Circumnuclear media of quiescent supermassive black holes

We calculate steady-state, one-dimensional hydrodynamic profiles of hot gas in slowly accreting ('quiescent') galactic nuclei for a range of central black hole masses M_•, parametrized gas heating rates, and observationally motivated stellar density profiles. Mass is supplied to the circumnuclear medium by stellar winds, while energy is injected primarily by stellar winds, supernovae, and black hole feedback. Analytic estimates are derived for the stagnation radius (where the radial velocity of the gas passes through zero) and the large-scale gas inflowrate, M˙, as a function of M_• and the gas heating efficiency, the latter being related to the star formation history. We assess the conditions under which radiative instabilities develop in the hydrostatic region near the stagnation radius, both in the case of a single burst of star formation and for the average star formation history predicted by cosmological simulations. By combining a sample of measured nuclear X-ray luminosities, L_X, of nearby quiescent galactic nuclei with our results for M˙(M_•), we address whether the nuclei are consistent with accreting in a steady state, thermally stable manner for radiative efficiencies predicted for radiatively inefficiency accretion flows. We find thermally stable accretion cannot explain the short average growth times of low-mass black holes in the local Universe, which must instead result from gas being fed in from large radii, due either to gas inflows or thermal instabilities acting on larger, galactic scales. Our results have implications for attempts to constrain the occupation fraction of upermassive black holes in low-mass galaxies using the mean L_X-M_• correlation, as well as the predicted diversity of the circumnuclear densities encountered by relativistic outflows from tidal disruption events.