Size estimates for intervening C IV absorbers from high resolution   spectroscopy of APM 0827+5255
  A new analysis of Keck/HIRES observations of the broad absorption line QSO APM 0827+5255 indicates that a number of intervening C IV absorbers give rise to absorption lines for which the observed optical depths for 1548, 1550 A doublet components are not in the expected 2:1 ratio. To compensate for the effect, a local adjustment of the zero-level is required. We model this effect as coverage of one line of sight to this gravitationally lensed QSO and perform a set of simulations to select a sample of lines for which our model provides an explanation for the effect. We use lines in this sample to obtain estimates for minimum C IV absorber sizes from total coverage and the separations of the lines of sight for a range of lens redshifts, z_l, and two cosmologies. We also obtain best estimates for overall sizes from a statistical `hit and miss' approach. For z_l=0.7 our results set a lower limit to sizes of C IV absorbers of ~0.3 h_72^{-1} kpc (~0.5 h_72^{-1} kpc) for Omega_M=1, Omega_Lambda=0 (Omega_M=0.3, Omega_Lambda=0.7), in agreement with other results from similar work but are limited by sample size and the uncertainty in z_l. Our method can be used to detect lensed QSOs and to probe absorber sizes when separate spectra cannot be obtained for each line of sight. 
Panayiotis Tzanavaris, Robert F. Carswell (Institute of Astronomy,   Cambridge)