Visible and invisible molecular gas in collisional debris of galaxies

TL;DR

This paper reviews the presence and origins of molecular gas in the extended regions of collisional debris from interacting galaxies, highlighting recent discoveries of diffuse CO and the possibility of hidden molecular components.

Contribution

It synthesizes recent observational findings on molecular gas in galaxy debris and discusses the implications for understanding gas origins and dark matter in these regions.

Findings

Detection of large quantities of molecular gas in tidal debris.

Presence of diffuse CO challenging previous assumptions.

Potential existence of 'invisible' molecular gas accounting for missing mass.

Abstract

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.