Tidal Dwarf Galaxies: from Gas to Stars to Galaxies

Tidal Dwarf Galaxies are the only galaxies in formation in the local universe. As such, they provide a unique laboratory to study how the proto-galactic gas condenses and how the onset of star formation comes about. The study of how star formation, and particularly the star formation efficiency (SFE), is affected by the large-scale environment is difficult. It is generally accepted that stars form from molecular gas, making the measure of molecular gas a critical ingredient. The molecular gas is typically and probably best traced by the CO lines, such that it is difficult to estimate the molecular gas mass in small galaxies as they are generally metal-deficient. Due to their high metallicity, Tidal Dwarf Galaxies provide a possibly unique means of studying star formation in a dwarf galaxy environment (dominated by the gas mass, little or no rotation) and comparing with spiral galaxies (rotating disks dominated by the stellar mass). We can also probe the dark matter content of the disk material from which they form. While it is not possible to exclude the presence of some dark matter, there is much less than in classical dwarf galaxies and the observations are consistent with its absence in Tidal Dwarf Galaxies