The most abundant molecule H2 molecule does not have significant emission at the cold temperatures that are typical in molecular clouds (< 30 K). Hence, the emission from CO molecule, the second-most abundant molecule, is commonly used to trace molecular gas. Molecular gas is typically concentrated toward the centres of galaxies and its surface density decreases with galactic radius (Young and Scoville (1991); Wong and Blitz (2002)). The gas phase changes from mostly molecular in the central regions to more atomic in the outer regions (Sofue et al (1995); Koda et al (2016); Sofue and Nakanishi (2016)). These trends apparently continue into the outskirts, as Hi disks often extend beyond the edges of optical disks (Bosma (1981)).
We may infer the properties of gas in the outskirts by extending our knowledge from the inner disks. Recently, Koda et al (2016) concluded that the Hi - H2 gas phase transition between spiral arm and interarm regions changes as a function of radius in the MW and other nearby galaxies. In the molecule-dominant inner parts, the gas remains highly molecular as it moves from an interarm region into a spiral arm and back into the next interarm region. Stellar feedback does not dissociate molecules much, and perhaps the coagulation and fragmentation of molecular clouds dominate the evolution of the ISM at these radii. The trend differs in the outer regions where the gas phase is atomic on average. The Hi gas is converted to H2 in spiralarm compression and goes back into the Hi phase after passing spiral arms. These different regimes of ISM evolution are also seen in the LMC, M33, and M51, depending on the dominant gas phase there (Heyer and Terebey (1998); Engargiola et al (2003); Koda et al (2009); Fukui et al (2009); Tosaki et al (2011); Colombo et al (2014)).
Even in regions of relatively low gas densities, a natural fluctuation may occasionally lead to gravitational collapse into molecular gas and clouds. For example, many low-density dwarf galaxies show some molecular gas and star formation. However, some stimulus, such as spiral arm compression, seems necessary to accelerate the Hi to H2 phase transition. In addition to such internal stimuli, there are external stimuli, such as interactions with satellite galaxies, which may also trigger the phase transition into molecular gas in the outskirts.