Fitting the Shadows: Star Formation Scaling Relations in the Low Surface Brightness Regime

Classical low surface brightness (LSB) galaxies pose an important challenge to galaxy evolution models. While they are found to host large reservoirs of atomic hydrogen, they display low stellar and star-formation surface densities. Global star formation scaling relations characterize trends in the star formation behaviour of galaxies; when used to compare populations or classes of galaxies, deviations in the observed trends can be used to probe predicted differences in physical conditions. In this work we utilize the well-studied Star Forming Main Sequence and integrated Kennicutt-Schmidt Relations to characterize star formation in the LSB regime, and compare the observed trends to relations for a normal star-forming galaxies. Using a comprehensive cross-matched sample of 277 LSB galaxies from the GALEX-SDSS-WISE Legacy Catalog Release 2 and the Arecibo Legacy Fast Arecibo L-band Feed Array Catalog, we gain an in-depth view of the star formation process in the LSB regime. HI-selected LSB galaxies follow very similar trends in atomic gas-to-stellar mass ratio and the star forming main sequence to their high surface brightness counterparts. However, while LSB galaxies host comparably large atomic gas reservoirs, they prove to be largely inefficient in converting this gas to stars with a median depletion time $t_{dep} \approx 18$ Gyr. These results are discussed in relation to previous studies which find that LSB galaxies host low atomic gas densities and are largely deficient in molecular gas, which suggest that the faint appearance of LSB galaxies may be the result of physical conditions on the sub-kpc scale.