Simulating the evolution of optically dark HI clouds in the Virgo
  cluster : will no-one rid me of this turbulent sphere ?
  Most detected neutral atomic hydrogen (HI) at low redshift is associated with
optically bright galaxies. However, a handful of HI clouds are known which
appear to be optically dark and have no nearby potential progenitor galaxies,
making tidal debris an unlikely explanation. In particular, 6 clouds identified
by the Arecibo Galaxy Environment Survey are interesting due to the combination
of their small size, isolation, and especially their broad line widths atypical
of other such clouds. A recent suggestion is that these clouds exist in
pressure equilibrium with the intracluster medium, with the line width arising
from turbulent internal motions. Here we explore that possibility by using the
FLASH code to perform a series of 3D hydro simulations. Our clouds are modelled
using spherical Gaussian density profiles, embedded in a hot, low-density gas
representing the intracluster medium. The simulations account for heating and
cooling of the gas, and we vary the structure and strength of their internal
motions. We create synthetic HI spectra, and find that none of our simulations
reproduce the observed cloud parameters for longer than about 100 Myr : the
clouds either collapse, disperse, or experience rapid heating which would cause
ionisation and render them undetectable to HI surveys. While the turbulent
motions required to explain the high line widths generate structures which
appear to be inherently unstable, making this an unlikely explanation for the
observed clouds, these simulations demonstrate the importance of including the
intracluster medium in any model seeking to explain the existence of these
objects.
