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Register a new userDynamics of radiatively inefficient flows accreting onto radiatively efficient black hole objects
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Daniel Proga
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I present results from numerical simulations of gas dynamics outside luminous accretion disks in active galactic nuclei. The gas, gravitationally captured by a super massive black hole, can be driven away by the energy and momentum of the radiation emitted during black hole accretion. Assuming axisymmetry, I study how the mass accretion and outflow rates, and the flow dynamics respond to changes in radiation heating relative to radiation pressure. I find that for a 10^8 MSUN black hole with the accretion luminosity of 0.6 of the Eddington luminosity the flow settles into a steady state and has two components: (1) an equatorial inflow and (2) a bipolar inflow/outflow with the outflow leaving the system along the disk rotational axis. The inflow is a realization of a Bondi-like accretion flow. The second component is an example of a non-radial accretion flow which becomes an outflow once it is pushed close to the rotational axis where thermal expansion and radiation pressure accelerate it outward. The main result of this preliminary work is that although the above two-component solution is robust, its properties are sensitive to the geometry and spectral energy distribution of the radiation field.
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We study low-density axisymmetric accretion flows onto black holes (BHs) with two-dimensional hydrodynamical simulations, adopting the $alpha$-viscosity prescription. When the gas angular momentum is low enough to form a rotationally supported disk within the Bondi radius ($R_{rm B}$), we find a global steady accretion solution. The solution consists of a rotational equilibrium distribution at $rsim R_{rm B}$, where the density follows $rho propto (1+R_{rm B}/r)^{3/2}$, surrounding a geometrically thick and optically thin accretion disk at the centrifugal radius, where thermal energy generated by viscosity is transported via strong convection. Physical properties of the inner solution agree with those expected in convection-dominated accretion flows (CDAF; $rho propto r^{-1/2}$). In the inner CDAF solution, the gas inflow rate decreases towards the center due to convection ($dot{M}propto r$), and the net accretion rate (including both inflows and outflows) is strongly suppressed by several orders of magnitude from the Bondi accretion rate $dot{M}_{rm B}$ The net accretion rate depends on the viscous strength, following $dot{M}/dot{M}_{rm B}propto (alpha/0.01)^{0.6}$. This solution holds for low accretion rates of $dot{M}_{rm B}/dot{M}_{rm Edd}< 10^{-3}$ having minimal radiation cooling, where $dot{M}_{rm Edd}$ is the Eddington rate. In a hot plasma at the bottom ($r<10^{-3}~R_{rm B}$), thermal conduction would dominate the convective energy flux. Since suppression of the accretion by convection ceases, the final BH feeding rate is found to be $dot{M}/dot{M}_{rm B} sim 10^{-3}-10^{-2}$. This rate is as low as $dot{M}/dot{M}_{rm Edd} sim 10^{-7}-10^{-6}$ inferred for SgrA$^*$ and the nuclear BHs in M31 and M87, and can explain the low luminosities in these sources, without invoking any feedback mechanism.
We present magnetohydrodynamic simulations of a resistive accretion disk continuously launching transmagnetosonic, collimated jets. We time-evolve the full set of magnetohydrodynamic equations, but neglect radiative losses in the energetics (radiatively inefficient). Our calculations demonstrate that a jet is self-consistently produced by the interaction of an accretion disk with an open, initially bent large-scale magnetic field. A constant fraction of heated disk material is launched in the inner equipartition disk regions, leading to the formation of a hot corona and a bright collimated, super-fastmagnetosonic jet. We illustrate the complete dynamics of the ``hot near steady-state outflow (where thermal pressure $simeq$ magnetic pressure) by showing force balance, energy budget and current circuits. The evolution to this near stationary state is analyzed in terms of the temporal variation of energy fluxes controlling the energetics of the accretion disk. We find that unlike advection-dominated accretion flow, the energy released by accretion is mainly sent into the jet rather than transformed into disk enthalpy. These magnetized, radiatively inefficient accretion-ejection structures can account for under-luminous thin disks supporting bright fast collimated jets as seen in many systems displaying jets (for instance M87).
