Lyman-α Heating of Inhomogeneous High-redshift Intergalactic Medium

The intergalactic medium (IGM) prior to the epoch of reionization consists mostly of neutral hydrogen gas. Lyman-α (Lyα) photons produced by early stars resonantly scatter off hydrogen atoms, causing energy exchange between the radiation field and the gas. This interaction results in moderate heating of the gas due to the recoil of the atoms upon scattering, which is of great interest for future studies of the pre-reionization IGM in the H i 21 cm line. We investigate the effect of this Lyα heating in the IGM with linear density, temperature and velocity perturbations. Perturbations smaller than the diffusion length of photons could be damped due to heat conduction by Lyα photons. The scale at which damping occurs and the strength of this effect depend on various properties of the gas, the flux of Lyα photons and the way in which photon frequencies are redistributed upon scattering. To find the relevant length scale and the extent to which Lyα heating affects perturbations, we calculate the gas heating rates by numerically solving linearized Boltzmann equations in which scattering is treated by the Fokker-Planck approximation. We find that (i) perturbations add a small correction to the gas heating rate, and (ii) the damping of temperature perturbations occurs at scales with comoving wavenumber k & 10 Mpc, which are much smaller than the Jeans scale and thus unlikely to substantially affect the observed 21 cm signal. Subject headings: intergalactic medium — large-scale structure of universe — radiative transfer