Dual black holes in merger remnants. I: linking accretion to dynamics
  We study the orbital evolution and accretion history of massive black hole
(MBH) pairs in rotationally supported circumnuclear discs up to the point where
MBHs form binary systems. Our simulations have high resolution in mass and
space which, for the first time, makes it feasible to follow the orbital decay
of a MBH either counter- or co-rotating with respect to the circumnuclear disc.
We show that a moving MBH on an initially counter-rotating orbit experiences an
"orbital angular momentum flip" due to the gas-dynamical friction, i.e., it
starts to corotate with the disc before a MBH binary forms. We stress that this
effect can only be captured in very high resolution simulations. Given the
extremely large number of gas particles used, the dynamical range is
sufficiently large to resolve the Bondi-Hoyle-Lyttleton radii of individual
MBHs. As a consequence, we are able to link the accretion processes to the
orbital evolution of the MBH pairs. We predict that the accretion rate is
significantly suppressed and extremely variable when the MBH is moving on a
retrograde orbit. It is only after the orbital angular momentum flip has taken
place that the secondary rapidly "lights up" at which point both MBHs can
accrete near the Eddington rate for a few Myr. The separation of the double
nucleus is expected to be around ~10 pc at this stage. We show that the
accretion rate can be highly variable also when the MBH is co-rotating with the
disc (albeit to a lesser extent) provided that its orbit is eccentric. Our
results have significant consequences for the expected number of observable
double AGNs at separations of <100 pc.
