Astronomy & Astrophysics manuscript no. paper c ⃝ESO 2017 August 25, 2017 Solving the conundrum of intervening strong Mg ii absorbers towards GRBs and QSOs L. Christensen 1 , S. D. Vergani 2, 3 , S. Schulze 4 , N. Annau 5, 1 , J. Selsing 1 , J. P. U. Fynbo 1 , A. de Ugarte Postigo 6, 1 , R. Cañameras 1 , S. Lopez 7 , D. Passi 7 , P. Cortés-Zuleta 7 , S. L. Ellison 5 , V. D’Odorico 8 , G. Becker 9 , T. A. M. Berg 5 , Z. Cano 6 , S. Covino 10 , G. Cupani 8 , V. D’Elia 11, 12 , P. Goldoni 13 , A. Gomboc 14 , F. Hammer 2 , K. E. Heintz 15, 1 , P. Jakobsson 15 , J. Japelj 16 , L. Kaper 16 , D. Malesani 1 , P. Møller 17 , P. Petitjean 3 , V. Pugliese 16 , R. Sánchez-Ramírez 6, 18 , N. R. Tanvir 19 , C. C. Thöne 6 , K. Wiersema 19 , G. Worseck 20 ⋆ 1 Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-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, F-75014 Paris, France 4 Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 7610001, 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, E-18008, Granada, Spain 7 Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago 8 INAF- Osservatorio Astronomico di Trieste, Via Tiepolo 11, I-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, I-00040 Monteporzio Catone, Italy 12 ASI-Science Data Centre, Via del Politecnico snc, I-00133 Rome, Italy 13 APC, Univ. Paris Diderot, CNRS/IN2P3, CEA/Irfu, Obs. de Paris, Sorbonne Paris Cité, 75013 P 14 Centre for Astrophysics and Cosmology, University of Nova Gorica, Vipavska 11c, 5270 Ajdovšˇ cina, 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, D-85748 Garching bei München, Germany 18 INAF, Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, I-00133 Roma, Italy 19 Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK 20 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany Received ; accepted ABSTRACT Previous studies have shown that the incidence rate of intervening strong Mg ii absorbers towards GRBs were a factor of 2 − 4 higher than towards QSOs. Exploring the similar sized and uniformly selected legacy data sets XQ-100 and XSGRB, each consisting of 100 QSO 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 W λ2976 r > 1 Å towards GRBs and QSOs in the redshift range 0.2 ≲ z ≲ 5. With large and similar sample sizes, and path length coverages of Δz = 57.8 and 254.4 for GRBs and QSOs, 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 SDSS quasar spectra, while for QSO absorbers observed with X-shooter we find an excess factor of 1.4 ± 0.2 relative to SDSS QSOs. Conversely, the incidence rates agree at all redshifts with reported high spectral resolution QSO 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 QSO samples. Intervening strong Mg ii absorbers towards GRBs are therefore neither unusually frequent, nor unusually strong. Key words. Quasars: absorption lines – Gamma-ray burst: general – Galaxies: halos 1. Introduction Luminous point sources like gamma-ray bursts (GRBs) and quasi-stellar objects (QSOs) are efficient probes of the gaseous ⋆ Based on observations collected at the European Southern Obser- vatory, 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. material along their lines of sight. Both classes of objects are currently detected out to z > 7 (Tanvir et al. 2009; Salvaterra et al. 2009; Mortlock et al. 2011) and thus probe a very long path length through the universe. Absorption lines at various red- shifts in the spectra of the background sources provide us with methods to explore the high-redshift universe in absorption even though the galaxies that cause the absorption lines are not de- tected in emission. Since both GRBs and QSOs probe interven- ing material randomly, it was a puzzling discovery a decade ago Article number, page 1 of 10