  Every star-forming galaxy has a halo of metal-enriched gas extending out to
at least 100 kpc, as revealed by the absorption lines this gas imprints on the
spectra of background quasars. However, quasars are sparse and typically probe
only one narrow pencil beam through the intervening galaxy. Close quasar pairs
and gravitationally lensed quasars have been used to circumvent this inherently
one-dimensional technique, but these objects are rare and the structure of the
circum-galactic medium remains poorly constrained. As a result, our
understanding of the physical processes that drive the re-cycling of baryons
across the lifetime of a galaxy is limited. Here we report integral-field
(tomographic) spectroscopy of an extended background source -a bright giant
gravitational arc. We can thus coherently map the spatial and kinematic
distribution of Mg II absorption -a standard tracer of enriched gas- in an
intervening galaxy system at redshift 0.98 (i.e., ~8 Gyr ago). Our
gravitational-arc tomography unveils a clumpy medium in which the
absorption-strength decreases with increasing impact parameter, in good
agreement with the statistics towards quasars; furthermore, we find strong
evidence that the gas is not distributed isotropically. Interestingly, we
detect little kinematic variation over a projected area of ~600 kpc squared,
with all line-of-sight velocities confined to within a few tens of km/s of each
other. These results suggest that the detected absorption originates from
entrained recycled material, rather than in a galactic outflow.
