Density effect on multiwavelength luminosities on star-formation regions in NGC 3184 and NGC 3938

Aims. We analyzed the regions of star formation in the spiral galaxies NGC 3184 and NGC 3938 from archive images for a wide range of wavelengths (NUV from GALEX, Hα from JKT and KPNO, 8 and 24 μm from Spitzer, and CO from BIMA). Methods. We used the Clump Find Algorithm to extract the properties of the star-forming tracers, identifiable as emission regions at each wavelength. Results. We obtained a power-law relation between the luminosity and the emission region volume that scales as expected, L ∝ V ,f or the Hα and NUV emission, but the luminosity varies far more rapidly with the volume for the dust (8 and 24 μm) and molecular gas emitting regions in CO. This is interpreted as a change in the emissivity with the size of the cloud, either by an augmentation of the overall density or due to the presence of high density clumps, with high local emissivity coefficients. Although the clumpy nature of molecular gas may be unsurprising, the clumpy nature of mid-infrared emission regions, which could be explained by newly formed high to intermediate mass stars embedded in the dust providing the heating, is clearly revealed in both galaxies.