Constraints on the relative sizes of intervening Mg II-absorbing clouds and quasar emitting regions^⋆

¹ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
e-mail: unclellama@gmail.com
² Steward Observatory, University of Arizona, 933 N. Cherry Avenue, 85721 Tucson, AZ, USA

Received: 1 April 2012
Accepted: 31 July 2012

Abstract

Context. A significantly higher incidence of strong (rest equivalent width W_r > 1 Å) 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 an 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 performed 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 at the CDS. We measured the sight-line number density of strong Mg ii absorbers superimposed on and off the quasar C iv λ 1550 and C iii]  λ 1909 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 10¹⁷ cm for the characteristic size of an Mg ii absorbing cloud.