The Evolution of the Effective Equation of State of the Intergalactic Medium

Abstract

We develop a method to extract the ``effective equation of state'' of the intergalactic medium from the Doppler (b) parameter distribution of the low-density Lyα forest. We test the method on numerical simulations and then apply it to published observations of the Lyα forest at redshifts z~=0-4. We find that the effective equation of state is close to isothermal at redshift z~3, indicating that a second reheating of the intergalactic medium took place at z~3. This reheating can plausibly be identified with the reionization of He II observed to occur at z~3.