Environment of the gamma-ray burst GRB 971214: A giant HII region surrounded by a galactic supershell

Among a number of gamma-ray bursts whose host galaxies are known, GRB 971214 stands out for its high redshift (z greater than or equal to 3) and the Ly alpha emission line having a P Cygni-type profile, which is interpreted to be a direct consequence of the expanding supershell. From a profile-fitting analysis, we estimate the expansion velocity of the supershell (v(exp), = 1500 km s(-1)) and the neutral column density (N-HI = 10(20) cm (-2)). The redshift z = 3.418 of the host galaxy proposed by Kulkarni et al. in 1998 has been revised to be z = 3.425 from our profile analysis. The observed Ly alpha profile is fitted well by a Gaussian curve, which yields the Ly alpha luminosity L-Ly alpha = (1.8 +/- 0.8) x 10(42) ergs s(-1). Assuming that the photon source is a giant H II region, we deduce the electron number density in the H II region n(e) = (40 +/- 10)(L/L-Ly alpha)(0.5)(R/100 pc)(-1.5) cm(-3), which corresponds to the illumination by about 10(4) O5 stars. We estimate the star formation rate to be R-SF = 7 +/- 3 M. yr(-1) with the internal and the Galactic extinction corrected. The theory on the evolution of supernova remnants is used to propose that the supershell is at the adiabatic phase, with its radius R = 18E(53)(1/2) pc, its age t = 4.7 x 10(3)E(53)(1/2) yr, and the density of the ambient medium n(t) = 5.4E(53)(-1/2) cm(-3), where E-53 = E/10(53) ergs; we estimate the kinetic energy of the supershell to be E-k = 7.3 x 10(52)E(53) ergs. These values are consistent with the hypothesis that the supershell is the remnant of a gamma-ray burst. We note similarities between supershells found in nearby galaxies and remote primeval galaxies and propose that the gamma-ray burst may have occurred in a giant H II region whose environment is similar to that in star-forming galaxies.