TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3399 SUBJECT: GRB 050509b: Physical Properties of the Candidate Host galaxy DATE: 05/05/10 00:53:14 GMT FROM: Hsiao-Wen Chen at MIT/CSR Jason X. Prochaska (UCO/Lick Obs.), J. S. Bloom (UCB), H.-W. Chen (MIT), & K. Hurley (UCB) report on behalf of the GRAASP collaboration: "We have further analyzed the longslit spectrum obtained for the candidate host galaxy of GRB 050509b Bloom et al. GCN 3386) by Prochaska et al. (GCN 3390), using Keck/DEIMOS. We estimate a more precise redshift of the galaxy at z = 0.2249 +/- 0.0008 by cross-correlating the spectrum with galaxy templates from SDSS. At this redshift, the host has an intrinsic K-band luminosity of ~ 1.6 x 10^11 L_sun as derived based on its 2MASS photometry of K = 14.1 mag. We attempt to constrain the underlying dark matter halo mass of the galaxy based on the observed absorption line profiles. Using the Ca H&K doublet, we estimate a velocity dispersion of ~ 340 km/sec. This corresponds to an enclosed dynamical mass of 7.2 x 10^11 h^{-1} M_sun at the radius of the OT r = 26.6 h^-1 kpc. The spectrum of the galaxy is presented at http://www.graasp.org/Data/050509b. This message can be cited."