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Keywords:

  • accretion, accretion discs;
  • hydrodynamics;
  • radiation mechanisms: general;
  • methods: numerical;
  • galaxies: nuclei

ABSTRACT

We investigate non-spherical behaviour of gas accreting on to a central supermassive black hole. Assuming optically thin conditions, we include radiative cooling and radiative heating by the central X-ray source. Our simulations are performed using the 3D smoothed particle hydrodynamic (SPH) code gadget-3 and are compared to theoretical predictions as well as to 1D simulations performed using the grid code zeus. As found in earlier 1D studies, our 3D simulations show that the accretion mode depends on the X-ray luminosity (LX) for a fixed density at infinity and accretion efficiency. In the low LX limit, gas accretes in a stable, spherically symmetric fashion. In the high LX limit, the inner gas is significantly heated up and expands, reducing the central mass inflow rate. The expanding gas can turn into a strong enough outflow capable of expelling most of the gas at larger radii. For some intermediate LX, the accretion flow becomes unstable developing prominent non-spherical features. Our detailed analysis and tests show that the key reason for this unstable non-spherical nature of the flow is thermal instability (TI). Small perturbations of the initially spherically symmetric accretion flow that is heated by the intermediate LX quickly grow to form cold and dense clumps surrounded by overheated low-density regions. The cold clumps continue their inwards motion forming filamentary structures, while the hot infalling gas slows down because of buoyancy and can even start outflowing through the channels in between the filaments. We measured various local and global properties of our solutions. In particular, we found that the ratio between the mass inflow rates of the cold and hot gas is a dynamical quantity depending on several factors: time, spatial location and LX; and ranges between 0 and 4. We briefly discuss astrophysical implications of such TI-driven fragmentation of accreting gas on the formation of clouds in narrow- and broad-line regions of active galactic nuclei, the formation of stars and the observed variability of the active galactic nuclei luminosity.