Mr. Yu Niinou (Kyoto Univ.) visit our group and give colloquium speech

Oct 17, 2008

“Lyman-alpha emission of long GRB host galaxies and the metallicity dependent GRB production”

Theoretical and observational studies suggest that long gamma-ray bursts (GRBs) preferentially occur in low-metallicity environment. Lyman-alpha equivalent width is a property of galaxies that is closely connected to the metal abundance of the galaxies, and hence Lyman-alpha emission luminosity of GRB host galaxies can provide us with an important clue to the metallicity dependence of long GRB production.

 Accroding to the optical follow up observations of high redshift (> 2) long GRBs, most of the detected host galaxies of GRBs have strong Lyman-alpha emission lines, while the fraction of such galaxies in Lyman-break galaxies selected by galaxy surveys at similar redshifts is less than 30 %. Using a model of hierarchical galaxy formation, we predict the distribution of various properties of GRB host galaxies including Lyman alpha luminosity at a given redshift, assuming several models of metallicity dependence of GRB production rate. The predictions are then compared with available observational results. We find that the larger mean Lyman alpha luminosity of GRB host galaxies than that for LBGs can indeed be reproduced by a model in which GRBs occur only in low metallicity environments. However, the currently available sample of host galaxies is not large enough to exclude the case of metallicity-independent GRB production rate. Future observations of GRB host galaxies will put more strict constraint on the metallicity dependence of long GRB production, and hence on the progenitor of long GRBs.