WHAT DETERMINES THE INCIDENCE AND EXTENT OF Mg II ABSORBING GAS AROUND GALAXIES?

We study the connections between ongoing 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 Mg II absorbers at large galactic radii (greater than or similar to 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 starformation rate (sSFR) for the 94 galaxies described by Chen et al.'s 2010 paper. We find that the extent of Mg II absorbing gas, R-Mg (II), scales with M-star and sSFR, following R-Mg (II) alpha M-star(0.28) x 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 Mg II absorbers arise in infalling clouds which will subsequently fuel star formation in the galaxies. The additional scaling relation of R-Mg (II) 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 Mg II absorbers along common sight lines provides an empirical characterization of halo gaseous radius versus halo mass. A comparison study of R-Mg (II) 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.