Luminosity functions and IMF variations from large samples of HII regions and molecular clouds

TL;DR

This study investigates the relationship between GMC and HII region luminosity functions and initial mass functions, finding no evidence for a maximum stellar mass varying with cloud or cluster mass.

Contribution

It provides a detailed analysis of luminosity functions and IMF variations using large samples of HII regions and GMCs in local galaxies.

Findings

CO luminosity functions are steeper than Hα functions.

Outer disk regions show steepening of luminosity functions.

No evidence found for a maximum stellar mass depending on cloud or cluster mass.

Abstract

Large high-quality samples of HII regions and their parent Giant Molecular Clouds (GMC) are now available for local galaxies. It is therefore possible to investigate links between the CO and H$\alpha$ luminosity functions and whether massive stars form in GMCs of all masses. The CO luminosity functions (LF), representing the distribution of GMC masses, are consistently steeper than the H$\alpha$ luminosity functions. The CO LF invariably steepens in the outer disk where fewer massive GMCs are present beyond the median cloud galactocentric distance. The H$\alpha$ LF also steepens in the outer disk for most of the galaxies examined. Using Salpeter, Kroupa, and Chabrier Initial Mass Functions (IMF) along with stellar mass-luminosity-radius relations, we compute numerically the bolometric luminosity and H$\alpha$ emission from young star clusters. The cluster masses are linked to the GMC mass by assuming that the cluster mass is a constant fraction (3\%) of the parent cloud mass. In particular, results for a fully stochastic IMF are compared to suggestions that very massive stars only form in massive clusters or clouds. Within the limits of the observations -- no small molecular clouds or low-luminosity HII regions can be detected at the typical $\sim 10$~Mpc distance of the sample galaxies -- we find no evidence for a maximum stellar mass which varies with cloud or cluster mass.