Characterizing the pressure smoothing scale of the intergalactic medium

The thermal state of the intergalactic medium (IGM) at z < 6 constrains the nature and timing of cosmic reionization events, but its inference from the Lyα forest is degenerate with the 3D structure of the IGM on ∼100 kpc scales, where, analogous to the classical Jeans argument, the pressure of the T ≃ 10⁴ K gas supports it against gravity. We simulate the IGM using smoothed particle hydrodynamics, and find that, at z < 6, the gas density power spectrum does not exhibit the expected filtering scale cutoff, because dense gas in collapsed halos dominates the small-scale power masking pressure smoothing effects. We introduce a new statistic, the real-space Lyα flux, F_real, which naturally suppresses dense gas, and is thus robust against the poorly understood physics of galaxy formation, revealing pressure smoothing in the diffuse IGM. The F_real power spectrum is accurately described by a simple fitting function with cutoff at λ_F, allowing us to rigorously quantify the pressure smoothing scale for the first time: we find λ_F = 79 kpc (comoving) at z = 3 for our fiducial thermal model. This statistic has the added advantage that it directly relates to observations of correlated Lyα forest absorption in close quasar pairs, recently proposed as a method to measure the pressure smoothing scale. Our results enable one to quantify the pressure smoothing scale in simulations, and ask meaningful questions about its dependence on reionization and thermal history. Accordingly, the standard description of the IGM in terms of the amplitude T₀ and slope γ of the temperature–density relation $T=T_0{\left(\rho /\overline{\rho }\right)}^{\gamma -1}$ should be augmented with a third pressure smoothing scale parameter λ_F.