Constraints on the relative sizes of intervening Mg II-absorbing clouds
  and quasar emitting regions
  Context: A significantly higher incidence of strong (rest equivalent width
W_r > 1 {\AA}) intervening Mg II absorption is observed along gamma-ray burst
(GRB) sight-lines relative to those of quasar sight-lines. A geometrical
explanation for this discrepancy has been suggested: the ratio of the beam size
of the source to the characteristic size of a Mg II absorption system can
influence the observed Mg II equivalent width, if these two sizes are
comparable. Aims: We investigate whether the differing beam sizes of the
continuum source and broad-line region of Sloan Digital Sky Survey (SDSS)
quasars produce a discrepancy between the incidence of strong Mg II absorbers
illuminated by the quasar continuum region and those of absorbers illuminated
by both continuum and broad-line region light. Methods: We perform a
semi-automated search for strong Mg II absorbers in the SDSS Data Release 7
quasar sample. The resulting strong Mg II absorber catalog is available online.
We measure the sight-line number density of strong Mg II absorbers superimposed
on and off the quasar C IV 1550 {\AA} and C III] 1909 {\AA} emission lines.
Results: We see no difference in the sight-line number density of strong Mg II
absorbers superimposed on quasar broad emission lines compared to those
superimposed on continuum-dominated spectral regions. This suggests that the Mg
II-absorbing clouds typically observed as intervening absorbers in quasar
spectra are larger than the beam sizes of both the continuum-emitting regions
and broad line-emitting regions in the centers of quasars, corresponding to a
lower limit of the order of 10^17} cm for the characteristic size of a Mg II
absorbing cloud.
