The redshift determination of GRB 990506 and GRB 000418 with the Echellete Spectrograph Imager on Keck

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.