  We investigate the spatially-resolved star formation relation using a
galactic disk formed in a comprehensive high-resolution (3.8 pc) simulation.
Our new implementation of stellar feedback includes ionizing radiation as well
as supernova explosions, and we handle ionizing radiation by solving the
radiative transfer equation rather than by a subgrid model. Photoheating by
stellar radiation stabilizes gas against Jeans fragmentation, reducing the star
formation rate. Because we have self-consistently calculated the location of
ionized gas, we are able to make spatially-resolved mock observations of star
formation tracers, such as H-alpha emission. We can also observe how stellar
feedback manifests itself in the correlation between ionized and molecular gas.
Applying our techniques to the disk in a galactic halo of 2.3e11 Msun, we find
that the correlation between star formation rate density (estimated from mock
H-alpha emission) and molecular hydrogen density shows large scatter,
especially at high resolutions of <~ 75 pc that are comparable to the size of
giant molecular clouds (GMCs). This is because an aperture of GMC size captures
only particular stages of GMC evolution, and because H-alpha traces hot gas
around star-forming regions and is displaced from the molecular hydrogen peaks
themselves. By examining the evolving environment around star clusters, we
speculate that the breakdown of the traditional star formation laws of the
Kennicutt-Schmidt type at small scales is further aided by a combination of
stars drifting from their birthplaces, and molecular clouds being dispersed via
stellar feedback.
