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

TL;DR

This study investigates the sizes of Mg II-absorbing clouds relative to quasar emission regions, finding that clouds are larger than the beam sizes of both the continuum and broad-line regions, which explains the similar incidence of absorbers across different spectral regions.

Contribution

The paper provides observational evidence that Mg II-absorbing clouds are larger than quasar emission regions, constraining their size to at least 10^17 cm, and tests the geometrical explanation for absorption discrepancies.

Findings

No difference in Mg II absorber incidence on emission lines versus continuum regions.

Mg II clouds are larger than the quasar beam sizes, at least 10^17 cm.

Supports the geometrical model explaining absorption incidence rates.

Abstract

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.