The Role of Gas in the Merging of Massive Black Holes in Galactic Nuclei. I. Black Hole Merging in a Spherical Gas Cloud

Abstract

Using high-resolution SPH numerical simulations, we investigate the effects of gas on the inspiral and merger of a massive black hole binary. This study is motivated by both observational and theoretical work that indicate the presence of large amounts of gas in the central regions of merging galaxies. N-body simulations have shown that the coalescence of a massive black hole binary eventually stalls in a stellar background. However, our simulations suggest that the massive black hole binary will finally merge if it is embedded in a gaseous background. Here we present results in which the gas is assumed to be initially spherical with a relatively smooth distribution. In the early evolution of the binary, the separation decreases due to the gravitational drag exerted by the background gas. In the later stages, when the binary dominates the gravitational potential in its vicinity, the medium responds by forming an ellipsoidal density enhancement whose axis lags behind the binary axis, and this offset produces a torque on the binary that causes continuing loss of angular momentum and is able to reduce the binary separation to distances where gravitational radiation is efficient. Assuming typical parameters from observations of Ultra Luminous Infrared Galaxies, we predict that a black hole binary will merge within 10 7 yrs; therefore these results imply that in a merger of gas-rich galaxies, any massive central black holes will coalesce soon after the galaxies merge. Our work thus supports scenarios of massive black -2hole evolution and growth where hierarchical merging plays an important role. The final coalescence of the black holes leads to gravitational radiation emission that would be detectable out to high redshift by LISA. We show that similar physical effects, which we simulate with higher resolution than previous work, can also be important for the formation of close binary stars.

Mergers in Double-Peaked [O Iii] Active Galactic Nuclei

“…The population of binary SMBHs depends critically on the efficiency of their coalescence (the so-called Final Parsec Problem; Begelman et al 1980); moreover, whether the binary stalls over the final few parsecs (e.g., Milosavljević & Merritt 2001), coalesces (e.g., Escala et al 2004), or recoils (e.g., Madau & Quataert 2004) has key implications for future gravitational wave detection and the demography of SMBHs.…”

Effects of a Supermassive Black Hole Binary on a Nuclear Gas Disk

“…This process may have an important role in the merging process of SMBHs. Escala et al (2005) studied the effect of the hydrodynamic drag force from dense gas on the evolution of a SMBH binary using numerical simulations. They have shown that after the SMBHs gradually fall into the galactic central dense gas region through dynamical friction, the effect of hydrodynamic drag becomes very large in the central region.…”

“…After the galaxies merge, the SMBHs sink into the center by dynamical friction between the SMBHs and field stars (Ebisuzaki et al 2001;Escala et al 2004Escala et al , 2005. These SMBHs form a SMBH binary and finally merge due to the emission of gravitational waves (Matsubayashi et al 2004;Enoki et al 2004;Escala et al 2004Escala et al , 2005.…”

“…These SMBHs form a SMBH binary and finally merge due to the emission of gravitational waves (Matsubayashi et al 2004;Enoki et al 2004;Escala et al 2004Escala et al , 2005. In explaining this process, Escala et al (2004Escala et al ( , 2005 have shown the important role of dynamical interaction between SMBHs and gas, especially in very gas-rich regions. Kazantzidis et al (2005) have made simulations of the merging process of two disk galaxies with SMBHs at each galactic center.…”

Untitled

“…Our scheme also neglects the role of gaseous-rather than stellar dynamical-processes in driving the evolution of an MBH binary (see, e.g., Escala et al 2004). We will try to address the effect of some these uncertainties on the predicted GWB in x 4.…”