Feeding Supermassive Black Holes through collisional cascades
thesisposted on 12.06.2019, 16:05 by Christian Faber
The processes driving gas accretion on to supermassive black holes (SMBHs) are still poorly understood. Angular momentum conservation prevents gas within 10 pc of the black hole from reaching radii 103 pc where viscous accretion becomes efficient. In this thesis I present simulations of the collapse of a clumpy shell of sweptup isothermal gas, which is assumed to have formed as a result of feedback from a previous episode of AGN activity. The gas falls towards the SMBH forming clumps and streams, which intersect, collide, and often form a disc. These collisions promote partial cancellations of angular momenta, resulting in further infall and more collisions. This continued collisional cascade generates a tail of gas with sufficiently small angular momenta and provides a viable route for gas inflow to sub-parsec scales. The efficiency of this process hardly depends on details, such as gas temperature, initial virial ratio and power spectrum of the gas distribution, as long as it is not strongly rotating. In order to assess the result more quantitatively, I reduce the numerically motivated inner boundary and find that the inner structure is affected to about 4 times the inner boundary radius in the case of eccentric inflows. In this context I also discuss some tentative evidence that the collisional cascade may minimise any pre-existing preferential orientation of the angular momentum. Finally I present a preliminary analysis on the prevalence of disc disruption and destructions and the affect of dense, self-gravitating clumps in discs. These findings may provide an explanation for the missing star formation disc of the O-stars inside the central parsec of our Milky Way and the discrepancy between the total mass required to form the observed stars and the at least by a magnitude higher mass that must have eventually fed SgrA?, created an wide-angle outflow and subsequently caused the Fermi bubbles.