THE BARYON CONTENT OF DARK MATTER HALOS: EMPIRICAL CONSTRAINTS FROM Mg II ABSORBERS

We study the extent and covering fraction of cool baryons around galaxies of different luminosity and mass, based on a survey of Mg II lambda lambda 2796, 2803 absorption features near known galaxies. The initial sample consists of 13 galaxy and absorber pairs and 10 galaxies that do not produce Mg II absorption lines to within sensitive upper limits. The redshifts of the galaxy and absorber pairs range from z = 0.2067 to 0.892 with a median of z = 0.3818. We find that galaxies at larger impact parameters produce on average weaker Mg II absorbers. This anticorrelation is substantially improved when accounting for the intrinsic luminosities of individual galaxies. In addition, there exists a distinct boundary at rho - R-gas, beyond which no Mg II absorbers are found. A maximum likelihood analysis shows that the observations are best described by an isothermal density profile and a scaling relation R-gas = 91(L-B/L-B double dagger)(0.35 +/- 0.05) h(-1) kpc [or 69 h(-1) kpc at W(2796) -0.3 angstrom] with a mean covering factor of - 80%-86%. Together with the scaling relation between halo mass and galaxy luminosity inferred from halo occupation studies, this scaling of Rgas indicates that gas radius is a fixed fraction of the dark matter halo radius. We compare our results with previous studies and discuss the implications of our analysis for constraining the baryon content of galactic halos and for discriminating between competing scenarios for understanding the nature of the extended gas.