A newly discovered DLA and associated Ly-alpha emission in the spectra
  of the gravitationally lensed quasar UM 673A,B
  The sightline to the brighter member of the gravitationally lensed quasar
pair UM 673A,B intersects a damped Lyman-alpha system (DLA) at z = 1.62650
which, because of its low redshift, has not been recognised before. Our high
quality echelle spectra of the pair, obtained with HIRES on the Keck I
telescope, show a drop in neutral hydrogen column density N(H I) by a factor of
at least 400 between UM 673A and B, indicating that the DLA's extent in this
direction is much less than the 2.7 kpc separation between the two sightlines
at z = 1.62650. By reassessing this new case together with published data on
other quasar pairs, we conclude that the typical size (radius) of DLAs at these
redshifts is R ~ (5 +/- 3) kpc, smaller than previously realised. Highly
ionized gas associated with the DLA is more extended, as we find only small
differences in the C IV absorption profiles between the two sightlines.
  Coincident with UM 673B, we detect a weak and narrow Ly-alpha emission line
which we attribute to star formation activity at a rate SFR >~ 0.2 M_solar/yr.
The DLA in UM 673A is metal-poor, with an overall metallicity Z_DLA ~ 1/30
Z_solar, and has a very low internal velocity dispersion. It exhibits some
apparent peculiarities in its detailed chemical composition, with the elements
Ti, Ni, and Zn being deficient relative to Fe by factors of 2-3. The [Zn/Fe]
ratio is lower than those measured in any other DLA or Galactic halo star,
presumably reflecting somewhat unusual previous enrichment by stellar
nucleosynthesis. We discuss the implications of these results for the nature of
the galaxy hosting the DLA.
