Interaction between dark matter sub-halos & galactic gaseous disk
  We investigate the idea that the interaction of Dark Matter (DM) sub-halos
with the gaseous disk of galaxies can be the origin for the observed holes and
shells found in their neutral hydrogen (HI) distributions. We use high
resolution hydrodynamic simulations to show that pure DM sub-halos impacting a
galactic disk are not able to produce holes; on the contrary, they result in
high density regions in the disk. However, sub-halos containing a small amount
of gas (a few percent of the total DM mass of the sub-halo) are able to
displace the gas in the disk and form holes and shells. The size and lifetime
of these holes depend on the sub-halo gas mass, density and impact velocity. A
DM sub-halo of mass $10^8$ $M_{\odot}$ and a gas mass fraction of $\sim 3$%, is
able to create a kpc scale hole, with a lifetime similar to those observed in
nearby galaxies. We also register an increase in the star formation rate at the
rim of the hole, again in agreement with observations. Even though the
properties off these simulated structures resemble those found in observations
we find that the number of predicted holes (based on mass and orbital
distributions of DM halos derived from cosmological N-body simulations) falls
short compared to the observations. Only a handful of holes are produced per
Giga year. This leads us to conclude that DM halo impact is not the major
channel through which these holes are formed.
