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[129.02] Spectroscopic signatures of star formation in Damped Ly\alpha absorbers

G. Shaw (Univerity of Kentucky), G. J. Ferland (University of Kentucky), R. Srianand (IUCAA)

Damped Ly\alpha Absorbers (DLAs) are those absorption-line systems with large neutral hydrogen column density, N(H I) \geq 2\times10²⁰cm^-2, seen against the emission of background QSOs. They are believed to be the progenitors of present day disk galaxies. In our galaxy or in the Magellanic clouds, all the clouds with N(H I)\geq 2\times10²⁰cm^-2, have N(H₂)> 10¹⁹cm^-2. But roughly 80% of the DLAs do not have detected H₂. We use chemistry and spectroscopy of molecular hydrogen, together with the fine structure excitations of C I and C II, to determine the physical properties of those z ~ 2 DLAs with molecular hydrogen. We expanded the plasma simulation code Cloudy to do all the relevant formation, destruction, and physical processes involving molecular hydrogen self consistently with the thermal and ionization equilibrium of the gas. We find that the QSO dominated metagalactic radiation is sufficient to maintain a low H₂ column density for gas density < 0.1 cm^-3. For denser gas, a local radiation field, likely due to the star formation, is needed to reproduce the small H₂ column densities.

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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.