What Determines the Incidence and Extent of MgII Absorbing Gas Around
  Galaxies?
  We study the connections between on-going star formation, galaxy mass, and
extended halo gas, in order to distinguish between starburst-driven outflows
and infalling clouds that produce the majority of observed MgII absorbers at
large galactic radii (>~ 10 h^{-1} kpc) and to gain insights into halo gas
contents around galaxies. We present new measurements of total stellar mass
(M_star), H-alpha emission line strength (EW(H-alpha)), and specific star
formation rate (sSFR) for the 94 galaxies published in H.-W. Chen et al.
(2010). We find that the extent of MgII absorbing gas, R_MgII, scales with
M_star and sSFR, following R_MgII \propto M_star^{0.28}\times sSFR^{0.11}. The
strong dependence of R_MgII on M_star is most naturally explained, if more
massive galaxies possess more extended halos of cool gas and the observed MgII
absorbers arise in infalling clouds which will subsequently fuel star formation
in the galaxies. The additional scaling relation of R_MgII with sSFR can be
understood either as accounting for extra gas supplies due to starburst
outflows or as correcting for suppressed cool gas content in high-mass halos.
The latter is motivated by the well-known sSFR--M_star} inverse correlation in
field galaxies. Our analysis shows that a joint study of galaxies and MgII
absorbers along common sightlines provides an empirical characterization of
halo gaseous radius versus halo mass. A comparison study of R_MgII around red-
and blue-sequence galaxies may provide the first empirical constraint for
resolving the physical origin of the observed sSFR--M_star} relation in
galaxies.
