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 I) 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 approximate to 0.06 Z(circle dot), 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) greater than or similar to 10(2) cm(-3), in the vicinity of the burst, with a size of similar to 4 pc. The C IV absorption system spans a velocity range of about 10(3) 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 less than or similar to 25 M-circle dot and a metallicity less than or similar to 0.1 Z(circle dot), 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 approximate to 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.