  Using the Echellete Spectrograph Imager (ESI) on the Keck II 10-m telescope
we have measured the redshifts of the host galaxies of gamma-ray bursts GRB
990506 and GRB 000418, z=1.30658 +/- 0.00004 and 1.1181 +/- 0.0001,
respectively. Thanks to the excellent spectral resolution of ESI we resolved
the [O II] 3727 doublet in both cases. The measured redshift of GRB 990506 is
the highest known for a dark burst GRB, though entirely consistent with the
notion that dark and non-dark bursts have a common progenitor origin. The
relative strengths of the [O II], He I, [Ne III], and H gamma emission lines
suggest that the host of GRB 000418 is a starburst galaxy, rather than a LINER
or Seyfert 2. Since the host of GRB 000418 has been detected at sub-millimeter
wavelengths these spectroscopic observations suggest that the sub-millimeter
emission is due to star-formation (as opposed to AGN) activity. The [O
II]-derived unobscured star-formation rates are 13 and 55 M_solar/yr for the
hosts of GRB 990506 and GRB 000418, respectively. In contrast, the
star-formation rate of the host of GRB 000418 derived from sub-millimeter
observations is twenty times larger.

Astrophysics