NGC 253 and a proposed sequence for nuclear starbursts

The modest-luminosity, isolated starburst galaxy NGC 253 shows strong shocked molecular hydrogen emission from a 100 pc region centered on its nucleus. This emission can arise in an early stage of a nuclear starburst where supernova explosions lead to large amounts of shocked molecular gas through collisions of dense molecular clouds and cloud implosion. Other properties of NGC 253 also indicate it is at a relatively early stage in its starburst. A sequence of starburst development is suggested; examples along this sequence from young to old would include NGC 5253, NGC 253, M82, NGC 4736, and M31. This sequence is based on the expected evolution of a broad variety of observed parameters, and it is consistent with the observed relation between nonthermal radio and thermal infrared luminosities. 48 references.