Spectroscopy of GRB 050505 atz= 4.275: A log N(Hi) = 22.1 DLA Host Galaxy and the Nature of the Progenitor

We present the discovery of the optical afterglow of GRB 050505 and anoptical absorption spectrum obtained with the Keck I 10-m telescope. Thespectrum exhibits three redshifted absorption systems with the highest, atz=4.2748, arising in the GRB host galaxy. The host absorption system is markedby a damped Ly-alpha (DLA) feature with a neutral hydrogen column density oflogN(HI)=22.05+/-0.10, higher than that of any QSO-DLA detected to date, butsimilar to several other recent measurements from GRB spectra. In addition, wedetect absorption lines from both low- and high-ionization species from whichwe deduce a metallicity, Z~0.06 Z_solar, with a depletion pattern that isroughly similar to that of the Galactic warm halo, warm disk, or disk+halo.More importantly, we detect strong absorption from SiII* indicating a denseenvironment, n_H>10^2 cm^-3, in the vicinity of the burst, with a size of about4 pc. In addition, the CIV absorption system spans a velocity range of about1000 km/s, which is not detected in any other absorption feature. We show thatthe most likely interpretation for this wide velocity range is absorption inthe wind from the progenitor star. In this context, the lack of correspondingSiIV absorption indicates that the progenitor had a mass of <25 M_solar and ametallicity <0.1 Z_solar, and therefore required a binary companion to ejectits hydrogen envelope prior to the GRB explosion. Finally, by extending theGRB-DLA sample to z~4.3 we show that these objects appear to follow a similarmetallicity-redshift relation as in QSO-DLAs, but with systematically highermetallicities. It remains to be seen whether this trend is simply due to thehigher neutral hydrogen columns in GRB-DLAs, or if it is a manifestation ofdifferent star formation properties in GRB-DLAs. [abridged]Comment: 19 pages, 8 figures, 2 tables; submitted to Ap