Molecular gas in Tidal Dwarf Galaxies: Exploring the conditions for star
  formation
  Tidal Dwarf Galaxies (TDGs), produced from material expelled in galactic
interactions, are well--suited to test the laws of star formation (SF) due to
their simple structure, high metallicity -- making CO a reliable tracer of the
molecular gas content -- and recent SF. Here, we study the conditions for the
onset of SF and for the rate at which SF proceeds once above a threshold in a
small sample of TDGs. We use data for the gas (atomic and molecular) surface
density and SF rate per area to test the laws of SF found for spiral and dwarf
galaxies in this more extreme environment. We find in general a good agreement
with the Schmidt law found for the total gas and for the molecular gas but note
that higher resolution CO observations are necessary to clarify some possible
discrepancies. We find, down to a scale of $\sim$1 kpc, in general a good
agreement between the peaks of SF and of the molecular gas, but also find in
some objects surprisingly large quantities of molecular gas at places where no
SF is occuring. A high column density of molecular gas is therefore not a
sufficient condition for the onset of SF. We find that the kinematical
properties of the gas are also relevant: in two objects our observations showed
that SF only occured in regions with a narrow line width.
