Abstract:
The sightline to the brighter member of the gravitationally lensed quasar pair UM 673A,B intersects a damped Lyman-alpha system (DLA) at z ~ 1.6.The neighbouring sightline, however, exhibits a drop in neutral Hydrogencolumn density by at least a factor of 400, over just 3 kpc. By reassessing this new case together with published data on other QSO pairs,we constrain the typical size of these neutral gas reservoirs at moderate redshifts. Furthermore, we detect a weak and narrow Lyman-alpha emission line which we attribute to star formation activity in the host galaxy of the DLA. We also find the DLA in UM 673A to be metal-poor, with an overall metallicity Z_DLA ~ 1/30 Z_solar. By studying the properties of these low-metallicity systems at high redshift, we may uncover some important clues into the nucleosynthesis from some of the first structures to form in the Universe.