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[97.01] Evidence for a Cool Neutral Medium in Damped Lyman-alpha Systems

J.C. Howk (UCSD)

I discuss the use of the relative populations of the fine-structure excited states of Si II and C II as temperature diagnostics in high-redshift damped Lyman-alpha systems (DLAs). The upper ²P_3/2 states of these ions are populated through collisions with electrons and hydrogen as well as through direct pumping by CMB photons. The ratio of Si II* to C II* is dependent on the temperature of the gas, given the difference in the excitation energies for each ion; it depends only weakly on the density (so long as the densities in the absorber are not near the critical density of either ion). I demonstrate the application of this diagnostic, showing that the ISM in high-redshift DLAs must contain a cold neutral phase, implying they may be capable of forming stars.

This work is presented in Howk, Wolfe, and Prochaska (2004).

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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.