Spectroscopy of GRB 050505 at z = 4.275: A log N(H I) = 22.1 DLA Host Galaxy and the Nature of the Progenitor

We present optical spectroscopy of the afterglow of GRB 050505 obtained with the Keck I 10 m telescope. The spectrum exhibits three absorption systems with the highest, at z = 4.2748, arising in the host galaxy. The host absorption is marked by a DLA with log N(H ) = 22.05 ± 0.10, higher than that of any QSO-DLA detected to date but similar to several other recent measurements from GRB spectra. We further deduce a metallicity of Z ≈ 0.06 Z_☉, with a depletion pattern that is similar to that of the Galactic warm halo or warm disk. More importantly, we detect strong absorption from Si II* indicating a dense environment, n_H ≳ 10² cm^-3, in the vicinity of the burst, with a size of ~4 pc. The C IV absorption system spans a velocity range of about 10³ km s^-1, most likely arising in the progenitor stellar wind. In this context the lack of corresponding Si IV absorption indicates that the progenitor had a mass ≲25 M_☉ and a metallicity ≲0.1 Z_☉, and therefore required a binary companion to eject its hydrogen envelope prior to the GRB explosion. Finally, by extending the GRB-DLA sample to z ≈ 4.3 we show that these objects appear to follow a similar metallicity-redshift relation as in QSO-DLAs, but with systematically higher metallicities. It remains to be seen whether this trend is simply due to the higher neutral hydrogen columns in GRB-DLAs and/or sight lines which probe star-forming regions, or if it is a manifestation of different star formation properties in GRB-DLAs.