Hierarchical Star Formation in Nearby LEGUS Galaxies

TL;DR

This study investigates the hierarchical structure of star-forming regions in nearby galaxies, revealing common unit structures and the role of turbulence and self-gravity in star cluster formation across different scales.

Contribution

It provides new insights into the hierarchical organization of star formation in galaxies and links structural properties to starburst activity and cluster formation processes.

Findings

Starburst dwarfs have steeper size and flux distributions.

Hierarchical structures are common across multiple galaxies.

Self-similarity suggests turbulence-driven star formation.

Abstract

Hierarchical structure in ultraviolet images of 12 late-type LEGUS galaxies is studied by determining the numbers and fluxes of nested regions as a function of size from ~1 to ~200 pc, and the number as a function of flux. Two starburst dwarfs, NGC 1705 and NGC 5253, have steeper number-size and flux-size distributions than the others, indicating high fractions of the projected areas filled with star formation. Nine subregions in 7 galaxies have similarly steep number-size slopes, even when the whole galaxies have shallower slopes. The results suggest that hierarchically structured star-forming regions several hundred parsecs or larger represent common unit structures. Small galaxies dominated by only a few of these units tend to be starbursts. The self-similarity of young stellar structures down to parsec scales suggests that star clusters form in the densest parts of a turbulent medium that also forms loose stellar groupings on larger scales. The presence of super star clusters in two of our starburst dwarfs would follow from the observed structure if cloud and stellar subregions more readily coalesce when self-gravity in the unit cell contributes more to the total gravitational potential.