Cloud coalescence: a dynamical instability affecting multiphase
  environments
  The mass and size distributions are the key characteristics of any
astrophysical objects, including the densest clumps comprising the cold phase
of multiphase environments. In our recent papers, we showed how individual
clouds of various sizes form and evolve in AGN. In particular, we showed that
large clouds undergo damped oscillations as a response to their formation
process. Here we followup this investigation, addressing how different size
clouds interact. We find that smaller clouds become trapped in the advective
flows generated by larger clouds. The explanation for this behavior leads to a
rather remarkable conclusion: even in the absence of gravity, complexes of
clouds are dynamically unstable. In an idealized environment (e.g., one free of
turbulence and magnetic fields) a perfectly symmetric arrangement of static
clouds will remain static, but any small spatial perturbation will lead to all
clouds coalescing into a single, large cloud, given enough time. Using
numerical simulations, we investigate the main factors that determine the rate
of coalescence. Besides the cloud separation distance, we find that the
transient response of clouds to a disturbance is the primary factor. Turbulent
motions in the flow can easily suppress this tendency for spatially
well-separated clouds to coalesce, so it is as yet unclear if this phenomenon
can occur in nature. Nevertheless, this work casts strong doubts on a recent
hypothesis that large clouds are prone to fragmentation.
