AN EMPIRICAL CHARACTERIZATION OF EXTENDED COOL GAS AROUND GALAXIES USING Mg II ABSORPTION FEATURES

We report results from a survey of MgII absorbers in the spectra of background quasi-stellar objects (QSOs) that are within close angular distances to a foreground galaxy at z < 0.5, using the Magellan Echellette Spectrograph. We have established a spectroscopic sample of 94 galaxies at a median redshift of < z > = 0.24 in fields around 70 distant background QSOs (z(QSO) > 0.6), 71 of which are in an isolated environment with no known companions and located at rho less than or similar to 120 h(-1) kpc from the line of sight of a background QSO. The rest-frame absolute B-band magnitudes span a range from M(B) - 5 log h = -16.4 to M(B) - 5 log h = -21.4 and rest-frame B(AB) - R(AB) colors range from B(AB) - R(AB) approximate to 0 to B(AB) - R(AB) approximate to 1.5. Of these isolated galaxies, we find that 47 have corresponding MgII absorbers in the spectra of background QSOs and rest-frame absorption equivalent width W(r)(2796) = 0.1-2.34 angstrom, and 24 do not give rise to MgII absorption to sensitive upper limits. Our analysis shows that (1) W(r) (2796) declines with increasing distance from isolated galaxies but shows no clear trend in group environments; (2) more luminous galaxies possess more extended MgII absorbing halos with the gaseous radius scaled by B-band luminosity according to R(gas) = 75 x (L(B)/L(B))((0.35 +/- 0.03)) h(-1) kpc; (3) there is little dependence between the observed absorber strength and galaxy intrinsic colors; and (4) within R(gas), we find a mean covering fraction of approximate to 70% for absorbers of W(r)(2796) >= 0.3 angstrom and approximate to 80% for absorbers of W(r) (2796) >= 0.1 angstrom. The results confirm that extended MgII absorbing halos are a common and generic feature around ordinary galaxies and that the gaseous radius is a fixed fraction of the dark matter halo radius. The lack of correlation between W(r) (2796) strength and galaxy colors suggests a lack of physical connection between the origin of extended MgII halos and recent star formation history of the galaxies. Finally, we discuss the total gas mass in galactic halos as traced by MgII absorbers. We also compare our results with previous studies.