Abstract
In the low density intergalactic medium (IGM) that gives rise to the Lyman-alpha forest, gas temperature and density are tightly correlated. The velocity scale of thermal broadening and the Hubble flow across the gas Jeans scale are of similar magnitude (Hlambda_J ~ sigma_th). To separate the effects of gas pressure support and thermal broadening on the Lya forest, we compare spectra extracted from two smoothed particle hydrodynamics (SPH) simulations evolved with different photoionization heating rates (and thus different Jeans scales), imposing different temperature-density relations on the evolved particle distributions. The turnover scales in the flux power spectrum and flux autocorrelation function are determined mainly by thermal broadening rather than pressure. However, the insensitivity to pressure arises partly from a cancellation effect with a sloped temperature-density relation (T ~ rho^{0.6} in our simulations): the high density peaks in the colder, lower pressure simulation are less smoothed by pressure support than in the hotter simulation, and it is this higher density gas that experiences the strongest thermal broadening. Changes in thermal broadening and pressure support have comparably important effects on the flux probability distribution (PDF), which responds directly to the gas overdensity distribution rather than the scale on which it is smooth. Tests on a lower resolution simulation show that our statistical results are converged even at this lower resolution. While thermal broadening generally dominates the longitudinal structure in the Lya forest, we show in Paper II that pressure support determines the transverse coherence of the forest observed towards close quasar pairs. [ABRIDGED]
Highlights
The Lyman-α forest, caused by Lyα absorption of neutral hydrogen atoms along the line of sight to some distant source, was originally described in terms of discrete intervening gas “clouds” (Lynds 1971; Sargent et al 1980), analogous to clouds in the Galactic interstellar medium
We have investigated the relative importance of pressure support and thermal broadening in determining the longitudinal structure of the Lyman-α forest
We have imposed different temperature-density relations on the simulation outputs to isolate physical effects, extracted spectra from the dark matter distribution to extend our investigation to the pressureless case, and compared the fiducial simulation (2 × 2883 particles) to a lower resolution simulation (2 × 1443) to quantify numerical resolution effects
Summary
The Lyman-α forest, caused by Lyα absorption of neutral hydrogen atoms along the line of sight to some distant source (usually a quasar), was originally described in terms of discrete intervening gas “clouds” (Lynds 1971; Sargent et al 1980), analogous to clouds in the Galactic interstellar medium. Because the Jeans length is expected to vary with the gas temperature and density, a Zel’dovich-like scheme can be used when smoothing the dark matter distribution in order to model these effects (Viel et al 2002). Because the higher pressure associated with hotter gas would smooth the IGM in three dimensions, using closely paired lines of sight to probe this coherence scale has been proposed as an alternative route for inferring the temperature-density relation While here we find that thermal broadening dominates pressure support in setting the level of longitudinal structure in the Lyα forest, in Paper II we show that the gas Jeans length dominates the level of coherence transverse to the line of sight.
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