Circumnuclear media of quiescent supermassive black holes

Abstract

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 inflow rate,Ṁ , 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 forṀ (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 SMBHs 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.

High-energy cosmic ray nuclei from tidal disruption events: Origin, survival, and implications

“…The CNM gas density in the galaxy nuclear region is still unclear. It can originate from the stellar winds, and their profile depends on the detailed distribution of stars and star formation history in the galaxy nuclear region [84,85]. We assume that the CNM density is constant in our analysis, which is reasonable for galaxy cores [84].…”

On the charge of the Galactic centre black hole

“…1. For the case when the heating rate due to fast outflows vw is larger than the stellar velocity dispersion σ , vw σ , the stagnation radius can be approximately expressed as (Generozov et al 2015),…”

“…where Γ is the inner power-law slope of the stellar brightness profile, where we consider two limiting cases, the core profile with Γ = 0.1 and the cusp profile with Γ = 0.8. The quantity ν = −dρ/dr|R stag is the gas density power-law slope at RStag, which according to the numerical analysis of Generozov et al (2015) is ν ≈ 1/6[(4Γ + 3)]. According to the estimates in Eq.…”

A generalized Bondi accretion model for the galactic centre

“…In other words the cooling timescale ∼ m 2 p c 2 s /ρΛ must be shorter than the advection time ∼ r/u, i.e. One effect of cooling, when the above conditions are satisfied, is to give rise thermal instabilities (Generozov et al 2015), in which some of the gas undergoes runaway cooling and condenses' into dense clumps. At high temperatures, thermal Bremsstrahlung is the dominant cooling mechanism and the Λ ∝ T 1/2 dependence of the cooling function gas is thermally stable.…”