4.4. Gamma-Ray-Bursters

A new and exciting demonstration of extragalactic "power" has recently emerged with the realization that Gamma-Ray-Bursters (GRBs) are apparently the most powerful cosmic explosions; observing their optical or radio afterglows can give us an indirect glimpse of a distant host galaxy. Not all bursters are successfully tracked for days or weeks after the outburst, but a reasonable fraction do point to distant (z 1) star-burst galaxy hosts. So, for this review we note that occasional luminous afterglows may signal the locations of star-forming young galaxies at z > 4.

The detailed physics of the situation is unclear, but there are now believable scenarios suggesting that the GRBs originate from the collapse of a massive star or even a stellar merger. So sites of active SF may be one of the "usual suspects", much as Type II SNe may be sited in young-star-rich locations. With the improved ability to locate GRBs we do find several annual opportunities to follow the afterglows as they decay; occasionally a redshift from an afterglow spectrum rich in UV interstellar lines (shifted to the visible) is obtained. The highest conventional spectroscopic redshift measured to date is z = 3.42 (Kulkarni et al. 1998).

Because many GRBs are very luminous (for a short time interval) we note that the possibility exists to derive "photo-z's" or obtain low-resolution spectra of even more distant GRBs - perhaps with a little help from their galaxy hosts. Indeed Andersen et al. (2000) suggest a GRB at z ~ 4.5 from the afterglow's broad-band colors. At higher redshifts we will need photometry and/or spectroscopy in the near-IR. The J-band at ~ 1.2 µm will take the strong spectral discontinuity anticipated at Ly (1216 Å, rest) to z ~ 9! Of course our present abilities to obtain good S/N infrared spectra would be taxed by all but the earliest bright GRB afterglows; spectroscopy in the first minutes may be needed!