Extreme Variation in Star Formation Efficiency Across a Compact,
  Starbursting Disk Galaxy
  We report on the internal distribution of star formation efficiency in IRAS
08339+6517 (hereafter IRAS08), using $\sim$200~pc resolution CO(2-1)
observations from NOEMA. The molecular gas depletion time changes by 2
orders-of-magnitude from disk-like values in the outer parts to less than
10$^8$~yr inside the half-light radius. This translates to a star formation
efficiency per free-fall time that also changes by 2 orders-of-magnitude,
reaching 50-100\%, different than local spiral galaxies and typical assumption
of constant, low star formation efficiencies. Our target is a compact, massive
disk galaxy that has SFR 10$\times$ above the $z=0$ main-sequence; Toomre
$Q\approx0.5-0.7$ and high gas velocity dispersion ($\sigma_{mol}\approx
25$~km~s$^{-1}$). We find that IRAS08 is similar to other rotating, starburst
galaxies from the literature in the resolved
$\Sigma_{SFR}\propto\Sigma_{mol}^N$ relation. By combining resolved literature
studies we find that distance from the main-sequence is a strong indicator of
the Kennicutt-Schmidt powerlaw slope, with slopes of $N\approx1.6$ for
starbursts from 100-10$^4$~M$_{\odot}$~pc$^{-2}$. Our target is consistent with
a scenario in which violent disk instabilities drive rapid inflows of gas. It
has low values of Toomre-$Q$, and also at all radii the inflow timescale of the
gas is less than the depletion time, which is consistent with the flat
metallicity gradients in IRAS08. We consider these results in light of popular
star formation theories, in general observations of IRAS08 find the most
tension with theories in which star formation efficiency is a constant. Our
results argue for the need of high spatial resolution CO observations are a
larger number of similar targets.
