Molecular superbubbles in the starburst galaxy NGC 253

The central 2; 1 kpc of the starburst galaxy NGC 253 have been imaged using the Submillimeter Array at a 60 pc resolution in the J = 2 - 1 transitions of (CO)-C-12, (CO)-C-13, and (CO)-O-18, as well as in the 1.3 mm continuum. Molecular gas and dust are distributed mainly in a circumnuclear disk of similar to 500 pc radius, with warm (similar to 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 similar to 100 pc diameters and have large velocity widths of 80 - 100 km s(-1), suggestive of expansion at similar to 50 km s(-1) . Modeled as an expanding bubble, each shell has an age of similar to 0.5 Myr and needed kinetic energy of similar to 1; 10(46) J, as well as mean mechanical luminosity of similar to 1; 10(33) W, for its formation. The large energy allows each to be called a superbubble. A similar to 10(6) M circle dot 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 being globally disturbed in the form of the well-known superwind.