FUELING NUCLEAR ACTIVITY IN DISK GALAXIES - STARBURSTS AND MONSTERS

We study the evolution of the gas distribution in a globally unstable galactic disk with a particular emphasis on the gasdynamics in the cen tral kiloparsec and the fueling activity there. The two-component self -gravitating disk is embedded in a responsive halo of comparable mass. The gas and stars are evolved using a three-dimensional hybrid smooth ed particle hydrodynamics/N-body code and the gravitational interactio ns are calculated using a hierarchical TREE algorithm. A massive ''sta r formation'' is introduced when the gas becomes Jeans unstable and lo cally exceeds the critical density of approximately 100 M. pc-3. The n ewly formed OB stars deposit energy in the gas by means of radiation-d riven winds and supernovae. This energy is partially thermalized (effi ciency of a few percent); the rest is radiated away. Models without st ar formation are evolved for a comparison. The effect of a massive obj ect at the disk center is studied by placing a ''seed'' black hole (BH ) of 5 x 10(7) M. with an accretion radius of 20 pc. The tendency of t he system to form a massive object spontaneously '' is tested in model s without the BH. We find that for models without star formation the b ar- or dynamical friction-driven inflows lead to 1) domination of the central kpc by a few massive clouds that evolve into a single object p robably via a cloud binary system, with and without a '' seed '' BH, 2 ) accretion onto the BH which has a sporadic character, and 3) formati on of remnant disks around the BH with a radius of 60-80 pc which resu lt from the capture and digestion of clouds. For models with star form ation, we find that 1) the energy input into the gas induces angular m omentum loss and inflow rates by a factor < 3, 2) the star formation i s concentrated mainly at the apocenters of the gaseous circulation in the stellar bar and in the nuclear region, 3) the nuclear starburst ph ase appears to be very luminous approximately 10(45)-10(46) erg s-1 an d episodic with a typical single burst duration of approximately 10(7) yr, and 4) the starburst phase coincides with both the gas becoming d ynamically important and the catastrophic growth of the BH. It ends wi th the formation of cold residual < 1 kpc radius gas disks. Models wit hout the '' seed '' BH form < 1 kpc radius fat disks which dominate th e dynamics. Gaseous bars follow, drive further inflow and ma fission i nto a massive cloud binary system at the center.