Bondi accretion of slowly rotating gas onto a supermassive black hole

1. Bondi accretionof slowly rotating gasonto a supermassive black hole • N.A. Inogamov1, R.A. Sunyaev2,3 • 1Landau Institute for Theoretical Physics, RAS • 2Max Planck Institut für Astrophysik, Garching • 3Space Research Institute, Russian Academy of Sciences (RAS)

2. Bondi radius • cs2 = G M / rB – thermal velocities are of the order of gravitational velocities at the Bondi radius. • This condition defines the Bondi radius: • rB = G M / cs2 • Above the rB gravity • is less important, • while below • it dominates • For M87 rB~0.1 kpc= • 100 pc~105 rg • rg~1015 cm for M87

3. Feeding of BH Motion of point mass in a gravity field and gas-dynamics motion

4. Flow structure Gaseous surrounding • Main ingredients: (1) Outside gas— Bondi radius—Atmosphere. • Why atmosphere? Because the CFB slows down accretion. –No SW ! • (2) CFB is CentriFugal Barrier • Leakage through CFB feeds TWO Disks • (3) There is the External disk which withdraws angular momentum of mass accreting onto BH • *** *** *** *** *** *** • To create thin CFB -- cooling of gas is necessary  EHC. • EHC + Rad.cooling

5. Simplified scheme of accretion through obstacle (= through the CFB) and resulting splitting into two disks. Effective cooling of gas is necessary • Deviation from radial accretion and subsonic settling of gas into the torus with radius rc. Weakly rotating gas increases its angular velocity v' up to Keplerian velocity as it approaches the torus. • The torus is thin, since its temperature is much smaller than virial temperature thanks to EHC and radiative loses. • In the torus the settling flow branches into two opposite flows. One is going to the left to the SMBH, while the other is forming the inverse disk

6. Thin torus cooled by combined action of EHC(= electron heat conduction) and bremsstrahlung • Arrival of accreting mass into torus at the centrifugal radius rcand diffusive separation of this flux into two sides - to the left to the SMBH through the disk-1 and ADAF and to the right into the disk-2. The disk-2 returns matter back from a deep gravity potential well. This lifting process is a result of angular momentum recoil from the disk-1 to disk-2.

7. Structure of inverse disk Pressure (the left axis) and density (the right axis) profiles of the external disk (this is the inverse disk or disk-2 in the previous pages). Typical CFB position is x = r/rB = 0.01-0.03

8. Thickness of the inverse disk • Thickness of the inverse disk and density of gas in it. • The disk is thin. Indeed, at Bondi-radius its thickness is ~ 10-4 from rB.

9. Heat sources of disk-2 and its radiative loses • Radiative loses and viscous heating. • Erad = n2 h Λ(T), where Λ(T) is cooling function taken from McKee, Cowie 1977

10. Conclusion • The Hubble Space Telescope discovered the Keplerian ring of relatively cold gas radiating in H-beta and optical forbidden lines of oxygen, sulfur, nitrogen and other ions around the central object in M87. The analysis of the profile of these lines permitted to find the mass of the central black hole in this giant elliptical galaxy Mbh ~ 3x109Msun, and to find the distance of this ring from the black hole. It is obvious that the temperature of the gas in this ring is of the order of 10-30 000 K. In our previous paper we reminded that M87 has an extremely low "spin" of the stellar population and mentioned that the gas accreting onto the black hole has a relatively low angular momentum. In the simplest scenario of the gas accretion with low angular momentum the quasi spherical flow should stop near the centrifugal radius defined by the value of the average angular momentum of matter captured near the outer boundary of the Bondi flow. If there are effective cooling processes (saturated electron thermal conductivity, bremsstrahlung or strong Compton cooling) the rather thin torus of the rapidly rotating matter should form near the centrifugal radius. In the presence of effective viscosity the two flows might originate from this torus. The first flow toward the black hole might lead to the formation of a disklike structure formed by accreting matter. The additional disklike outflow might be formed due to the necessity to carry the exccess of the angular momentum out. The radial velocity of this outflow is decreasing with distance from the black hole. In this paper we try to explain the observed Keplerian ring around M87 as an appearance of the outer regions of this outflow disk. According to our estimates this disk might be placed significantly farther than the torus near the centrifugal radius and rather close to the Bondi radius. This scenerios is similar to that proposed by Kolykhalov and Sunyaev, 1980 for bright quasars and active galactic nuclei where the outflowing disk might lead to the formation of the massive selfgravitating torus responsible for the formation of the ring of new born stars. In the case of M87, where the accretion rate is relatively low today, the outflowing disk has very low surface density and is unable to create the ring of stars, but possibly is able to produce a ring of gas bright in the H-beta and optical forbidden lines.