Fueling Gas to the Central Region of Galaxies

Supplying gas to the galactic central regions is one of key ingredients for AGN activity. I will review various fueling mechanisms for a R ≈ 0.1 kpc region, determined mainly by numerical simulations over the last decade. I will also comment on the bars-within-bars mechanism. Observations suggest that the stellar bar is not a sufficient condition for gas fueling. Moreover, considering the various factors for the onset of gas accretion, stellar bars would not even be a necessary condition. I introduce recent progress obtained through our twoand three-dimensional, high resolution hydrodynamical simulations of the ISM in the central 0.1–1 kpc region of galaxies. Possible structure of the obscuring molecular tori around AGNs is also shown. The nuclear starburst is an important factor in determining the structure of the molecular tori and the mass accretion rate to the nucleus. It is natural that the ISM in the central 100 pc region is a highly inhomogeneous and turbulent structure. As a result, gas accretion to the central parsec region should be time dependent and stochastic. The conventional picture of gas fueling and the AGN unified model may be modified in many respects. 1.1 Conventional Picture of the Fueling Problem Accretion of gas to the supermassive black hole in the galactic center is the source of all AGN activity. A long-standing issue concering this gas supply is the “fueling problem” — that is, the question of how to remove the large angular momentum of the gas in a galactic disk and funnel it into the accretion disk in the central AU region. This was one of the main topics at the “Mass-transfer Induced Activity in Galaxies” conference held in Lexington in 1993 (Shlosman 1994). The well known cartoon by E. S. Phinney of a baby being fed by a huge spoon, published in the workshop’s proceedings, well represents the essence of the fueling problem. To power an AGN luminosity of ∼ 1010−11 L⊙, we need a mass accretion rate of ∼ 0.1M⊙ yr−1 with a ∼ 10% energy conversion rate. Therefore, to maintain the AGN activity during its lifetime of 108 yr, a large amount of the gas, 107 M⊙ = 0.1M⊙ yr−1 ×108 yr, must be funneled into the black hole. The galactic disk is probably a reservoir of the gas, and since the time scale of 108 yr is comparable to the rotational time scale of galaxies, it is natural to postulate that the gas is accumulated from the galactic disk. A number of mechanisms for removing the angular momentum of the gas have been proposed. Among them, the use of gravitational torques due to galaxy-galaxy interactions (e.g., major/minor