We consider the angular momentum exchange at the corotation resonance between a two-dimensional gaseous disk and a uniformly rotating external potential, assuming that the disk flow is adiabatic. We first consider the linear case for an isolated resonance, for which we give an expression of the corotation torque that involves the pressure perturbation, and which reduces to the usual dependence on the vortensity gradient in the limit of a cold disk. Although this expression requires the solution of the hydrodynamic equations, it provides some insight into the dynamics of the corotation region. In the general case, we find an additional dependence on the entropy gradient at corotation. This dependence is associated to the advection of entropy perturbations. These are not associated to pressure perturbations. They remain confined to the corotation region, where they yield a singular contribution to the corotation torque. In a second part, we check our torque expression by means of customized two-dimensional hydrodynamical simulations. In a third part, we contemplate the case of a planet embedded in a Keplerian disk, assumed to be adiabatic. We find an excess of corotation torque that scales with the entropy gradient, and we check that the contribution of the entropy perturbation to the torque is in agreement with the expression obtained from the linear analysis. We finally discuss some implications of the corotation torque expression for the migration of low mass planets in the regions of protoplanetary disks where the flow is radiatively inefficient on the timescale of the horseshoe U-turns.
We explore the MRI driven dynamo in a radiatively inefficient accretion flow (RIAF) using the mean field dynamo paradigm. Using singular value decomposition (SVD) we obtain the least squares fitting dynamo coefficients $alpha$ and $gamma$ by comparing the time series of the turbulent electromotive force and the mean magnetic field. Our study is the first one to show the poloidal distribution of these dynamo coefficients in global accretion flow simulations. Surprisingly, we obtain a high value of the turbulent pumping coefficient $gamma$ which transports the mean magnetic flux radially outward. This would have implications for the launching of magnetised jets which are produced efficiently in presence a large-scale poloidal magnetic field close to the compact object. We present a scenario of a truncated disc beyond the RIAF where a large scale dynamo-generated poloidal magnetic field can aid jet-launching close to the black hole. Magnitude of all the calculated coefficients decreases with radius. Meridional variations of $alpha_{phi phi}$, responsible for toroidal to poloidal field conversion, is very similar to that found in shearing box simulations using the `test field (TF) method. By estimating the relative importance of $alpha$-effect and shear, we conclude that the MRI driven large-scale dynamo, which operates at high latitudes beyond a disc scale height, is essentially of the $alpha-Omega$ type.
We explore the effect of momentum-driven winds representing radiation pressure driven outflows from accretion onto supermassive black holes in a set of numerical hydrodynamical simulations. We explore two matched sets of cosmological zoom-in runs of 24 halos with masses ~$10^{12.0}-10^{13.4}$ M_sun run with two different feedback models. Our `NoAGN model includes stellar feedback via UV heating, stellar winds and supernovae, photoelectric heating and cosmic X-ray background heating from a meta-galactic background. Our fiducial `MrAGN model is identical except that it also includes a model for black hole seeding and accretion, as well as heating and momentum injection associated with the radiation from black hole accretion. Our MrAGN model launches galactic outflows which result in both `ejective feedback - the outflows themselves which drive gas out of galaxies - and `preventative feedback, which suppresses the inflow of new and recycling gas. As much as 80 % of outflowing galactic gas can be expelled, and accretion can be suppressed by as much as a factor of 30 in the MrAGN runs when compared with the NoAGN runs. The histories of NoAGN galaxies are recycling-dominated, with ~70% of material that leaves the galaxy eventually returning, and the majority of outflowing gas re-accretes on 1 Gyr timescales without AGN feedback. Outflowing gas in the MrAGN runs has higher characteristic velocity (500 - 1,000 km/s versus 100-300 km/s for outflowing NoAGN gas) and travels as far as a few Mpcs. Only ~10% of ejected material is re-accreted in the MrAGN galaxies.
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