AAS 199th meeting, Washington, DC, January 2002
Session 29. Intergalactic Medium and QSO Absorption Line Systems
Oral, Monday, January 7, 2002, 10:00-11:30am, Georgetown West

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[29.04] What Are Damped Lyman-Alpha Absorbers?

J.C. Charlton (Penn State)

There is a puzzle in the overall picture of quasar absorption line systems. Damped Lyman-alpha absorption traces regions with the very highest column densities of neutral hydrogen. However, at low redshifts they are located in a variety of galaxy morphologies; dwarf and and low surface brightness galaxy hosts are as common as luminous galaxies (>0.05L*). On the other hand, absorbers with more modest neutral hydrogen column densities, i.e. the Lyman limit absorbers, are almost always found within 40 kiloparsecs of luminous galaxies. Dwarf and low surface brightness galaxies apparently do not present a significant cross-section for Lyman limit absorption, but they do produce a substantial fraction of the damped Lyman-alpha absorption which is much less common.

I hypothesize that damped Lyman-alpha absorption is typically produced within a relatively high density pocket of material in its host. This hypothesis is supported by the observation of narrow 21-cm emission components in several DLAs, and by the presence of a separate very narrow low ionization phase (Doppler parameter < 1 km/s) required to explain the column density ratio of Mg I to Mg II. Dwarf galaxy hosts of DLAs would be unable to retain the majority of their gas after the initial burst of star formation, thus most regions that would produce Lyman limit absorption would be evacuated. However, the denser DLA pockets would remain behind, explaining the differences between the hosts of the DLA and Lyman limit absorber populations.

This research is funded by NASA and by the National Science Foundation.

The author(s) of this abstract have provided an email address for comments about the abstract: charlton@astro.psu.edu

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