ALMA $^{12}$CO (J=1--0) imaging of the nearby galaxy M83: Variations in
  the efficiency of star formation in giant molecular clouds
  We present results of the $^{12}$CO (1--0) mosaic observations of the nearby
barred-spiral galaxy M83 obtained with the Atacama Large
Millimeter/submillimeter Array (ALMA). The total flux is recovered by combining
the ALMA data with single-dish data obtained using the Nobeyama 45-m telescope.
The combined map covers a $\sim$13 kpc$^{2}$ field that includes the galactic
center, eastern bar, and spiral arm with a resolution of \timeform{2''.03}
$\times$ \timeform{1''.1} ($\sim$45 pc $\times$ $\sim$25 pc). With a resolution
comparable to typical sizes of giant molecular clouds (GMCs), the CO
distribution in the bar and arm is resolved into many clumpy peaks that form
ridge-like structures. Remarkably, in the eastern arm, the CO peaks form two
arc-shaped ridges that run along the arm and exhibit a distinct difference in
the activity of star formation: the one on the leading side has numerous HII
regions associated with it, whereas the other one on the trailing side has only
a few. To see whether GMCs form stars with uniform star formation efficiency
(SFE) per free-fall time (SFEff), GMCs are identified from the data cube and
then cross-matched with the catalog of HII regions to estimate the star
formation rate for each of them. 179 GMCs with a median mass of 1.6 $\times$
10$^{6}$ $M_{\odot}$ are identified. The mass-weighted average SFEff of the
GMCs is $\sim$9.4 $\times$ 10$^{-3}$, which is in agreement with models of
turbulence regulated star formation. Meanwhile, we find that SFEff is not
universal within the mapped region. In particular, one of the arm ridges shows
a high SFEff with a mass-weighted value of $\sim$2.7 $\times$ 10$^{-2}$, which
is higher by more than a factor of 5 compared to the inter-arm regions. This
large regional variation in SFEff favors the recent interpretation that GMCs do
not form stars at a constant rate within their lifetime.
