ALMA Reveals the Molecular Medium Fueling the Nearest Nuclear Starburst
  We use ALMA to derive the mass, length, and time scales associated with the
nuclear starburst in NGC 253. This region forms ~2 M_sun/yr of stars and
resembles other starbursts in scaling relations, with star formation consuming
the gas reservoir 10 times faster than in galaxy disks. We present observations
of CO, the high effective density transitions HCN(1-0), HCO+(1-0), CS(2-1), and
their isotopologues. We identify ten clouds that appear as peaks in line
emission and enhancements in the HCN-to-CO ratio. These clouds are massive
(~10^7 M_sun) structures with sizes (~30 pc) similar to GMCs in other systems.
Compared to disk galaxy GMCs, they show high line widths (~20-40 km/s) given
their size, with implied Mach numbers ~90. The clouds also show high surface
(~6,000 M_sun/pc^2) and volume densities (n_H2~2,000 cm^-3). Given these,
self-gravity can explain the line widths. This short free fall time (~0.7 Myr)
helps explain the more efficient star formation in NGC 253. We also consider
the starburst region as a whole. The geometry is confused by the high
inclination, but simple models support a non-axisymmetric, bar-like geometry
with a compact, clumpy region of high gas density embedded in an extended CO
distribution. Even for the whole region, the surface density still exceeds that
of a disk galaxy GMC. The orbital time (~10 Myr), disk free fall time (<~ 3
Myr), and disk crossing time (<~ 3 Myr) are each much shorter than in a normal
spiral galaxy disk. Some but not all aspects of the structure correspond to
predictions from assuming vertical dynamical equilibrium or a marginally stable
rotating disk. Finally, the CO-to-H2 conversion factor implied by our cloud
calculations is approximately Galactic, contrasting with results showing a low
value for the whole starburst region. The contrast provides resolved support
for the idea of mixed molecular ISM phases in starburst galaxies.
