We report results from a survey of Mg II absorbers in the spectra of background 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 \langle z \rangle = 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 \lesssim 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 } \approx 0 to B _ { AB } - R _ { AB } \approx 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 \times ( L _ { B } / L _ { B _ { * } } ) ^ { ( 0.35 \pm 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 \langle \kappa _ { 0.3 } \rangle \approx 70 % for absorbers of \mbox { $W _ { r } ( 2796 ) $ } \geq 0.3 Å and \langle \kappa _ { 0.1 } \rangle \approx 80 % for absorbers of \mbox { $W _ { r } ( 2796 ) $ } \geq 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 .