On The Metal Aversion of LGRBs

Abstract

Recently, it has been suggested that the metallicity aversion of long-duration gamma-raybursts (LGRBs) is not intrinsic to their formation, but rather a consequence of theanti-correlation between star-formation and metallicity seen in the general galaxypopulation. To investigate this proposal, we compare the metallicity of the hosts ofLGRBs, broad-lined Type Ic (Ic-bl) supernovae (SNe), and Type II SNe to each other and tothe metallicity distribution of star-forming galaxies using the SDSS to represent galaxiesin the local universe and the TKRS for galaxies at intermediate redshifts. The differingmetallicity distributions of the LGRB hosts and the star-formation in local galaxiesforces us to conclude that the low-metallicity preference of LGRBs is not primarily drivenby the anti-correlation between star-formation and metallicity, but rather must beoverwhelmingly due to the astrophysics of the LGRBs themselves. Three quarters of our LGRBsample are found at metallicities below 12+log(O/H) < 8.6, whileless than a tenth of local star-formation is at similarly low metallicities. However, ourSN samples are statistically consistent with the metallicity distribution of the generalgalaxy population. Using the TKRS population of galaxies, we are able to exclude thepossibility that the LGRB host metallicity aversion is caused by the decrease in galaxymetallicity with redshift. The presence of the strong metallicity difference between LGRBsand Ic-bl SNe largely eliminates the possibility that the observed LGRB metallicity biasis a byproduct of a difference in the initial mass functions of the galaxy populations.Rather, metallicity below half-solar must be a fundamental component of the evolutionaryprocess that separates LGRBs from the vast majority of Ic-bl SNe and from the bulk oflocal star-formation.