Black Hole Accretion Rates

Class providing models of the mass accretion rates onto supermassive black holes from the spheroid, hot halo, and nuclear star cluster components. Accretion drives black hole growth and AGN feedback. Implementations typically adopt Bondi-type formulae, Eddington-limited accretion, or empirical models that parameterize the accretion rate as a function of the available gas supply and the potential well depth.

Default implementation: blackHoleAccretionRateStandard

Methods

rateAccretion

Computes the mass accretion rate onto a black hole from the spheroid, hot halo, and nuclear star cluster components, returning each contribution separately (in \(\mathrm{M}_\odot\) Gyr\(^{-1}\)).

  • class(nodeComponentBlackHole) blackHole [inout]

  • double precision rateMassAccretionSpheroid [out]

  • double precision rateMassAccretionHotHalo [out]

  • double precision rateMassAccretionNuclearStarCluster [out]

blackHoleAccretionRateSpheroidTracking

A black hole accretion rate class that drives black hole growth in proportion to the stellar mass growth rate of the host spheroid. The ratio of black hole growth to spheroid stellar mass growth is set by the [growthRatioToStellarSpheroid] parameter.

Parameters

  • [growthRatioToStellarSpheroid] (real; default 1.0d-3) — The ratio of the rates of black hole growth and spheroid stellar mass growth.

blackHoleAccretionRateStandard

Computes black hole accretion rates via Bondi-Hoyle accretion from spheroid gas, hot halo gas, and nuclear star cluster gas. Enhancement factors above the classical Bondi rate are set independently via [bondiHoyleAccretionEnhancementSpheroid], [bondiHoyleAccretionEnhancementHotHalo], and [bondiHoyleAccretionEnhancementNuclearStarCluster]. The gas temperature assumed for each component and whether to restrict hot halo accretion to the hot-mode only are also configurable.

For accretion from the hot halo the Bondi radius is limited to the outer radius of the hot halo, and the resulting accretion rate is further limited to

\[\dot{M}_\mathrm{0,~hot~halo,~maximum} = M_\mathrm{hot}/\tau_\mathrm{sound~crossing},\]

where \(\tau_\mathrm{sound~crossing}=r_\mathrm{hot~halo~outer}/c_\mathrm{s}\), \(r_\mathrm{hot~halo~outer}\) is the outer radius of the hot halo, and \(c_\mathrm{s}\) is the speed of sound in the hot halo.

If [bondiHoyleAccretionHotModeOnly]\(=\)true then accretion occurs only from that fraction of the hot halo gas which was accreted in the “hot mode”. When the cold mode is explicitly tracked by the CGM component that fraction is taken to be unity (since the hot and cold reservoirs are then separate). Otherwise, a simple estimate of the hot mode fraction is made by the hotModeFraction method:

\[\begin{split}f_\mathrm{hot} = \left\{ \begin{array}{ll} 1 & \hbox{ if } x < 0.9 \\ y(x)^2[2 y(x) - 3]+1 & \hbox{ if } 0.9 \le x \le 1.0 \\ 0 & \hbox{ if } x > 1.0, \end{array} \right.\end{split}\]

where \(x = r_\mathrm{cool}/r_\mathrm{virial}\) and \(y(x)=[x-0.9]/[1.0-0.9]\). This smoothly switches off accretion from the hot halo as the cooling radius grows to the virial radius (i.e. as the halo leaves the slow cooling regime). The density of hot halo gas at the galactic center is scaled by \(f_\mathrm{hot}\) before the Bondi-Hoyle rate is evaluated.

(Default implementation)

Methods

hotModeFraction → double precision

Compute the fraction of a halo which is in the hot accretion mode.

  • type(treeNode) node [inout]

Parameters

  • [bondiHoyleAccretionEnhancementSpheroid] (real; default 5.0d0) — The factor by which the Bondi-Hoyle accretion rate of spheroid gas onto black holes is enhanced.

  • [bondiHoyleAccretionEnhancementHotHalo] (real; default 6.0d0) — The factor by which the Bondi-Hoyle accretion rate of hot halo gas onto black holes is enhanced.

  • [bondiHoyleAccretionEnhancementNuclearStarCluster] (real; default 5.0d0) — The factor by which the Bondi-Hoyle accretion rate of NSC gas onto black holes is enhanced.

  • [bondiHoyleAccretionHotModeOnly] (boolean; default .true.) — Determines whether accretion from the hot halo should only occur if the halo is in the hot accretion mode.

  • [bondiHoyleAccretionTemperatureSpheroid] (real; default 1.0d2) — The assumed temperature (in Kelvin) of gas in the spheroid when computing Bondi-Hoyle accretion rates onto black holes.

  • [bondiHoyleAccretionTemperatureNuclearStarCluster] (real; default 1.0d2) — The assumed temperature (in Kelvin) of gas in the NSC when computing Bondi-Hoyle accretion rates onto black holes.