Abstract
We present an analysis of the global and spatially-resolved Kennicutt-Schmidt
star formation relation in the FIRE (Feedback In Realistic Environments) suite
of cosmological simulations, including halos with $z = 0$ masses ranging from
$10^10$ -- $10^13$ M$_ødot$. We show that the Kennicutt-Schmidt (KS)
relation emerges robustly due to the effects of feedback on local scales,
independent of the particular small-scale star formation prescriptions
employed. This is true for the KS relation measured using all of the gas and
using only the dense (molecular) gas. We demonstrate that the time-averaged KS
relation is relatively independent of redshift and spatial averaging scale, and
that the star formation rate surface density is weakly dependent on metallicity
($Z^1/4$). Finally, we show that on scales larger than individual
giant molecular clouds, the primary condition that determines whether star
formation occurs is whether a patch of the galactic disk is thermally
Toomre-unstable (not whether it is self-shielding): once a patch can no longer
be thermally stabilized against fragmentation, it collapses, becomes
self-shielding, cools, and forms stars.
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