Star Formation in Tidal Dwarf Galaxies

Tidal Dwarf Galaxies (TDGs) are objects presently forming from gas which has been expelled from their parent galaxies during an interaction. We observed the CO emission of a sample of 11 TDGs, of which 8 were detected. The CO is found at the peak of the HI observations and has the same line velocity and width, indicating that the molecular gas is forming in situ instead of being torn from the parent galaxies. The presence of H$\alpha$ emission furthermore shows that stars are forming from this molecular gas. In order to investigate star formation in TDGs further, we compared their molecular gas content and star formation rate (SFR), traced by H$\alpha$, to those of spiral galaxies and classical dwarfs. The major difference between TDGs and classical dwarfs is the lower metallicity of the latter. The star formation efficiency (SFR per molecular gas mass) of TDGs lies in the range typical of spiral galaxies indicating that star formation is proceeding in a normal fashion from molecular gas.