Clustering of galaxies around gamma-ray burst sight-lines ^⋆

¹ Max-Planck–Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching bei München, Germany
e-mail: vsudilov@mpe.mpg.de
² GEPI, Observatoire de Paris, CNRS, Univ. Paris Diderot, 5 place Jules Jannsen, 92195 Meudon, France
³ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
⁴ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁵ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁶ Institute of Experimental and Applied Physics, Czech Technical University in Prague, Horská 3a/22, 128 00 Prague 2, Czech Republic

Received: 6 February 2013
Accepted: 25 February 2013

Abstract

There is evidence of an overdensity of strong intervening MgII absorption line systems distributed along the lines of sight toward gamma-ray burst (GRB) afterglows relative to quasar sight-lines. If this excess is real, one should also expect an overdensity of field galaxies around GRB sight-lines, as strong MgII tends to trace these sources. In this work, we test this expectation by calculating the two point angular correlation function of galaxies within 120′′ (~470 h^-1₇₁ Kpc at ⟨ z ⟩  ~ 0.4) of GRB afterglows. We compare the gamma-ray burst optical and near-infrared detector (GROND) GRB afterglow sample – one of the largest and most homogeneous samples of GRB fields – with galaxies and active galactic nuclei found in the COSMOS-30 photometric catalog. We find no significant signal of anomalous clustering of galaxies at an estimated median redshift of z ~ 0.3 around GRB sight-lines, down to K_AB < 19.3. This result is contrary to the expectations from the MgII excess derived from GRB afterglow spectroscopy, although many confirmed galaxy counterparts to MgII absorbers may be too faint to detect in our sample – especially those at z > 1. We note that the addition of higher sensitivity Spitzer/IRAC or HST/WFC3 data for even a subset of our sample would increase this survey’s depth by several orders of magnitude, simultaneously increasing statistics and enabling the investigation of a much larger redshift space.