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

We report results from a survey of Mg ii 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 ρ ≲ 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 − 5log h = −16.4 to M B − 5log h = −21.4 and rest-frame B AB − R AB colors range from B AB − R AB ≈ 0 to B AB − R AB ≈ 1.5. Of these "isolated" galaxies, we find that 47 have corresponding Mg ii absorbers in the spectra of background QSOs and rest-frame absorption equivalent width W r (2796) = 0.1–2.34 Å, and 24 do not give rise to Mg ii 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 Mg ii absorbing halos with the gaseous radius scaled by B -band luminosity according to R gas = 75 × ( 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 〈κ 0.3 〉 ≈ 70% for absorbers of W r (2796) ⩾ 0.3 Å and 〈κ 0.1 〉 ≈ 80% for absorbers of W r (2796) ⩾ 0.1 Å. The results confirm that extended Mg ii 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 Mg ii halos and recent star formation history of the galaxies. Finally, we discuss the total gas mass in galactic halos as traced by Mg ii absorbers. We also compare our results with previous studies.