The Molecular Clouds Fueling a 1/5 Solar Metallicity Starburst
  Using the Atacama Large Millimeter/submillimeter Array, we have made the
first high spatial and spectral resolution observations of the molecular gas
and dust in the prototypical blue compact dwarf galaxy II Zw 40. The CO(2-1)
and CO(3-2) emission is clumpy and distributed throughout the central
star-forming region. Only one of eight molecular clouds has associated star
formation. The continuum spectral energy distribution is dominated by free-free
and synchrotron: at 870$\mu m$, only 50% of the emission is from dust. We
derive a CO-to-H$_2$ conversion factor by several methods including a new
method that uses simple photodissocation models and resolved CO line intensity
measurements to derive a relationship that uniquely predicts $\alpha_{CO}$ for
a given metallicity. We find that the CO-to-H$_2$ conversion factor is 4 to 35
times that of the Milky Way (18.1 to 150.5 M$_\odot$ / (K km/s pc$^2$)). The
star formation efficiency of the molecular gas at least 10 times higher than
that found in normal spiral galaxies, which is likely due to the
burst-dominated star formation history of II Zw 40 rather than an intrinsically
higher efficiency. The molecular clouds within II Zw 40 resemble those in other
strongly interacting systems like the Antennae: overall they have high
size-linewidth coefficients and molecular gas surface densities. These
properties appear to be due to the high molecular gas surface densities
produced in this merging system rather than to increased external pressure.
Overall, these results paint a picture of II Zw 40 as a complex, rapidly
evolving system whose molecular gas properties are dominated by the large-scale
gas shocks from its on-going merger.
