  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.
