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Studying star formation beyond the optical radius of galaxies allows us totest empirical relations in extreme conditions with low average gas density andlow molecular fraction. Previous studies discovered galaxies with extendedultraviolet (XUV) disks, which often contain star forming regions with lowerHalpha-to-far-UV (FUV) flux ratios compared to inner disk star forming regions.However, most previous studies lack measurements of molecular gas, which ispresumably the component of the interstellar medium out of which stars form. Weanalyzed published CO measurements and upper limits for fifteen star formingregions in the XUV or outer disk of three nearby spiral galaxies and a new COupper limit from the IRAM 30 m telescope in one star forming region at r = 3.4r_25 in the XUV disk of NGC 4625. We found that the star forming regions are ingeneral consistent with the same molecular-hydrogen Kennicutt-Schmidt law thatapplies within the optical radius, independent of whether we used Halpha or FUVas the star formation rate (SFR) tracer. However, a number of the CO detectionsare significantly offset towards higher SFR surface density for their molecularhydrogen surface density. Deeper CO data may enable us to use the presence orabsence of molecular gas as an evolutionary probe to break the degeneracybetween age and stochastic sampling of the initial mass function as theexplanation for the low Halpha-to-FUV flux ratios in XUV disks.

Testing the Molecular-Hydrogen Kennicutt-Schmidt Law in the Low-Density Environments of Extended Ultraviolet Disk Galaxies