PROBING THE LONG GAMMA-RAY BURST PROGENITOR BY Lyα EMISSION OF HOST GALAXIES

Long gamma-ray bursts (GRBs) have been suggested to occur preferentially in low-metallicity environment. We discuss the possibility and theoretical aspects of using Ly alpha emission properties of long GRB host galaxies as a metallicity indicator of high-redshift GRB environments, where direct metallicity measurements are not easy. We propose to use the fraction of Lya emitters (LAEs) in long GRB host galaxies as a function of UV luminosity, which can be compared with star formation rate weighted LAE fraction of Lyman break galaxies as the standard in the case of no metallicity dependence. There are two important effects of metallicity dependence of long GRB rate to change the LAE fraction of host galaxies. One is the enhancement of intrinsic Lya equivalent width (EW) by stronger ionizing UV luminosity of low-metallicity stellar population, and the other is extinction by interstellar dust to change the observable EW. Based on a latest theoretical model of LAEs that reproduce observations, we argue that the latter is likely to work in the opposite direction to the former, i.e., to decrease LAE fraction if GRBs preferentially occur in low-metallicity environments, because of the clumpy interstellar medium effect. The high LAE fraction of GRB host galaxies indicated by observations is quantitatively explained by the LAE model if GRBs occur when Z less than or similar to 0.1 Z(circle dot), although this result is still indicative because of the limited statistics and theoretical uncertainties. This result demonstrates that the LAE statistics of GRB hosts may give us useful information in the future.