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Hierarchy and size distribution function of star formation regions in the spiral galaxy NGC 628

Hierarchical structures and size distribution of star formation regions in the nearby spiral galaxy NGC 628 are studied over a range of scale from 50 to 1000 pc using optical images obtained with 1.5 m telescope of the Maidanak Observatory. We found hierarchically structured concentrations of star formation regions in the galaxy, smaller regions with a higher surface brightness are located inside larger complexes having a lower surface brightness. We illustrate this hierarchy by dendrogram, or structure tree of the detected star formation regions, which demonstrates that most of these regions are combined into larger structures over several levels. We found three characteristic sizes of young star groups: 65 pc (OB associations), 240 pc (stellar aggregates) and 600 pc (star complexes). The cumulative size distribution function of star formation regions is found to be a power law with a slope of approximately -1.5 on scales appropriate to diameters of associations, aggregates and complexes. This slope is close to the slope which was found earlier by B. Elmegreen et al. for star formation regions in the galaxy on scales from 2 to 100 pc.