Measuring the equation of state of the intergalactic medium
Abstract
Numerical simulations indicate that the smooth, photoionized intergalactic medium (IGM) responsible for the low column density Lyα forest follows a welldefined temperaturedensity relation, which is well described by a power law TT_{0}(ρρ)^{γ1}. We demonstrate that such an equation of state results in a powerlaw cutoff in the distribution of linewidths (bparameters) as a function of column density (N) for the low column density (N<~10^{14.5}cm^{2}) absorption lines. This explains the existence of the lower envelope that is clearly seen in scatter plots of the b(N) distribution in observed QSO spectra. Even a strict powerlaw equation of state will not result in an absolute cutoff because of line blending and contamination by unidentified metal lines. We develop an algorithm to determine the cutoff, which is insensitive to these narrow lines. We show that the parameters of the cutoff in the b(N) distribution are strongly correlated with the parameters of the underlying equation of state. We use simulations to determine these relations, which can then be applied to the observed cutoff in the b(N) distribution to measure the equation of state of the IGM. We show that systematics that change the b(N) distribution, such as cosmology (for a fixed equation of state), peculiar velocities, the intensity of the ionizing background radiation and variations in the signaltonoise ratio, do not affect the measured cutoff. We argue that physical processes that have not been incorporated in the simulations, e.g. feedback from star formation, are unlikely to affect the results. Using Monte Carlo simulations of Keck spectra at z=3, we show that determining the slope of the equation of state will be difficult, but that the amplitude can be determined to within 10per cent, even from a single QSO spectrum. Measuring the evolution of the equation of state with redshift will allow us to put tight constraints on the reionization history of the Universe.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 November 1999
 DOI:
 10.1046/j.13658711.1999.02956.x
 arXiv:
 arXiv:astroph/9906271
 Bibcode:
 1999MNRAS.310...57S
 Keywords:

 EQUATION OF STATE;
 HYDRODYNAMICS;
 INTERGALACTIC MEDIUM;
 QUASARS: ABSORPTION LINES;
 COSMOLOGY: THEORY;
 Astrophysics
 EPrint:
 16 pages, 19 figures, accepted for publication in MNRAS