Orbital parameters (distance between the SMBHs, semi-major axis and eccentricity) of a SMBHB formed in a representative N-body simulation of a galactic merger with parameters drawn from progenitors of likely PTA sources in the IllustrisTNG100-1 cosmological simulation. Mergers are selected from the merger trees of the 100 most massive galaxies at z = 0, based on galaxy mass ratio (major mergers with mass ratio 1 : 4 or higher) and redshift (z ≤ 2). The dashed lines indicate the critical separation a_f and the corresponding eccentricity e_f at the time in the evolution marking approximately the end of the SMBH inspiral due to DF and the beginning of the hardening phase. Dots represent a and e computed from the apoastron-periastron separation of the two SMBHs before pairing in a bound binary.