[Paper Review] PHANGS-MUSE: The H II region luminosity function of local star-forming galaxies

Francesco Santoro, Kathryn Kreckel+31 more|arXiv (Cornell University)|Nov 17, 2021

Galaxies: Formation, Evolution, Phenomena126 references69 citations

TL;DR

This study presents the first large-scale extragalactic H ii region Hα luminosity function (LF) using 23,000 resolved H ii regions from the PHANGS-MUSE survey, achieving 67 pc spatial resolution across 19 nearby spiral galaxies. It finds that the LF slope α = −1.73 ± 0.15 decreases with increasing star formation rate surface density (ΣSFR), driven by enhanced young star clustering at high gas densities, while variations between spiral arm and inter-arm regions suggest spiral arms amplify molecular cloud mass contrast, and ionization parameter differences indicate age-driven evolution in H ii regions.

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.

Motivation & Objective

• To measure the extragalactic H ii region Hα luminosity function (LF) with unprecedented statistical power and spatial resolution.
• To investigate how global galaxy properties (e.g. ΣSFR, metallicity, morphology) and local environment (spiral arms vs. inter-arm) affect the LF slope α.
• To determine whether variations in the LF slope are driven by physical processes such as star formation clustering, feedback, or H ii region aging.
• To link the H ii region LF to the underlying initial mass function (IMF) and star formation efficiency via comparison with ALMA and HST data.

Proposed method

• Used MUSE integral field spectroscopy to detect and characterize 23,000 H ii regions at ~67 pc resolution in 19 nearby spiral galaxies.
• Applied maximum likelihood estimation (MLE) to fit the H ii region LF without binning, avoiding histogram-related biases.
• Classified H ii regions by ionization parameter q to probe age-dependent evolution effects.
• Compared LF slopes across galaxy regions (inner/outer disk, spiral arms vs. inter-arm) and correlated with ΣSFR, metallicity, and morphology.
• Assessed completeness and crowding effects using simulated H ii region catalogs to validate LF measurements.

Experimental results

Research questions

• RQ1 How does the H ii region Hα luminosity function slope α vary with the galaxy’s star formation rate surface density (ΣSFR)?
• RQ2 What causes variations in the LF slope between spiral arm and inter-arm regions in galaxies with visible spiral structure?
• RQ3 How do differences in ionization parameter q relate to the age and evolutionary state of H ii regions?
• RQ4 To what extent do gas-phase metallicity and dust content influence the H ii region LF?
• RQ5 Can variations in the LF slope be linked to star formation clustering or feedback timescales?

Key findings

• The mean H ii region Hα luminosity function slope is α = −1.73 ± 0.15, consistent with previous studies of similar galaxies.
• The LF slope α decreases (becomes flatter) with increasing ΣSFR, indicating a higher relative number of bright H ii regions at higher star formation surface densities.
• This trend is attributed to enhanced clustering of young stars at high gas surface densities, supporting theoretical predictions of increased star formation efficiency.
• No significant difference in LF slope is found between inner and outer disk regions, suggesting uniform star formation physics across radial gradients.
• For galaxies with spiral arms, the LF is shallower in arm regions than in inter-arm regions, likely due to increased molecular cloud mass contrast driven by spiral density waves.
• H ii regions with high ionization parameter q have a shallower LF than those with low q, indicating that high-q regions are younger and less evolved, with age being the dominant driver of ionization parameter variations.