MOLECULAR-HYDROGEN EMISSION IN NGC-4536

We present near-infrared images of the 2-mu m continuum and H-2 nu = 1 -0 S(1) 2.12-mu m line emission from the starburst galaxy NGC 4536. Th e morphologies show striking differences: the continuum is strongly pe aked at the nucleus, while the molecular hydrogen clearly shows subsid iary peaks. These coincide with emission peaks observed in the radio a nd 10.8-mu m continuum, supporting the hypothesis of an edge-on ring e xperiencing star formation. Consideration of the possible excitation m echanisms of the 1-0 S(1) line has led us to conclude that both supern ovae and OB stars are important. Shacks in cloud collisions may also c ontribute to the extended H-2 regions. We postulate that the star form ation at the nucleus has come to an end, so that supernovae now domina te the emission, while in the ring star formation is still active and OB stars make a significant contribution. The implications of this are that if the nuclear star formation has occurred first, it must also b e fuelled first. We show that this is possible by considering bar-indu ced gas infall: when the bar first forms, gas is channelled in towards the centre; however, when the nuclear mass has built up sufficiently, inner Lindblad resonances form and the inflow will cease in the circu mnuclear regions. Thus star formation in the ring is fuelled at a late r time than in the nucleus.