We study metal abundances in the z = 0.9313 damped Ly \alpha system observed in the two lines-of-sight , A and B , toward the gravitationally-lensed double QSO HE0512 - 3329 .
Spatially resolved Space Telescope Imaging Spectrograph spectra constrain the neutral-gas column density to be N ( H i ) = 10 ^ { 20.5 } cm ^ { -2 } in both A and B. UV-Visual Echelle Spectrograph spectra ( spectral resolution FWHM = 9.8 km s ^ { -1 } ) show , in contrast , significant line-of-sight differences in the column densities of Mn ii and Fe ii ; these are not due to observational systematics .
We find that [ Mn/H ] = -1.44 and [ Fe/H ] = -1.52 in damped Ly \alpha system A , while [ Mn/H ] -0.98 and [ Fe/H ] > -1.32 , and possibly as high as [ Fe/H ] \approx - 1 in damped Ly \alpha system B .
A careful assessment of possible systematic errors leads us to conclude that these transverse differences are significant at a 5 \sigma level or greater .
Although nucleosynthesis effects may also be at play , we favor differential dust-depletion as the main mechanism producing the observed abundance gradient .
The transverse separation is 5 h ^ { -1 } _ { 70 } kpc at the redshift of the absorber , which is also likely to be the lensing galaxy .
The derived abundances therefore probe two opposite sides of a single galaxy hosting both damped Ly \alpha systems .
This is the first time firm abundance constraints have been obtained for a single damped system probed by two lines-of-sight .
The significance of this finding for the cosmic evolution of metals is discussed .