Scaling Relations between Gas and Star Formation in Nearby Galaxies
  High resolution, multi-wavelength maps of a sizeable set of nearby galaxies
have made it possible to study how the surface densities of HI, H2 and star
formation rate (Sigma_HI, Sigma_H2, Sigma_SFR) relate on scales of a few
hundred parsecs. At these scales, individual galaxy disks are comfortably
resolved, making it possible to assess gas-SFR relations with respect to
environment within galaxies. Sigma_H2, traced by CO intensity, shows a strong
correlation with Sigma_SFR and the ratio between these two quantities, the
molecular gas depletion time, appears to be constant at about 2Gyr in large
spiral galaxies. Within the star-forming disks of galaxies, Sigma_SFR shows
almost no correlation with Sigma_HI. In the outer parts of galaxies, however,
Sigma_SFR does scale with Sigma_HI, though with large scatter. Combining data
from these different environments yields a distribution with multiple regimes
in Sigma_gas - Sigma_SFR space. If the underlying assumptions to convert
observables to physical quantities are matched, even combined datasets based on
different SFR tracers, methodologies and spatial scales occupy a well define
locus in Sigma_gas - Sigma_SFR space.
