Gas and Dust Properties in the Afterglow Spectra of the Third-Highest-Redshift GRB 050730

Gamma-ray bursts (GRBs) have proven to be excellent probes of the distant Universe. The high luminosities of GRB afterglows allow absorption line studies of the interstellar medium at high redshift to at least z=4.5. Deep observations of afterglow positions have detected host galaxies in almost all cases. Most hosts are compact, low-metallicity, actively star-forming galaxies and are found to have low intrinsic extinction. However, in a few cases, radio/submillimeter observations of hosts give a star-formation rate which is of order a few to ~100 larger than rates derived from optical estimators. This may be caused by strong dust obscuration, but neither spectra nor colours of hosts show strong internal extinction. Afterglow spectroscopy provides a unique window on the nearby environment of GRBs, allowing astronomers to probe absorbing dust and gas properties in more detail.

During the afterglow phase of GRB 050730 a team of astronomers acquired spectra using ISIS spectrograph on the William Herschel Telescope. Two observations were done sequentially, with exposure times of 1260 and 1800 seconds. They found that the spectrum shows a Damped Lyman-Alpha (DLA) system with the highest measured hydrogen column to date: N(HI)=22.1±0.1 at the third-highest GRB redshift z=3.968. They also find the host to be a low metallicity galaxy, with low dust content.

These results were prepared by a team of astronomers led by R. L. C. Starling (Astronomical Institute 'Anton Pannekoek', University of Amsterdam) and submitted in the form of a paper to astro-ph only ten days after data acquisition. The research paper can be downloaded from astro-ph: 0508237.

The WHT ISIS combined, normalised spectrum of the afterglow (midpoint 0.14 days), and 1σ error spectrum (lower curve). Overlaid is the best-fitting DLA profile (solid line) and its errors (dashed lines). For more information please read the submitted paper astro-ph/0508237.