ALMA Observations toward the starburst dwarf galaxy NGC 5253: I.
  Molecular cloud properties and scaling relations
  We present high-spatial-resolution ($\sim 0\farcs2$, or $\sim$3\,pc) CO(2--1)
observations of the nearest young starburst dwarf galaxy, NGC\,5253, taken with
the Atacama Large Millimeter/submillimeter Array. We have identified 118
molecular clouds with average values of 4.3\,pc in radius and 2.2\,\kms\, in
velocity dispersion, which comprise the molecular cloud complexes observed
previously with $\sim$100\,pc resolution. We derive for the first time in this
galaxy the $I{\rm (CO)}$--$N$(H$_2$) conversion factor, $X$ =
$4.1^{+5.9}_{-2.4}\times10^{20}$\,cm$^{-2}$(K\,\kms)$^{-1}$, based on the
virial method. The line-width and mass-to-size relations of the resolved
molecular clouds present an offset on average toward higher line-widths and
masses with respect to quiescent regions in other nearby spiral galaxies and
our Galaxy. The offset in the scaling relation reaches its maximum in regions
close to the central starburst, where velocity dispersions are $\sim$ 0.5 dex
higher and gas mass surface densities are as high as $\Sigma_{\rm H_2}$ =
10$^3$\,\Msol\,pc $^{-2}$. These central clouds are gravitationally bound
despite the high internal pressure. A spatial comparison with star clusters
found in the literature enables us to identify six clouds that are associated
with young star clusters. Furthermore, the star formation efficiencies (SFEs)
of some of these clouds exceed those found in star-cluster-forming clouds
within our Galaxy. We conclude that once a super star cluster is formed, the
parent molecular clouds are rapidly dispersed by the destructive stellar
feedback, which results in such a high SFE in the central starburst of
NGC\,5253.
