CHARACTERIZING THE PRESSURE SMOOTHING SCALE OF THE INTERGALACTIC MEDIUM

Abstract

The thermal state of the intergalactic medium (IGM) at z < 6 constrains the\nnature and timing of cosmic reionization events, but its inference from the\nLy-alpha forest is degenerate with the 3-D structure of the IGM on ~100 kpc\nscales, where, analogous to the classical Jeans argument, the pressure of the\nT~$10^4$ K gas supports it against gravity. We simulate the IGM using smoothed\nparticle hydrodynamics, and find that, at z < 6, the gas density power spectrum\ndoes not exhibit the expected Jeans filtering cutoff, because dense gas in\ncollapsed halos dominates the small-scale power masking pressure smoothing\neffects. We introduce a new statistic, the real-space Ly-alpha flux,\n$F_\\mathrm{real}$, which naturally suppresses dense gas, and is thus robust\nagainst the poorly understood physics of galaxy formation, revealing pressure\nsmoothing in the diffuse IGM. The $F_\\mathrm{real}$ power spectrum is\naccurately described by a simple fitting function with cutoff at $\\lambda_F$,\nallowing us to rigorously quantify the filtering scale for the first time: we\nfind $\\lambda_F$ = 79 kpc (comoving) at z=3 for our fiducial thermal model.\nThis statistic has the added advantage that it directly relates to observations\nof correlated Ly-alpha forest absorption in close quasar pairs, recently\nproposed as a method to measure the filtering scale. Our results enable one to\nquantify the filtering scale in simulations, and ask meaningful questions about\nits dependence on reionization and thermal history. Accordingly, the standard\ndescription of the IGM in terms of the amplitude $T_0$ and slope $\\gamma$ of\nthe temperature-density relation $T = T_0\\Delta^{\\gamma-1}$ should be augmented\nwith a third filtering scale parameter $\\lambda_F$.\n