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Title:
An Imaging and Spectroscopic Study of Four Strong Mg II Absorbers Revealed by GRB 060418
Authors:
Pollack, L. K.; Chen, H.-W.; Prochaska, J. X.; Bloom, J. S.
Affiliation:
AA(Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA ), AB(Department of Astronomy & Astrophysics and Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA ), AC(Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA ; University of California Observatories/Lick Observatory, University of California, Santa Cruz, CA 95064, USA ), AD(Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA ; Sloan Research Fellow.)
Publication:
The Astrophysical Journal, Volume 701, Issue 2, pp. 1605-1615 (2009). (ApJ Homepage)
Publication Date:
08/2009
Origin:
IOP
ApJ Keywords:
gamma rays: bursts, intergalactic medium, ISM: abundances, ISM: kinematics and dynamics
DOI:
10.1088/0004-637X/701/2/1605
Bibliographic Code:
2009ApJ...701.1605P

Abstract

We present results from an imaging and spectroscopic study of four strong Mg II absorbers of W(2796) gsim 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 lsim 3farcs5 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 lsim 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 sun yr-1 kpc-2.

Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

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