Abstract
We suggest a new picture of supermassive black hole (SMBH) growth in galaxy centers. Momentum-driven feedback from an accreting hole gives significant orbital energy but little angular momentum to the surrounding gas. Once central accretion drops, the feedback weakens and swept-up gas falls back towards the SMBH on near-parabolic orbits. These intersect near the black hole with partially opposed specific angular momenta, causing further infall and ultimately the formation of a small-scale accretion disk. The feeding rates into the disk typically exceed Eddington by factors of a few, growing the hole on the Salpeter timescale and stimulating further feedback. Natural consequences of this picture include (i) the formation and maintenance of a roughly toroidal distribution of obscuring matter near the hole; (ii) random orientations of successive accretion disk episodes; (iii) the possibility of rapid SMBH growth; (iv) tidal disruption of stars and close binaries formed from infalling gas, resulting in visible flares and ejection of hypervelocity stars; (v) super-solar abundances of the matter accreting on to the SMBH; and (vi) a lower central dark-matter density, and hence annihilation signal, than adiabatic SMBH growth implies. We also suggest a simple sub-grid recipe for implementing this process in numerical simulations.
Highlights
The relation between supermassive black holes (SMBHs) and their host galaxies is a major theme of current astrophysics
We suggest a new picture of supermassive black hole (SMBH) growth in galaxy centers
The feeding rates into the disk typically exceed Eddington by factors of a few, growing the hole on the Salpeter timescale and stimulating further feedback. Natural consequences of this picture include (i) the formation and maintenance of a roughly toroidal distribution of obscuring matter near the hole; (ii) random orientations of successive accretion disk episodes; (iii) the possibility of rapid SMBH growth; (iv) tidal disruption of stars and close binaries formed from infalling gas, resulting in visible flares and ejection of hypervelocity stars; (v) super–solar abundances of the matter accreting on to the SMBH; and (vi) a lower central dark–matter density, and annihilation signal, than adiabatic SMBH growth implies
Summary
The relation between supermassive black holes (SMBHs) and their host galaxies is a major theme of current astrophysics. For binding energies the situation is reversed: a hole of mass 108M⊙ has ηc2M ∼ 1061 erg, while the bulge binding energy is ∼ σ2Mbulge ∼ 1058 erg for a typical velocity dispersion σ ≃ 200 km s−1 (this disparity is even bigger for smaller SMBH if these follow the scaling relations) This suggests that the cause of black hole accretion involves its effects on the galaxy, i.e. feedback. With fg the local gas fraction, the wind shocks are able to move far away from the hole (King 2003, 2005), beyond the critical radius Rcool ∼ 0.5 kpc where the radiation field of the accreting black hole becomes too dilute to cool the shocked wind This expands adiabatically (‘energy–driven’ flow), sweeping the host ISM before it at high speed (∼ 1000km s−1) and largely clearing the galaxy bulge of gas (Zubovas & King 2012). This terminates black hole growth, leaving the hole near the mass (2)
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