On the nature of the heat source for damped Lyα systems

We investigate the heat source of the neutral gas comprising damped Lyalpha systems. Unlike the Lyalpha 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 mum cooling rate inferred from damped Lyalpha 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 Lyalpha system toward Q2206 - 19, the local source is FUV emission from the associated galaxy found by Moller et al. ( 2002): the mean intensity inferred from photometry is in good agreement with the intensity J(nu)(stars) required to explain the cooling rate. The FUV mean intensity predicted for a cold neutral medium (CNM) model, J(nu)(stars) = (1.7(-1.0)(+2.7)) x 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 Lyalpha systems. This may explain why this is the only confirmed damped Lyalpha system yet detected in emission at z > 1.9. We argue that in most damped Lyalpha systems with detected C I*I absorption, J(nu)(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 Lyalpha 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(nu)(stars) values, suggesting that the majority of damped Lyalpha 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.