The quenching of star formation in accretion-driven clumpy turbulent tori of active galactic nuclei

摘要

Galactic gas-gas collisions involving a turbulent multiphase interstellar medium (ISM) share common ISM properties: dense extraplanar gas visible in CO, large linewidths (?50?km?s-1), strong mid-infrared H2 line emission, low star formation activity, and strong radio continuum emission. Gas-gas collisions can occur in the form of ram pressure stripping caused by the rapid motion of a spiral galaxy within the intracluster medium, galaxy head-on collisions, compression of the intragroup gas and/or galaxy ISM by an intruder galaxy which flies through the galaxy group at a high velocity, or external gas accretion on an existing gas torus in a galactic center. We suggest that the common theme of all these gas-gas interactions is adiabatic compression of the ISM leading to an increase of the turbulent velocity dispersion of the gas. The turbulent gas clouds are then overpressured and star formation is quenched. Within this scenario we developed a model for turbulent clumpy gas disks where the energy to drive turbulence is supplied by external infall or the gain of potential energy by radial gas accretion within the disk. The cloud size is determined by the size of a continuous (C-type) shock propagating in dense molecular clouds with a low ionization fraction at a given velocity dispersion. We give expressions for the expected volume and area filling factors, mass, density, column density, and velocity dispersion of the clouds. The latter is based on scaling relations of intermittent turbulence whose open parameters are estimated for the circumnuclear disk in the Galactic center. The properties of the model gas clouds (~0.1?pc, ?~100?M⊙, Δv???6?km?s-1) and the external mass accretion rate necessary for the quenching of the star formation rate due to adiabatic compression (??~?1?10???M⊙?yr-1) are consistent with those derived from high-resolution H2 2.12?μm line observations. Based on these findings, a scenario for the evolution of gas tori in galactic centers is proposed and the implications for star formation in the Galactic center are discussed.