2009ApJ...701.1605P

We present results from an imaging and spectroscopic study of four strong Mg II absorbers of W(2796) ≳ 1 Å revealed by the afterglow of GRB 060418 at z_GRB= 1.491. These absorbers, at z = 0.603, 0.656, 1.107, and z_GRB, exhibit large ion abundances that suggest neutral gas columns characteristic of damped Lyα systems. The imaging data include optical images obtained using Low-Resolution Imaging Spectrometer (LRIS) on the Keck I telescope and using Advanced Camera for Surveys on board Hubble Space Telescope, and near-infrared H-band images obtained using Persson's Auxiliary Nasmyth Infrared Camera on the Magellan Baade Telescope and K'-band images obtained using NIRC2 with laser guide star adaptive optics on the Keck II telescope. These images reveal six distinct objects at Δ θ ≲ 3".5 of the afterglow's position, two of which exhibit well-resolved mature disk morphology, one shows red colors, and three are blue compact sources. Follow-up spectroscopic observations using LRIS confirm that one of the disk galaxies coincides with the Mg II absorber at z = 0.656. The observed broadband spectral energy distributions of the second disk galaxy and the red source indicate that they are associated with the absorbers at z = 0.603 and z = 1.107, respectively. These results show that strong Mg II absorbers identified in gamma-ray burst (GRB) afterglow spectra are associated with typical galaxies of luminosity ~0.1 - 1 L_* at impact parameter of ρ ≲ 10 h^–1 kpc. The close angular separation would preclude easy detections toward a bright quasar. Finally, we associate the remaining three blue compact sources with the GRB host galaxy, noting that they are likely star-forming knots located at projected distances of ρ = 2-12 h^–1 kpc from the afterglow. At the afterglow's position, we derive a 2σ upper limit to the underlying star-formation rate intensity of 0.0074 M_☉/yr/kpc2.