Hierarchical star formation in nearby galaxies
  Aims: The purpose of this work is to study the properties of the spatial
distribution of the young population in three nearby galaxies in order to
better understand the first stages of star formation.
  Methods: We used ACS/HST photometry and the "path-linkage criterion" in order
to obtain a catalog of young stellar groups (YSGs) in the galaxy NGC 2403. We
studied the internal distribution of stars in these YSGs using the Q parameter.
We extended these analyses to the YSGs detected in in NGC 300 and NGC 253 our
previous works. We built the young stars' density maps for these three
galaxies. Through these maps, we were able to identify and study young stellar
structures on larger scales.
  Results: We found 573 YSGs in the galaxy NGC 2403, for which we derived their
individual sizes, densities, luminosity function,and other fundamental
characteristics. We find that the vast majority of the YSGs in NGC 2403, NGC
300 and NGC 253 present inner clumpings, following the same hierarchical
behavior that we observed in the young stellar structures on larger scales in
these galaxies. We derived values of the fractal dimension for these structures
between ~ 1.5 and 1.6. These values are very similar to those obtained in other
star forming galaxies and in the interstellar medium, suggesting that the star
formation process is regulated by supersonic turbulence.
