Abundant molecular gas in tidal dwarf galaxies: On-going galaxy formation

We investigate the process of galaxy formation as can be observed in the on ly currently forming galaxies {the so-called Tidal Dwarf Galaxies, hereafte r TDGs {through observations of the molecular gas detected via its CO (Carb on Monoxide) emission. These objects are formed of material torn off of the outer parts of a spiral disk due to tidal forces in a collision between tw o massive galaxies. Molecular gas is a key element in the galaxy formation process, providing the link between a cloud of gas and a bona fide galaxy. We have detected CO in 8 TDGs (two of them have already been published in B raine et al. 2000, hereafter Paper I), with an overall detection rate of 80 %, showing that molecular gas is abundant in TDGs, up to a few 10(8) M-.. T he CO emission coincides both spatially and kinematically with the HI emiss ion, indicating that the molecular gas forms from the atomic hydrogen where the HI column density is high. A possible trend of more evolved TDGs havin g greater molecular gas masses is observed, in accord with the transformati on of HI into H-2. Although TDGs share many of the properties of small irre gulars, their CO luminosity is much greater (factor similar to 100) than th at of standard dwarf galaxies of comparable luminosity. This is most likely a consequence of the higher metallicity (greater than or similar to1/3 sol ar) of TDGs which makes CO a good tracer of molecular gas. This allows us t o study star formation in environments ordinarily inaccessible due to the e xtreme difficulty of measuring the molecular gas mass. The star formation e fficiency, measured by the CO luminosity per H alpha flux, is the same in T DGs and full-sized spirals. CO is likely the best tracer of the dynamics of these objects because some fraction of the HI near the TDGs may be part of the tidal tail and not bound to the TDG. Although uncertainties are large for individual objects, as the geometry is unknown, our sample is now of ei ght detected objects and we find that the "dynamical" masses of TDGs, estim ated from the CO line widths, seem not to be greater than the "visible" mas ses (HI + H-2 + a stellar component). Although higher spatial resolution CO (and HI) observations would help reduce the uncertainties, we find that TD Gs require no dark matter, which would make them the only galaxy-sized syst ems where this is the case. Dark matter in spirals should then be in a halo and not a rotating disk. Most dwarf galaxies are dark matter-rich, implyin g that they are not of tidal origin. We provide strong evidence that TDGs a re self-gravitating entities, implying that we are witnessing the ensemble of processes in galaxy formation: concentration of large amounts of gas in a bound object, condensation of the gas, which is atomic at this point, to form molecular gas and the subsequent star formation from the dense molecul ar component.