  The central 2x1 kpc of the starburst galaxy NGC 253 has been imaged using the
Submillimeter Array at a 60 pc resolution in the J=2-1 transitions of 12CO,
13CO, and C18O as well as in the 1.3 mm continuum. Molecular gas and dust are
mainly in the circumnuclear disk of ~500 pc radius, with warm (~40 K) and high
area-filling factor gas in its central part. Two gas shells or cavities have
been discovered in the circumnuclear disk. They have ~100 pc diameters and have
large velocity widths of 80-100 km/s, suggestive of expansion at ~50 km/s.
Modeled as an expanding bubble, each shell has an age of ~0.5 Myr and needed
kinetic energy of ~1E46 J as well as mean mechanical luminosity of ~1E33 W for
its formation. The large energy allows each to be called a superbubble. A ~10^6
Msun super star cluster can provide the luminosity, and could be a building
block of the nuclear starburst in NGC 253. Alternatively, a hypernova can also
be the main source of energy for each superbubble, not only because it can
provide the mechanical energy and luminosity but also because the estimated
rate of superbubble formation and that of hypernova explosions are comparable.
Our observations indicate that the circumnuclear molecular disk harboring the
starburst is highly disturbed on 100 pc or smaller scales, presumably by
individual young clusters and stellar explosions, in addition to globally
disturbed in the form of the well-known superwind.
