Direct collapse black hole formation from synchronized pairs of atomic
  cooling halos
  High-redshift quasar observations imply that supermassive black holes (SMBHs)
larger than $\sim 10^9 ~ M_\odot$ formed before $z=6$. That such large SMBHs
formed so early in the Universe remains an open theoretical problem. One
possibility is that gas in atomic cooling halos exposed to strong Lyman-Werner
(LW) radiation forms $10^4-10^6 ~ M_\odot$ supermassive stars which quickly
collapse into black holes. We propose a scenario for direct collapse black hole
(DCBH) formation based on synchronized pairs of pristine atomic cooling halos.
We consider halos at very small separation with one halo being a subhalo of the
other. The first halo to surpass the atomic cooling threshold forms stars. Soon
after these stars are formed, the other halo reaches the cooling threshold and
due to its small distance from the newly formed galaxy, is exposed to the
critical LW intensity required to form a DCBH. The main advantage of this
scenario is that synchronization can potentially prevent photoevaporation and
metal pollution in DCBH-forming halos. Since the halos reach the atomic cooling
threshold at nearly the same time, the DCBH-forming halo is only exposed to
ionizing radiation for a brief period. Tight synchronization could allow the
DCBH to form before stars in the nearby galaxy reach the end of their lives and
generate supernovae winds. We use N-body simulations to estimate the abundance
of DCBHs formed in this way. The largest source of uncertainty in our estimate
is the initial mass function (IMF) of metal free stars formed in atomic cooling
halos. We find that even for tight synchronization, the density of DCBHs formed
in this scenario could explain the SMBHs implied by $z=6$ quasar observations.
Metal pollution and photoevaporation could potentially reduce the abundance of
DCBHs below that required to explain the observations in other models that rely
on a high LW flux.
