The route to massive black hole formation via merger-driven direct
  collapse: a review
  In this paper we review a new scenario for the formation of massive black
hole seeds that relies on multi-scale gas inflows initiated by the merger of
massive gas-rich galaxies at $z > 6$, where gas has already achieved solar
composition. Hydrodynamical simulations undertaken to explore our scenario show
that supermassive, gravitationally bound gaseous disks, weighing a billion
solar masses and of a few pc in size, form in the nuclei of merger remnants in
less than $10^5$ yr. These could later produce a supermassive protostar or
supermassive star at their center via various mechanisms. Moreover, we present
a new analytical model, based on angular momentum transport in mass-loaded
gravitoturbulent disks. This naturally predicts that a nuclear disk accreting
at rates exceeding $1000 M_{\odot}$/yr, as seen in the simulations, is stable
against fragmentation irrespective of its metallicity. This is at variance with
conventional direct collapse scenarios, which require the suppression of gas
cooling in metal-free protogalaxies for gas collapse to take place. Such high
accretion rates reflect the high free-fall velocities in massive halos
appearing at $z < 10$, and occur naturally as a result of the efficient angular
momentum loss provided by mergers. We discuss the implications of our scenario
on the observed population of high-z quasars and on its evolution to lower
redshifts using a semi-analytical galaxy formation model. Finally, we consider
the intriguing possibility that the secondary gas inflows in the unstable disks
might drive gas to collapse into a supermassive black hole directly via the
General Relativistic radial instability. Such {\it dark collapse} route could
generate gravitational wave emission detectable via the future Laser
Interferometer Space Antenna (LISA). [Abridged]
