Hierarchical Star Formation in Nearby Legus Galaxies

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 seven 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.

The Hierarchical Distribution of the Young Stellar Clusters in Six Local Star-forming Galaxies

“…The same observational signature is also present in the age and positions of the young star clusters in the LMC (Efremov & Elmegreen 1998) and other local galaxies (Grasha et al 2017). Additionally, local star-forming environments in the MW display distributions that are consistent with predictions of hierarchically structured star formation (Bressert et al 2010) that is also observed in extragalactic studies, arising from the molecular cloud hierarchical structure (Elmegreen 2008;Elmegreen et al 2014).…”

“…Most structures within the hierarchy are themselves gravitationally unbound and the stellar components are expected to inherit their clustered substructure from the molecular clouds from which they are born (Scalo 1985). Recent analyses of 12 local galaxies (Elmegreen et al 2014) found that the clustering of star formation remains scalefree, up to the largest scales observable, for both starburst galaxies and more quiescent starforming galaxies. This result is consistent within the framework where the self-similar structure of the interstellar medium (ISM), regulated by turbulence, is believed to be the primary driver for the hierarchical nature of star formation (Elmegreen & Efremov 1996;Elmegreen et al 2014).…”

“…Recent analyses of 12 local galaxies (Elmegreen et al 2014) found that the clustering of star formation remains scalefree, up to the largest scales observable, for both starburst galaxies and more quiescent starforming galaxies. This result is consistent within the framework where the self-similar structure of the interstellar medium (ISM), regulated by turbulence, is believed to be the primary driver for the hierarchical nature of star formation (Elmegreen & Efremov 1996;Elmegreen et al 2014). Thus, extensive starforming regions of several hundred parsecs or larger are expected to represent common structures,related in both space and time, in a hierarchical manner that determines the structure and morphology of all galaxies.…”

The VMC Survey. XXII. Hierarchical Star Formation in the 30 Doradus-N158–N159–N160 Star-forming Complex

“…D 2 derived in this work is close to those for NGC 628 (D 2 = 1.5; Elmegreen et al 2006), NGC 6503 (D 2 = 1.7; Gouliermis et al 2015), and part of the galaxy sample of Elmegreen et al (2014), but it can deviate more than the measurement uncertainties compared with other ones. Elmegreen et al (2014) found that compared with large spiral galaxies or low surface brightness dwarfs, the starburst dwarfs or HII galaxies have larger projected fractal dimensions if they contain one or two dominant young stellar complexes; thus, the difference in D 2 may reflect different clustering under different conditions. Secondly, older stellar populations have a smaller degree of substructures due to dynamical evolutions (Gieles et al 2008;Bastian et al 2009;Parker et al 2014;Gouliermis et al 2015); this effect may also contributes to the difference in D 2 , since these studies rely on different stellar samples which may be complicated mixtures of populations of various ages.…”

“…On larger scales, a number of galaxies display galaxy-wide hierarchies formed by young stars (e.g. Bastian et al 2007Bastian et al , 2009; Gieles et al 2008;Elmegreen & Elmegreen 2001;Elmegreen et al 2006Elmegreen et al , 2014Gouliermis et al 2010Gouliermis et al , 2015. These studies show that hierarchical stellar structures, significant for a high degree of substructure and an absence of characteristic scales, exhibit self-similar and fractal properties.…”