An Empirical Characterization of Extended Cool Gas Around Galaxies Using
  MgII Absorption Features
  We report results from a survey of MgII 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 <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 <~ 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 MgII absorbers in the
spectra of background QSOs and rest-frame absorption equivalent width
W_r(2796)=0.1-2.34 A, and 24 do not give rise to MgII absorption to sensitive
upper limits. Our analysis shows that (1) Wr(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=75x(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 <kappa_0.3>~70% for absorbers
of Wr(2796)>=0.3 A and <kappa_0.1>~80% for absorbers of Wr(2796)>=0.1 A. The
lack of correlation between Wr(2796) 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.
