The Swift GRB Host Galaxy Legacy Survey - II. Rest-Frame NIR Luminosity
  Distribution and Evidence for a Near-Solar Metallicity Threshold
  We present rest-frame NIR luminosities and stellar masses for a large and
uniformly-selected population of GRB host galaxies using deep Spitzer Space
Telescope imaging of 119 targets from the Swift GRB Host Galaxy Legacy Survey
spanning 0.03 < z < 6.3, and determine the effects of galaxy evolution and
chemical enrichment on the mass distribution of the GRB host population across
cosmic history. We find strong evolution in the host luminosity distribution
between z~0.5 (median absolute NIR AB magnitude ~ -18.5, corresponding to M* ~
3x10^8 M_sun and z~1.5), but negligible variation between z~1.5 and z~5 (median
magnitude ~ -21.2, corresponding to M* ~ 5x10^9 M_sun). Dust-obscured GRBs
dominate the massive host population but are only rarely seen associated with
low-mass hosts, indicating that massive star-forming galaxies are universally
and (to some extent) homogeneously dusty at high-redshift while low-mass
star-forming galaxies retain little dust in their ISM. Comparing our luminosity
distributions to field surveys and measurements of the high-z mass-metallicity
relation, our results have good consistency with a model in which the GRB rate
per unit star-formation is constant in galaxies with gas-phase metallicity
below approximately the Solar value but heavily suppressed in more metal-rich
environments. This model also naturally explains the previously-reported
"excess" in the GRB rate beyond z>2; metals stifle GRB production in most
galaxies at z<1.5 but have only minor impact at higher redshifts. The
metallicity threshold we infer is much higher than predicted by single-star
models and favors a binary progenitor. Our observations also constrain the
fraction of cosmic star-formation in low-mass galaxies undetectable to Spitzer
to be a small minority at most redshifts (~10% at z~2, ~25% at z~3, and ~50% at
z=3.5-6.0).
