Transition of BH feeding from the quiescent regime into star-forming cold disc regime

We study the properties of rotating accretion flows on to supermassive black holes (SMBHs) using axisymmetric two-dimensional hydrodynamical simulations with radiative cooling and BH feedback. The simulations resolve the accretion dynamics of gas outside from the BH influence radius through an inner accretion disc. For lower Bondi accretion rates in units of the Eddington rate (M_B ≪ 10^-3 M_Edd), the BH feeding is suppressed due to turbulent motion by several orders of magnitude from the Bondi rate with outflows to the Bondi radius nearly balancing inflows. Thus, the radiative luminosity results in as low as ~10^-10-10^-7 LEdd, where LEdd is the Eddington luminosity. For higher rates of M_B ≳ 10^-3 M_Edd, the optically thin accreting gas cools via free-free emission and forms a geometrically thin disc, which feeds the BH efficiently and increases the radiative luminosity to ≳10^-3 L_Edd. The transitional behaviour of accreting BHs in galactic nuclei from radiatively inefficient phases to cold disc accretion naturally explains (1) the reason for the offset between the observed luminosities and theoretical predictions for nearby quiescent SMBHs, and (2) the conditions to fuel gas into the nuclear SMBH. In addition, the cold disc formed in galactic nuclei tends to be gravitationally unstable and leads to star formation when the Bondi rate is as high as M_B ≳ 10^-2M⊙ yr^-1. This is a plausible explanation of the correlation observed between star formation rates and BH feeding rates in Seyfert galaxies.