LGRB Metal Aversion

Author: John Graham

Abstract: Here I present results from a recent paper showing that LGRBs have a strong (but not absolute) intrinsic preference for low metallicity environments and soon to be published results showing that the higher SSFRs of LGRB hosts is a result of this metallicity preference. Recently, it has been suggested that the metallicity aversion of long-duration gamma-ray bursts (LGRBs) is not intrinsic to their formation, but rather a consequence of the anti-correlation between star-formation and metallicity seen in the general galaxy population. To investigate this, we compare the metallicity of LGRB, broad-lined Type Ic (Ic-bl) supernovae (SNe), and Type II SNe hosts to each other and to the metallicity distribution of star-forming galaxies using the SDSS to represent galaxies in the local universe and the TKRS for galaxies at intermediate redshifts. The differing metallicity distributions of the LGRB hosts and the star formation in local galaxies forces us to conclude that the low-metallicity preference of LGRBs is not primarily driven by the anti-correlation between star-formation and metallicity, but rather must be overwhelmingly due to the astrophysics of the LGRBs themselves. Three quarters of our LGRB sample are found at metallicities below 12+log(O/H) < 8.6, while less than a tenth of local star-formation is at similarly low metallicities. However, our SN samples are statistically consistent with the metallicity distribution of the general galaxy population. Using the TKRS population of galaxies, we are able to exclude the possibility that the LGRB host metallicity aversion is caused by the decrease in galaxy metallicity with redshift. The presence of the strong metallicity difference between LGRBs and Ic-bl SNe largely eliminates the possibility that the observed LGRB metallicity bias is 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 evolutionary process that separates LGRBs from the vast majority of Ic-bl SNe and from the bulk of local star-formation. Furthermore, we show that the distribution of specific-star-formation rate (SSFR) can be corrected for evolution with redshift. This results in a general galaxy population where the corrected SSFR vs. mass anti-correlation remains consistent across the 0 < z < 1 range. While the SNe populations remain consistent with the general galaxy population distribution, the LGRB population is notably biased towards the high SSFR low mass tail. We show that this bias is a function of the LGRB metal aversion by reproducing it with the low metallicity subset of the general galaxy population. We also show that a high SSFR bias in the LGRB population would not duplicate the observed metallicity bias. Therefor an intrinsic low metallicity preference among LGRBs remains the only explanation consistent with observations.