Superluminous supernova progenitors have a half-solar metallicity
  threshold
  Host galaxy properties provide strong constraints on the stellar progenitors
of superluminous supernovae. By comparing a sample of 19 low-redshift (z < 0.3)
superluminous supernova hosts to galaxy populations in the local Universe, we
show that sub-solar metallicities seem to be a requirement. All superluminous
supernovae in hosts with high measured gas-phase metallicities are found to
explode at large galactocentric radii, indicating that the metallicity at the
explosion site is likely lower than the integrated host value. We found that
superluminous supernovae hosts do not always have star-formation rates higher
than typical star-forming galaxies of the same mass. However, we confirm that
high absolute specific star-formation rates are a feature of superluminous
supernova host galaxies, but interpret this as simply a consequence of the
anti-correlation between gas-phase metallicity and specific star-formation rate
and the requirement of on-going star formation to produce young, massive stars
greater than ~ 10-20 M_sol. Based on our sample, we propose an upper limit of ~
0.5 Z_sol for forming superluminous supernova progenitors (assuming an N2
metallicity diagnostic and a solar oxygen abundance of 8.69). Finally, we show
that if magnetar powering is the source of the extreme luminosity then the
required initial spins appear to be correlated with metallicity of the host
galaxy. This correlation needs further work, but if it holds it is a powerful
link between the supernova parameters and nature of the progenitor population.
