Abstract
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 and morphology of the accreted
satellite galaxy. 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.
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