NGC 2403 and NGC 628 Halpha observation

The two galaxies exhibit a similar size-density relation, <n_e_>_rms_{prop.to}R_eq_^-0.3^, which breaks for R_eq_>50pc, show at most a weak dependence of <n_e_>_rms_ on galactocentric radius for NGC 2403, and no clear dependence of n_e_ or phi on these parameters. Combining these results with published data, <n_e_>_rms_ presents tentative scaling relations with the median HII region size, the fraction of large regions in the parent galaxy, and the star formation rate surface density.

These trends, if confirmed, would provide new constraints for massive cluster formation models and important clues for interpreting dependencies observed at high redshift, underscoring the necessity of consistently extending this analysis to larger samples.