PHANGS-MUSE: the HII region luminosity function of local star-forming galaxies

TL;DR

This study analyzes the luminosity function of over 23,000 HII regions in 19 nearby spiral galaxies, revealing how galaxy properties and spiral arm structures influence star-forming regions' luminosity distribution.

Contribution

It provides the first extensive analysis of HII region luminosity functions across multiple galaxies at high resolution, highlighting the impact of galaxy environment and spiral arms.

Findings

The mean LF slope is 1.73 with a standard deviation of 0.15.

LF slope decreases with higher star formation rate surface density.

Spiral arm regions have shallower LFs than inter-arm regions in most galaxies.

Abstract

We use an unprecedented sample of about 23,000 HII regions detected at an average physical resolution of 67pc in the PHANGS-MUSE sample to study the extragalactic HII region Ha luminosity function (LF). Our observations probe the star-forming disk of 19 nearby spiral galaxies with low inclination and located close to the star formation main sequence at z=0. The mean LF slope $\alpha$ in our sample is =1.73 with a $\sigma$ of 0.15. We find that $\alpha$ decreases with the galaxy's star formation rate surface density and argue that this is driven by an enhanced clustering of young stars at high gas surface densities. Looking at the HII regions within single galaxies we find that no significant variations occur between the LF of the inner and outer part of the star-forming disk, whereas the LF in the spiral arm areas is shallower than in the inter-arm areas for six out of the 13 galaxies with clearly visible spiral arms. We attribute these variations to the spiral arms increasing the molecular clouds arm--inter-arm mass contrast and find suggestive evidence that they are more evident for galaxies with stronger spiral arms. Furthermore, we find systematic variations in $\alpha$ between samples of HII regions with high and low ionization parameter q and argue that they are driven by the aging of HII regions.