The Metal Aversion of LGRBs
  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
proposal, we compare the metallicity of the hosts of LGRBs, broad-lined Type Ic
(Ic-bl) supernovae (SNe), and Type II SNe 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.
