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 (lambda/Deltalambda = 13000), we resolved the [O II] lambda727 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 Hgamma 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 submillimeter wavelengths, these spectroscopic observations suggest that the submillimeter emission is due to star formation ( as opposed to AGN) activity. The [O II]-derived unobscured star formation rates are 13 and 55 M-circle dot yr(-1) for the hosts of GRB 990506 and GRB 000418, respectively. In contrast, the star formation rate of the host of GRB 000418 derived from submillimeter observations is 20 times larger.