An Imaging and Spectroscopic Study of Four Strong MgII Absorbers
  Revealed By GRB060418
  We present results from an imaging and spectroscopic study of four strong
MgII absorbers of W(2796) >~ 1 Ang revealed by the afterglow of GRB060418 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 column densities characteristic of
damped Lya systems. The imaging data include optical images obtained using LRIS
on the Keck I telescope and using ACS on board HST, and near-infrared H-band
images obtained using PANIC on the Magellan Baade Telescope and K'-band images
obtained using NIRC2 with LGSAO on the Keck II telescope. These images reveal
six distinct objects at <~ 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 MgII absorber at
z=0.656. The observed broad-band 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
MgII absorbers identified in GRB afterglow spectra are associated with typical
galaxies of luminosity ~ (0.1-1) L* at impact parameter <~ 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 2-12 h^-1 kpc from the afterglow. At the afterglow's
position, we derive a 2-sigma upper limit to the underlying SFR intensity of
0.0074 M_sun yr^-1 kpc^-2.
