The central 2 \times 1 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 ^ { 12 } CO , ^ { 13 } CO , and C ^ { 18 } O as well as in the 1.3 mm continuum .
Molecular gas and dust are mainly in the circumnuclear disk of \sim 500 pc radius , with warm ( \sim 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 \sim 100 pc diameters and have large velocity widths of 80–100 km s ^ { -1 } , suggestive of expansion at \sim 50 km s ^ { -1 } .
Modeled as an expanding bubble , each shell has an age of \sim 0.5 Myr and needed kinetic energy of \sim 1 \times 10 ^ { 46 } J as well as mean mechanical luminosity of \sim 1 \times 10 ^ { 33 } W for its formation .
The large energy allows each to be called a superbubble .
A \sim 10 ^ { 6 } M _ { \odot } 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 .