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 Ly \alpha forest is degenerate with the 3-D structure of the IGM on \sim 100 kpc scales , where , analogous to the classical Jeans argument , the pressure of the T \simeq 10 ^ { 4 } { 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 Ly \alpha flux , F _ { \mathrm { 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 _ { \mathrm { real } } power spectrum is accurately described by a simple fitting function with cutoff at \lambda _ { \mathrm { F } } , allowing us to rigorously quantify the pressure smoothing scale for the first time : we find \lambda _ { \mathrm { 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 Ly \alpha 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 T _ { 0 } and slope \gamma of the temperature-density relation T = T _ { 0 } ( \rho / { \bar { \rho } } ) ^ { \gamma - 1 } should be augmented with a third pressure smoothing scale parameter \lambda _ { \mathrm { F } } .