The Relative Rate of LGRB Formation as a Function of Metallicity

TL;DR

This study quantifies how the likelihood of Long-duration Gamma-Ray Burst formation sharply decreases in environments with higher metallicity, revealing a significant cutoff at a specific metallicity level and a continued decline beyond it.

Contribution

It provides the first direct measurement of the relative LGRB formation rate as a function of metallicity, highlighting a sharp cutoff and further decline at higher metallicities.

Findings

LGRB formation rate drops dramatically above log(O/H)+12 ~ 8.3

LGRBs form 10 to 50 times more frequently below this metallicity

Rate may decrease by up to a factor of 100 from one-third solar to solar metallicity

Abstract

There is now strong evidence that Long-duration Gamma-Ray Bursts (LGRBs) are preferentially formed in low-metallicity environments. However, the magnitude of this effect, and its functional dependence on metallicity have not been well characterized. In our previous paper, Graham & Fruchter (2013), we compared the metallicity distribution of LGRB host galaxies to the that of star forming galaxies in the local universe. Here we build upon this work by in effect dividing one distribution by the other, and thus directly determine the relative rate of LGRB formation as a function of metallicity in the low-redshift universe. We find a dramatic cutoff in LGRB formation above a metallicity of log(O/H)}+12 ~ 8.3 in the KK04 scale, with LGRBs forming between ten and fifty times more frequently per unit star-formation below this cutoff than above. Furthermore, our data suggests that the rate of LGRB formation per unit star formation continues to fall above this break. We estimate the LGRB formation rate per unit star formation may drop by as much as a factor of one hundred between one-third solar and solar metallicity.