High‐Redshift Gamma‐Ray Bursts from Population III Progenitors

Detection of gamma-ray bursts (GRBs) from redshifts z > 7 would open a newwindow into the earliest epoch of cosmic star formation. We construct separatestar formation histories at high redshifts for normal (Pop I and II) stars, andfor predominantly massive (Pop III) stars. Based on these separate histories,we predict the GRB redshift distribution to be observed by the {\it Swift}mission. Regardless of whether Pop III progenitors are able to trigger GRBs, wefind that a fraction 10% of all bursts detected by {\it Swift} will originateat z > 5. This baseline contribution is due to Pop I/II star formation whichmust have extended out to high redshifts in rare massive galaxies that wereenriched by heavy elements earlier than the typical galaxies. In addition, weconsider the possible contribution of Pop III progenitors to the observable GRBrate. Pop III stars are viable progenitors for long-duration GRBs which aretriggered by the collapsar mechanism, as long as they can lose their outerenvelope through mass transfer to a companion star in a close binary. We findthat the likelihood of Pop III binaries to satisfy the conditions required bythe collapsar mechanism could be enhanced significantly relative to Pop I/IIbinaries. If Pop III binaries are common, {\it Swift} will be first observatoryto probe Pop III star formation at redshifts z > 7.Comment: 8 pages, 2 figures, revised version, ApJ in press (May 1, 2006 issue