Throughout the Chapter, we have highlighted that some stimuli seem necessary to accelerate the formation of molecular gas in galaxy outskirts. In the outskirts of the MW, stimuli include spiral arm compression, expanding shells from supernova remnants, and interactions with HVCs (Yasui et al (2006); Izumi et al (2014); Koda et al (2016)). These same processes are likely at play in the outskirts of extragalactic disk galaxies. In particular, spiral density waves, interactions, and/or cold accretion may stimulate molecule formation and the subsequent star formation activity in XUV disks (Thilker et al (2007); Bush et al (2008); Holwerda et al (2012)). Interactions and mergers likely cause the polar rings in the outskirts of S0 galaxies, although it may be more likely that the molecules form in the gas-rich companion before the merger (Bournaud and Combes (2003)). Finally, in groups and clusters, interactions and ram pressure stripping may accelerate molecular gas formation in some localized areas of galaxies even as the overall effect is to remove the star-forming fuel from the galaxies (Vollmer et al (2005); Jáchym et al (2014)). Galaxy outskirts offer opportunities to study the formation of molecular gas over a variety of conditions and will be the key to understanding if there are different modes of star formation.
Fundamental questions remain about the physical conditions of the ISM in the outskirts. Where is the molecular gas? What are the basic properties of the molecular clouds, e.g., the H2 volume density, H2 column density, temperature, mass, and size? How do these properties differ from the properties of molecular clouds in the inner regions of galaxies? Is the transition from Hi to H2 and the transition from H2 to stars more or less efficient in the outskirts? Are these phase transitions affected by different large-scale processes, stimuli, or environmental conditions compared to inner regions? Measurements of molecular gas properties often depend on assumptions about the gas properties themselves. Right now, those assumptions are based on our knowledge of molecular gas in inner disks. Those assumptions need to be revisited and adjusted continuously as we learn more about molecular gas in the outskirts. This iterative improvement of our knowledge is now starting in the field of galaxy outskirts.
Building on the research that has already been done, we have identified a number of specific studies that would begin to address the fundamental questions above. In the outskirts of the MW, we can study whether the relationship between the mass of the molecular cloud and the most massive associated star is different than in the inner MW. In the outskirts of extragalactic disk galaxies, we need to measure the mass and size functions of molecular clouds and compare to the MW results. In addition, theoretical studies can work towards predicting where and how molecular gas will form in the outskirts. To test these predictions, we encourage sensitive and wide-area mapping of CO and/or dust continuum emission. Higher resolution (cloud-scale) maps of Hi may also be required to accurately locate potential sites of molecular gas formation. After each discovery of molecular gas, subsequent multi-wavelength studies including excitation ladders of molecular line emission are necessary to refine our knowledge of the physical conditions of molecular gas there. In early-type galaxies, we should search for molecular gas in XUV disks, as XUV emission could be even more common in early-type galaxies than late-type galaxies (Moffett et al (2012)). We hope those researchers will take note and learn from the high failure rate of previous (published and unpublished) searches for molecular gas in the outskirts of disk galaxies.
Acknowledgements We are grateful to Françoise Combes, Jennifer Donovan Meyer, Natsuko Izumi, and Hiroyuki Nakanishi for their advice, reading suggestions, and comments. We also thank Natsuko Izumi for allowing us to use her figure for the distribution of molecular clouds in the outer MW (Fig. 4). JK thanks the NAOJ Chile observatory, a branch of the National Astronomical Observatory of Japan, and the Joint ALMA Observatory for hospitality during his sabbatical visit. JK acknowledges the support from NASA through grant NNX14AF74G.