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 central 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 smoothed particle hydrodynamics/N-body code and the gravitational interactions are calculated using a hierarchical TREE algorithm. A massive "star formation" is introduced when the gas becomes Jeans unstable and locally exceeds the critical density of ∼100 M_⊙ pc^-3. The newly formed OB stars deposit energy in the gas by means of radiation-driven winds and supernovae. This energy is partially thermalized (efficiency of a few percent); the rest is radiated away. Models without star formation are evolved for a comparison. The effect of a massive object at the disk center is studied by placing a "seed" black hole (BH) of 5 × 10⁷ M_⊙ with an accretion radius of 20 pc. The tendency of the system to form a massive object "spontaneously" is tested in models without the BH. We find that for models without star formation the bar- or dynamical friction-driven inflows lead to 1) domination of the central kpc by a few massive clouds that evolve into a single object probably via a cloud binary system, with and without a "seed" BH, 2) accretion onto the BH which has a sporadic character, and 3) formation of remnant disks around the BH with a radius of 60-80 pc which result from the capture and digestion of clouds. For models with star formation, we find that 1) the energy input into the gas induces angular momentum loss and inflow rates by a factor <3, 2) the star formation is concentrated mainly at the apocenters of the gaseous circulation in the stellar bar and in the nuclear region, 3) the nuclear starburst phase appears to be very luminous ∼10⁴⁵-10⁴⁶ erg s^-1 and episodic with a typical single burst duration of ∼10⁷ yr, and 4) the starburst phase coincides with both the gas becoming dynamically important and the catastrophic growth of the BH. It ends with the formation of cold residual <1 kpc radius gas disks. Models without the "seed" BH form <1 kpc radius fat disks which dominate the dynamics. Gaseous bars follow, drive further inflow, and may fission into a massive cloud binary system at the center.