SCYON Abstract

Received on June 5 2007

Hierarchical Star-Formation in M33: Fundamental properties of the star-forming regions

Authors	N. Bastian (¹), B. Ercolano (²), M. Gieles (³), E. Rosolowsky (²), R.A. Scheepmaker (⁴), R. Gutermuth (²), and Yu. Efremov (⁵)
Affiliation	(¹) University College London (²) Harvard-Smithsonian CfA (³) ESO-Santiago (⁴) Utrecht (⁵) Sternberg Institute, Moscow
Accepted by	Monthly Notices of the Royal Astronomical Society
Contact	bastian@star.ucl.ac.uk
URL	http://xxx.lanl.gov/abs/0706.0495
Links

Abstract

Star-formation within galaxies appears on multiple scales, from spiral structure, to OB associations, to individual star clusters, and often sub-structure within these clusters. This multitude of scales calls for objective methods to find and classify star-forming regions, regardless of spatial size. To this end, we present an analysis of star-forming groups in the local group spiral galaxy M33, based on a new implementation of the Minimum Spanning Tree (MST) method. Unlike previous studies which limited themselves to a single spatial scale, we study star-forming structures from the effective resolution limit (~20pc) to kpc scales. Once the groups are identified, we study their properties, e.g. size and luminosity distributions, and compare them with studies of young star clusters and giant molecular clouds (GMCs). We find evidence for a continuum of star-forming group sizes, which extends into the star-cluster spatial scale regime. We do not find a characteristic scale for OB associations, unlike that found in previous studies, and we suggest that the appearance of such a scale was caused by spatial resolution and selection effects. The luminosity function of the groups is found to be well represented by a power-law with an index, -2, the same as has been found for the luminosity and mass functions of young star clusters, as well as the mass function of GMCs. Additionally, the groups follow a similar mass-radius relation as GMCs. The size distribution of the groups is best described by a log-normal distribution, the peak of which is controlled by the spatial scale probed and the minimum number of sources used to define a group. We show that within a hierarchical distribution, if a scale is selected to find structure, the resulting size distribution will have a log-normal distribution. We find an abrupt drop of the number of groups outside a galactic radius of ~4kpc (although individual high-mass stars are found beyond this limit), suggesting a change in the structure of the star-forming ISM, possibly reflected in the lack of GMCs beyond this radius. Finally, we find that the spatial distribution of HII regions, GMCs, and star-forming groups are all highly correlated.