AAS 201st Meeting, January, 2003
Session 67. Cosmic Gas: Long Ago and Far Away
Oral, Tuesday, January 7, 2003, 2:00-3:30pm, 606-607

## [67.04] The Size, Density, and Mass of a Damped Lyman \alpha Absorber from HIRES/Keck Spectra of the Lensed Quasar 0957+561 A,B

C. W. Churchill, R.M. Mellon, J.C. Charlton (Penn State), S.S. Vogt (UCO/Lick/UCSC)

We studied the kinematics, ionization conditions, and physical sizes and masses of the clouds in the z=1.3911 damped Lyman \alpha absorber (DLA) in HIRES/Keck spectra of the lensed quasar 0957+561 A,B. The physical separation of the imaged sightlines is roughly 200h-1 kpc at the DLA. Assuming the clouds are in photoionization equilibrium, we find densities of ~10 cm-3, sizes of ~10 pc, and masses of no more than 500 M\odot (accounting for various cloud geometries); these properties are consistent with low density H I clouds in the ISM. These results imply that the covering factor of the DLA region, that which is associated with the damped neutral hydrogen, is roughly 0.1. \\

There are several velocity components in the Mg I, Mg II, and Fe II absorption lines. As found for several DLAs by other researchers, we find remarkable uniformity in the cloud ionic ratios and physical conditions with velocity. What we also find here is a remarkable spatial uniformity on a scale of 200 pc. \\

We present the observed properties of this DLA and the constraints it places on DLA properties. We point out the implications and quandaries that arise from the emerging picture in which DLAs are comprised of many highly uniform, small, and very low mass clouds. \\

Supported in part by NASA and the NSF.