Metal Abundances in a Damped Lyα System along Two Lines of Sight atz= 0.93

We study metal abundances in the z=0.9313 damped Lya system observed in thetwo lines-of-sight, A and B, toward the gravitationally-lensed double QSOHE0512-3329. Spatially resolved STIS spectra constrain the neutral-gas columndensity to be LogN(HI)=20.5 in both Aand B. UVES spectra (spectral resolutionFWHM=9.8 km/s) show, in contrast, significant line-of-sight differences in thecolumn densities of MnII and FeII; these are not due to observationalsystematics. We find that [Mn/H]=-1.44 and [Fe/H]=-1.52 in damped Lya system A,while [Mn/H]=-0.98 and [Fe/H]>-1.32, and possibly as high as [Fe/H] approx. -1in damped Lya system B. A careful assessment of possible systematic errorsleads us to conclude that these transverse differences are significant at a 5sigma level or greater. Although nucleosynthesis effects may also be at play,we favor differential dust-depletion as the main mechanism producing theobserved abundance gradient. The transverse separation is 5 kpc at the redshiftof the absorber, which is also likely to be the lensing galaxy. The derivedabundances therefore probe two opposite sides of a single galaxy hosting bothdamped Lya systems. This is the first time firm abundance constraints have beenobtained for a single damped system probed by two lines-of-sight. Thesignificance of this finding for the cosmic evolution of metals is discussed.Comment: In press at Ap