Extended Hi gas disks beyond optical edges are common around spiral galaxies, and as already discussed, some stimulus seems necessary to accelerate molecule formation there. In the group/cluster environment, galaxy interactions and interactions with the intergalactic medium (IGM) are triggers for the Hi to H2 phase transition. In the nearby M81 triplet (M82, M81, and NGC 3077), tidal interactions stretch the atomic gas in the outskirts into tidal spiral arms, leading to gravitational collapse to form molecular gas and stars (Brouillet et al (1992); Walter et al (2006)). Even an interaction with a minor partner can be a trigger, e.g., in the M51 system, CO emission is detected along the tidal arm/bridge between the main galaxy NGC 5194 and its companion NGC 5195 (Koda et al (2009)).
Interaction with the IGM in clusters is also important for the gas phase transition. Most Hi gas in galaxy outskirts is stripped away by the ram pressure from the IGM (van Gorkom (2004)), while the molecular gas, which resides mostly in inner disks, remains less affected (Kenney and Young (1989); Boselli et al (1997)). Some compression acts on the molecular gas near the transition from the molecular-dominant inner disks to the atomic-dominant outer disks, as the extents of molecular disks are smaller when the Hi in the outskirts is stripped away (Boselli et al (2014)).
The stripped gas in the outskirts is seen as multiphase and has been detected in Hi (e.g., Chung et al (2009)), Hα (e.g., Yagi et al (2010)), and X-rays (e.g., Wang et al (2004); Sun et al (2010)). Stripped molecular gas is found in NGC 4438 and NGC 4435, which are interacting galaxies in the Virgo cluster (Vollmer et al (2005)). CO emission has also been discovered in the trailing tails of the stripped gas from the disk galaxies ESO137-001 and NGC 4388 in the Norma and Virgo clusters, respectively (Jáchym et al (2014); Verdugo et al (2015)).
The ram pressure from the IGM can also heat up and excite H2 molecules, and H2 rotational emission lines are detected in the mid-infrared in spiral galaxies in the Virgo cluster (Wong et al (2014)). The emission from warm H2 is also detected over large scales in the intergalactic space of Stephan's Quintet galaxy group with the Spitzer Space Telescope (Appleton et al (2006)). An analysis of the rotational transition ladder of its ground vibrational state suggests the molecular gas has temperatures of 185 ± 30 K and 675 ± 80 K. This H2 emission coincides with and extends along the X-ray-emitting shock front that is generated by the galaxy NGC 7318b passing through the IGM at a high velocity.
A final example of the cluster environment affecting molecular gas formation is that CO has been detected in cooling flows in the outskirts of galaxies in cluster cores (e.g., Salomé et al (2006)). Clearly, the group and cluster environments produce some triggers for the formation of molecular gas in galaxy outskirts and therefore represent another extreme environment where we can test our understanding of the physics of the ISM and star formation.