ap

Compton Heating of the Intergalactic Medium by the Hard X-ray Background

High-resolution hydrodynamics simulations of the Ly-alpha forest in cold dark matter dominated cosmologies appear to predict line widths that are substantially narrower than those observed. Here we point out that Compton heating of the intergalactic gas by the hard X-ray background (XRB), an effect neglected in all previous investigations, may help to resolve this discrepancy. The rate of gain in thermal energy by Compton scattering will dominate over the energy input from hydrogen photoionization if the XRB energy density is 0.2x/ times higher than the energy density of the UV background at a given epoch, where x is the hydrogen neutral fraction in units of 1e-6 and is the mean X-ray photon energy in units of m_ec^2. The numerical integration of the time-dependent rate equations shows that the intergalactic medium approaches a temperature of about 1.5e4 K at z>3 in popular models for the redshift evolution of the extragalactic background radiation. The importance of Compton heating can be tested experimentally by measuring the Ly-alpha line-width distribution as a function of redshift, thus the Lyman-alpha forest may provide a useful probe of the evolution of the XRB at high redshifts.Comment: LaTeX, 10 pages, 2 figures, final version to be published in the Ap