Visible and invisible molecular gas in collisional debris of galaxies

Molecular gas has been searched for and found in unexpectedly large quantities in some collisional debris of interacting galaxies: HI-rich tidal tails, bridges and collisional rings. It was so far observed through millimeter observations of the CO line and detected towards or near regions of star-formation associated to dense condensations of the atomic hydrogen. The discovery of cool H2 at distances greater than 50 kpc from the parent (colliding) galaxies, whereas the external disk of spirals is generally considered to be CO-poor, raised question on its origin and favored the hypothesis of a local production out of collapsed HI clouds. However recent observations of a diffuse CO component along tidal debris have challenged this idea. Another recent puzzle is the measurement in the collisional debris of two interacting systems and four recycled objects of a missing mass, whereas no dark matter is expected there. One debated interpretation is that this unseen component is cold, "invisible" molecular gas initially present in the disk of spirals.