A Kennicutt-Schmidt Law for Intervening Absorption Line Systems
  We argue that most strong intervening metal absorption line systems, where
the rest equivalent width of the MgII 2796A line is >0.5A, are interstellar
material in, and outflowing from, star-forming disks. We show that a version of
the Kennicutt-Schmidt law is readily obtained if the MgII equivalent widths are
interpreted as kinematic broadening from absorbing gas in outflowing winds
originating from star-forming galaxies. Taking a phenomenological approach and
using a set of observational constraints available for star-forming galaxies,
we are able to account for the density distribution of strong MgII absorbers
over cosmic time. The association of intervening material with star-forming
disks naturally explains the metallicity and dust content of strong MgII
systems as well as their high HI column densities, and does not require the
advection of metals from compact star-forming regions into the galaxy halos to
account for the observations. We find that galaxies with a broad range of
luminosities can give rise to absorption of a given rest-equivalent width, and
discuss possible observational strategies to better quantify true
galaxy-absorber associations and further test our model. We show that the
redshift evolution in the density of absorbers closely tracks the star
formation history of the universe and that strong intervening systems can be
used to directly probe the physics of both bright and faint galaxies over a
broad redshift range. By identifying strong intervening systems with galaxy
disks and quantifying a version of the Kennicutt-Schmidt law that applies to
them, a new probe of the interstellar medium is found which provides
complementary information to that obtained through emission studies of
galaxies. Implications of our results for galaxy feedback and enrichment of the
intergalactic medium are discussed. [abridged]
