We present the first self-consistent prediction for the distribution of
formation timescales for close Supermassive Black Hole (SMBH) pairs following
galaxy mergers. Using ROMULUS25, the first large-scale cosmological simulation
to accurately track the orbital evolution of SMBHs within their host galaxies
down to sub-kpc scales, we predict an average formation rate density of close
SMBH pairs of 0.013 cMpc^-3 Gyr^-1. We find that it is relatively rare for
galaxy mergers to result in the formation of close SMBH pairs with sub-kpc
separation and those that do form are often the result of Gyrs of orbital
evolution following the galaxy merger. The likelihood and timescale to form a
close SMBH pair depends strongly on the mass ratio of the merging galaxies, as
well as the presence of dense stellar cores. Low stellar mass ratio mergers
with galaxies that lack a dense stellar core are more likely to become tidally
disrupted and deposit their SMBH at large radii without any stellar core to aid
in their orbital decay, resulting in a population of long-lived 'wandering'
SMBHs. Conversely, SMBHs in galaxies that remain embedded within a stellar core
form close pairs in much shorter timescales on average. This timescale is a
crucial, though often ignored or very simplified, ingredient to models
predicting SMBH mergers rates and the connection between SMBH and star
formation activity.Comment: 11 pages, 7 figures, accepted for publication in MNRA
