Star-forming accretion flows and the low-luminosity nuclei of giant elliptical galaxies

The luminosities of the centres of nearby elliptical galaxies are very low compared to models of thin disc accretion on 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 point out that, even if such processes are operating, a significant fraction of the accreting gas is prevented from reaching the central black hole because it condenses into stars in a gravitationally unstable disc. Star formation occurs inside the Bondi radius (typically ∼100 pc in giant ellipticals), but still relatively far from the black hole in terms of Schwarzschild radii. Star formation depletes and heats the gas disc, eventually leading to a marginally stable, but much reduced, accretion flow to the black hole. We predict the presence of cold (∼100 K), dusty gas discs, containing clustered Hα emission and occasional Type II supernovae, both resulting from the presence of massive stars. Star formation 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 rate inferred from the intensity of Hα emission is consistent with the Bondi accretion rate of the system. Star formation may therefore help to suppress accretion on to the central engines of massive ellipticals. We also discuss some implications for the fuelling of the Galactic Centre and quasar