  Radiation pressure from the absorption and scattering of starlight by dust
grains may be an important feedback mechanism in regulating star-forming
galaxies. We compile data from the literature on star clusters, star-forming
subregions, normal star-forming galaxies, and starbursts to assess the
importance of radiation pressure on dust as a feedback mechanism, by comparing
the luminosity and flux of these systems to their dust Eddington limit. This
exercise motivates a novel interpretation of the Schmidt Law, the LIR-L'CO
correlation, and the LIR-L'HCN correlation. In particular, the linear LIR-L'HCN
correlation is a natural prediction of radiation pressure regulated star
formation. Overall, we find that the Eddington limit sets a hard upper bound to
the luminosity of any star-forming region. Importantly, however, many normal
star-forming galaxies have luminosities significantly below the Eddington
limit. We explore several explanations for this discrepancy, especially the
role of "intermittency" in normal spirals - the tendency for only a small
number of subregions within a galaxy to be actively forming stars at any moment
because of the time-dependence of the feedback process and the luminosity
evolution of the stellar population. If radiation pressure regulates star
formation in dense gas, then the gas depletion timescale is 6 Myr, in good
agreement with observations of the densest starbursts. Finally, we highlight
the importance of observational uncertainties - namely, the dust-to-gas ratio
and the CO-H2 and HCN-H2 conversion factors - that must be understood before a
definitive assessment of radiation pressure as a feedback mechanism in
star-forming galaxies.
