Merging timescale for supermassive black hole binary in interacting galaxy NGC 6240

TL;DR

This study uses advanced N-body simulations with relativistic corrections to estimate the merging timescale of supermassive black hole binaries in galaxy NGC 6240, finding an upper limit of approximately 55 million years.

Contribution

The paper provides the first direct N-body simulation-based estimate of SMBH binary merging timescales in NGC 6240, incorporating relativistic effects and exploring parameter robustness.

Findings

Merging time less than ~55 Myr for the SMBH binary.

No strong correlation between merging time and black hole mass ratios.

Results are robust across different numerical parameters.

Abstract

One of the main possible way of creating the supermassive black hole (SMBH) is a so call hierarchical merging scenario. Central SMBHs at the final phase of interacting and coalescing host-galaxies are observed as SMBH binary (SMBHB) candidates at different separations from hundreds of pc to mpc. Today one of the strongest SMBHB candidate is a ULIRG galaxy NGC 6240 which was X-ray spatially and spectroscopically resolved by Chandra. Dynamical calculation of central SMBHBs merging in a dense stellar environment allows us to retrace their evolution from kpc to mpc scales. The main goal of our dynamical modeling was to reach the final, gravitational wave (GW) emission regime for the model BHs. We present the direct N-body simulations with up to one million particles and relativistic post-Newtonian corrections for the SMBHs particles up to 3.5 post-Newtonian terms. Generally speaking, the set of initial physical conditions can strongly effect of our merging time estimations. But in a range of our parameters, we did not find any strong correlation between merging time and BHs mass or BH to bulge mass ratios. Varying the model numerical parameters (such as particle number - N) makes our results quite robust and physically more motivated. From our 20 models we found the upper limit of merging time for central SMBHB is less than $\sim$55 Myr. This concrete number are valid only for our set of combination of initial mass ratios. Further detailed research of rare dual/multiple BHs in dense stellar environment (based on observations data) can clarify the dynamical co-evolution of central BHs and their host-galaxies.