TITLE: GCN CIRCULAR NUMBER: 11874 SUBJECT: GRB 110328A / Swift J164449.3+573451: Keck/DEIMOS Optical Spectroscopy DATE: 11/04/04 04:27:16 GMT FROM: S. Bradley Cenko at Caltech S. B. Cenko, D. A. Perley, J. S. Bloom (UC Berkeley), K. Hurley (SSL), J. X. Prochaska, J. Brodie, N. Singh, J. Arnold, A. Romanowsky, J. C. Forbes (UCO/Lick), and D. Forbes (Swinburne) report on behalf of a larger collaboration: We have obtained medium-resolution optical spectroscopy of the optical counterpart (Cenko et al., GCN 11827) of the high-energy transient GRB 110328A / Swift J164449.3+573451 (Cummings et al., GCN 11823) with the DEIMOS spectrograph mounted on the Keck II telescope. Observations began at 14:55 UT on 2011 Mar 31 and cover the wavelength range from 4500-9500 A. We identify strong nebular emission features associated with [O II], [O III], H-beta, H-alpha, [N II], and [S II] at a redshift consistent with that reported by Levan et al. (GCN 11833) and Thoene et al. (GCN 11834). Using a preliminary flux calibration, we find that the Balmer decrement (H-alpha / H-beta) is only marginally larger than the value expected for Case B recombination (consistent at the 2 sigma level). This would suggest that the observed red colors of the optical / NIR counterpart (e.g., Morgan et al., GCN 11845; Levan et al., GCN 11846) are either due to 1) an intrinsically red transient source, or 2) dust localized to the source of the transient emission (which does not affect the bulk of the ongoing star formation in the galaxy). Constructing a diagnostic diagram (e.g., Baldwin, Phillips, and Terlevich 1981, PASP, 93, 5) based on the observed ratio of the narrow emission lines, we find the source falls within the locus of star-forming galaxies, and thus does not appear to exhibit any evidence for past nuclear activity. A plot of the diagnostic diagram of the host, showing the empirical dividing line between star-forming galaxies and AGNs from Kauffmann et al (2003, MNRAS, 341, 33- solid line), the theoretical dividing lines from Kewley and Dopita (2002, ApJS, 142, 35 - dashed lines), and the empirical dividing lines from Ho, Filippenko, and Sargent (1997, ApJS, 112, 315), along with analogous measurements from the MPA-JHU value-added SDSS database, can be found at: http://astro.berkeley.edu/~cenko/public/grb/GRB110328A/bpt.png