On the nature of the heat source for damped Lyα systems

We investigate the heat source of the neutral gas comprising damped Lyα systems. Unlike the Lyα forest, where the extragalactic background radiation field ionizes and heats the gas, we find that grain photoelectric heating by the far-ultraviolet (FUV) background is not sufficient to balance the [C II] 158 μm cooling rate inferred from damped Lyα systems where C II* absorption is detected. In these systems, a local energy source is required. We show that in the case of the z = 1.919 damped Lyα system toward Q2206-19, the local source is FUV emission from the associated galaxy found by Møller et al. (2002): the mean intensity inferred from photometry is in good agreement with the intensity J _v^stars required to explain the cooling rate. The FUV mean intensity predicted for a cold neutral medium (CNM) model, J_v ^stars = (1.7_-1.0^+2.7) × 10^-18 ergs cm^-2 s^-1 Hz^-1 sr^-1 (95% c.l.), is the largest expected from our C II* study of 45 damped Lyα systems. This may explain why this is the only confirmed damped Lyα system yet detected in emission at z > 1.9. We argue that in most damped Lyα systems with detected C II* absorption, J_v ^stars is between 10^-19 and 10^-18 ergs cm ^-2 s^-1 Hz^-1 sr^-1 and heats the gas that is a CNM. By contrast, in most damped Lyα systems with upper limits on C II* absorption the gas is a warm neutral medium (WNM). Surprisingly, the upper limits are compatible with the same range of J_v ^stars values, suggesting that the majority of damped Lyα systems are heated by radiation fields generated by a limited range of star formation rates per unit area, between 10^-3 and 10^-2 M_⊙ yr^-1 kpc^-2. We also show that C II* absorption is unlikely to arise in gas that is ionized.