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

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