Direct Observational Test Rules Out Small MgII Absorbers

Abstract

Recent observations suggest the incidence of strong intervening MgII absorption systems along the line-of-sight to gamma ray burst (GRB) afterglows is significantly higher than expected from analogous quasar sightlines. One possible explanation is a geometric effect, arising because MgII absorbers only partially cover the quasar continuum regions, in which case MgII absorbers must be considerably smaller than previous estimates. We investigate the production of abnormal absorption profiles by partial coverage and conclude that the lack of any known anomalous profiles in observed systems, whilst constraining, cannot on its own rule out patchy MgII absorbers. In a separate test, we look for differences in the distribution function of MgII equivalent widths over quasar continuum regions and CIII] emission lines. We show that these anomalies should be observable in any scenario where MgII absorbers are very small, but they are not present in the data. We conclude that models invoking small MgII cloudlets to explain the excess of absorbers seen towards GRBs are ruled out.

Accepted for publication in MNRAS Letters. 5 pages, 2 figures

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• Casting light on the 'anomalous' statistics of Mg II absorbers toward Gamma-Ray Burst afterglows: the incidence of weak systems
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• Testing the Possible Intrinsic Origin of the Excess of Very Strong MgII Absorbers Along GRB Lines-of-Sight
• Incidence of MgII absorbers towards Blazars and the GRB/QSO puzzle
• Dusty MgII Absorbers: Implications for the GRB/Quasar Incidence Discrepancy
• Solving the conundrum of intervening strong MgII absorbers towards GRBs and quasars
• The Redshift Distribution of Intervening Weak MgII Quasar Absorbers and a Curious Dependence on Quasar Luminosity
• Constraints on the relative sizes of intervening Mg II-absorbing clouds and quasar emitting regions