Investigating MgII Absorption in Paired Quasar Sight-Lines
  We test whether the Tinker & Chen model of MgII absorption due to the gaseous halo around a galaxy can reproduce absorption in quasar pairs (both lensed and physical) and lensed triples and quads from the literature. These quasars exhibit absorption from a total of 38 MgII systems spanning z=0.043 - 2.066 with mean redshift <z>=1.099 and weighted mean rest-frame equivalent width of 0.87 Ang. Using the Tinker & Chen model to generate simulated sight-lines, we marginalize the unknown parameters of the absorbing galaxies: dark matter halo mass, impact parameter, and azimuthal angle on the sky. We determine the ability of the model to statistically reproduce the observed variation in MgII absorption strength between paired sight-lines for different values of the gas covering fraction f_c and the characteristic length scale ell_c, within which the variation in absorption equivalent widths between sight-lines exponentially decreases. We find a best-fit f_c=0.60 \pm 0.15 and ell_c<8/h_70 kpc (1\sigma confidence limits), with smaller f_c allowed at larger ell_c. At 99.7% confidence, we are able to rule out f_c>0.87 for all values of ell_c and the region where ell_c<1.0/h_70 kpc and f_c<0.3. 
Jesse A. Rogerson, Patrick B. Hall
astro-ph.CO