Star Captures by Quasar Accretion Disks: A Possible Explanation of theM‐σ Relation

A new theory of quasars is presented in which the matter of the thin accretion disk around the black hole is supplied by stars that crash on the disk. Stars in the central part of the host galaxy are randomly perturbed to highly radial orbits, and as they repeatedly cross the disk they lose orbital energy due to the drag force, eventually merging into the disk. Requiring the rate of stellar mass capture to equal the mass accretion rate into the black hole, a relation between the black hole mass and the stellar velocity dispersion is predicted of the form MBH \∝ sigma_*^(30/7). The normalization depends on various uncertain parameters such as the disk viscosity, but is consistent with observation for reasonable assumptions. We show that a seed central black hole in a newly formed stellar system can grow to this predicted mass by stellar captures on the accretion disk. Once this mass is reached, star captures can no longer maintain the disk at a high enough surface density and the quasar naturally turns off. The model provides a mechanism to deliver mass to the accretion disk at small radius, probably solving the problem of gravitational instability to star formation in the disk at large radii. The theory has the problem that the average angular momentum of the captured stars is negligible, preventing them from forming a disk by themselves. We suggest that a quasar wind and the star tails lifted from the disk mix matter at small radius with matter deposited at large radius by evolved stars in a rotating stellar system, absorbing its angular momentum. These same star tails may account for the broad emission line region. We propose several observational tests and consequences of this theory.