  The intervening z=0.7443 Mg II absorption system in the spectrum of MC
1331+170 shows an unusual series of line pairs, each with velocity separations
of 30 km/s. These lines could be explained as the shells of expanding
superbubbles residing in the outer regions of an edge-on spiral galaxy visible
in the optical image of the MC 1331+170 field. The color and brightness of this
galaxy make it the most likely candidate z=0.7443 absorber, though two other
galaxies in the quasar field could also be contributing to the Mg II absorption
profile. Kinematic models of absorption from compact groups and galaxy pairs
produce profiles largely inconsistent with the observed Mg II spectrum.
Superbubbles would naturally generate more regular structures such as those
observed. Photoionization models of the superbubble shell are consistent with
the observed profile for many realistic physical conditions. In a pure
superbubble model, the large velocity spread of the Mg II absorption system is
inconsistent with the expected spread of a quiescent, rotating disk. This
requires unusual kinematics within the host galaxy, perhaps due to a recent
interaction.
