Star-forming accretion flows: an explanation for the low luminosity nuclei of giant elliptical galaxies

The luminosities of the centers of nearby elliptical galaxies are very low compared to models of thin disc accretion to their black holes at the Bondi rate, typically a few hundredths to a few tenths of a solar mass per year. This has motivated models of inefficiently-radiated accretion that invoke weak electron-ion thermal coupling, and/or inhibited accretion rates due to convection or outflows. Here we suggest an alternative: most of the accreting gas does not reach the central black hole because it condenses into stars in a gravitationally unstable disc. The star formation occurs far from the central engine, but inside the Bondi radius, which is typically ∼ 100 pc in giant ellipticals. The model predicts the presence of cold, dusty gas discs, as well as associated Hα emission, thermal emission from the dusty gas disc and type II supernovae, all of which result from the presence of young, massive stars. The model accounts for several features of the M87 system: a thin disc, traced by Hα emission, is observed on scales of about 100 pc, with features reminiscent of spiral arms and dust lanes; the star formation rates inferred from the intensity of Hα and infrared emission are consistent with the Bondi accretion rate of the system. Finally we discuss some implications of this model for the fueling of the Galactic center and quasars.