We present a study on the dynamics of massive black holes (BHs) in galaxy mergers, obtained from a series of high-resolution N-Body/SPH simulations. We show that the presence of a gaseous component is essential for the rapid formation of an eccentric (Keplerian) BH binary. The binary resides at the center of a massive (∼ 10 9M ⊙) turbulent nuclear disc resulting from the collision of the two gaseous discs present in the parent galaxies. Using physically and/or numerically motivated recipes, we follow the accretion history of the BHs during the merger. We find that the mass of the BHs increases as large central inflows of gas occur inside each galaxy, and that the mass ratio q BH varies with time indicating that the memory of its initial value may be lost. Given the uncertainties in the accretion recipes and the encountered strong degeneracy between numerical resolution and physical assumptions, we suggest here three possible paths followed by the galaxies and the BHs during a merger in order to fulfill the M BH versus σ relation: Adjustment, Symbiosis, and BH Dominance. In an extremely high resolution run, we resolved the turbulent gas pattern down to parsec scales, and found that BH feedback is expected to be effective near the end of the merger. We then trace the BH binary orbit down to a scale of 0.1 pc modeling the nuclear disc, resulting from the galaxy collision, as an equilibrium Mestel disc composed either of gas, gas and stars, or just stars. Under the action of dynamical friction against the rotating gaseous and/or stellar background the orbit circularizes. When this occurs, each BH is endowed with its own small-size (≪ 0.01 pc) accretion disc comprising a few percent of the BH mass. Double AGN activity is expected to occur on an estimated timescale of ≲ 10 Myrs, comparable to the inspiral time. The double nuclear point-like sources that may appear have typical separations of ≲ 10 pc, and are likely to be embedded in the still ongoing starburst. © 2007 American Institute of Physics.
Colpi, M., Callegari, S., Dotti, M., Kazantzidis, S., Mayer, L. (2007). On the inspiral of massive black holes in gas-rich galaxy mergers. In The Multicolored Landscape of Compact Objects and their Explosive Origins; Cefalu, Sicily; Italy; 11-24 June 2006 (pp. 705-714). AIP American Institute of Physics [10.1063/1.2774931].
On the inspiral of massive black holes in gas-rich galaxy mergers
COLPI, MONICA
;DOTTI, MASSIMO;
2007
Abstract
We present a study on the dynamics of massive black holes (BHs) in galaxy mergers, obtained from a series of high-resolution N-Body/SPH simulations. We show that the presence of a gaseous component is essential for the rapid formation of an eccentric (Keplerian) BH binary. The binary resides at the center of a massive (∼ 10 9M ⊙) turbulent nuclear disc resulting from the collision of the two gaseous discs present in the parent galaxies. Using physically and/or numerically motivated recipes, we follow the accretion history of the BHs during the merger. We find that the mass of the BHs increases as large central inflows of gas occur inside each galaxy, and that the mass ratio q BH varies with time indicating that the memory of its initial value may be lost. Given the uncertainties in the accretion recipes and the encountered strong degeneracy between numerical resolution and physical assumptions, we suggest here three possible paths followed by the galaxies and the BHs during a merger in order to fulfill the M BH versus σ relation: Adjustment, Symbiosis, and BH Dominance. In an extremely high resolution run, we resolved the turbulent gas pattern down to parsec scales, and found that BH feedback is expected to be effective near the end of the merger. We then trace the BH binary orbit down to a scale of 0.1 pc modeling the nuclear disc, resulting from the galaxy collision, as an equilibrium Mestel disc composed either of gas, gas and stars, or just stars. Under the action of dynamical friction against the rotating gaseous and/or stellar background the orbit circularizes. When this occurs, each BH is endowed with its own small-size (≪ 0.01 pc) accretion disc comprising a few percent of the BH mass. Double AGN activity is expected to occur on an estimated timescale of ≲ 10 Myrs, comparable to the inspiral time. The double nuclear point-like sources that may appear have typical separations of ≲ 10 pc, and are likely to be embedded in the still ongoing starburst. © 2007 American Institute of Physics.
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