Solving the conundrum of intervening strong Mg II absorbers towards gamma-ray bursts and quasars⋆
1 Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
e-mail: lise@dark-cosmology.dk
2 GEPI, Observatoire de Paris, PSL Research University, CNRS, place Jules Janssen, 92190 Meudon, France
3 Institut d’Astrophysique de Paris, Université Paris 6-CNRS, UMR7095, 98bis boulevard Arago, 75014 Paris, France
4 Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 7610001 Rehovot, Israel
5 Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada
6 Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
7 Departamento de Astronomía, Universidad de Chile, Casilla 36-D, 1058 Santiago, Italy
8 INAF–Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
9 Department of Physics and Astronomy, University of California, Riverside, CA 92521, USA
10 INAF–Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (LC), Italy
11 INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
12 ASI-Science Data Centre, via del Politecnico snc, 00133 Rome, Italy
13 APC, Univ. Paris Diderot, CNRS/IN2P3, CEA/Irfu, Obs. de Paris, Sorbonne Paris Cité, 75013 Paris, France
14 Centre for Astrophysics and Cosmology, University of Nova Gorica, Vipavska 11c, 5270 Ajdovščina, Slovenia
15 Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavík, Iceland
16 Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
17 European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
18 INAF, Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy
19 Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
20 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
21 Dept. of Astronomy, Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
Received: 15 June 2017
Accepted: 1 September 2017
Previous studies have shown that the incidence rate of intervening strong Mg ii absorbers towards gamma-ray bursts (GRBs) were a factor of 2–4 higher than towards quasars. Exploring the similar sized and uniformly selected legacy data sets XQ-100 and XSGRB, each consisting of 100 quasar and 81 GRB afterglow spectra obtained with a single instrument (VLT/X-shooter), we demonstrate that there is no disagreement in the number density of strong Mg ii absorbers with rest-frame equivalent widths 
Å towardsGRBs and quasars in the redshift range 0.1 ≲ z ≲ 5. With large and similar sample sizes, and path length coverages of Δz = 57.8 and 254.4 for GRBs and quasars, respectively, the incidences of intervening absorbers are consistent within 1σ uncertainty levels at all redshifts. For absorbers at z < 2.3, the incidence towards GRBs is a factor of 1.5 ± 0.4 higher than the expected number of strong Mg ii absorbers in Sloan Digital Sky Survey (SDSS) quasar spectra, while for quasar absorbers observed with X-shooter we find an excess factor of 1.4 ± 0.2 relative to SDSS quasars. Conversely, the incidence rates agree at all redshifts with reported high-spectral-resolution quasar data, and no excess is found. The only remaining discrepancy in incidences is between SDSS Mg ii catalogues and high-spectral-resolution studies. The rest-frame equivalent-width distribution also agrees to within 1σ uncertainty levels between the GRB and quasar samples. Intervening strong Mg ii absorbers towards GRBs are therefore neither unusually frequent, nor unusually strong.
Based on observations collected at the European Southern Observatory, Paranal, Chile, Program ID: 098.A-0055, 097.A-0036, 096.A-0079, 095.B-0811(B), 095.A-0045, 094.A-0134, 093.A-0069, 092.A-0124, 0091.C-0934, 090.A-0088, 089.A-0067, 088.A-0051, 087.A-0055, 086.A-0073, 085.A-0009 and 084.A-0260. XQ-100: 189.A-0424.
© ESO, 2017