Abstract
We use an unprecedented sample of about 23 000 H II regions detected at an average physical resolution of 67 pc in the PHANGS–MUSE sample to study the extragalactic H II region Hα 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, α, in our sample is =1.73 with a σ of 0.15. We find that α decreases with the galaxy’s star formation rate surface density, ΣSFR, and argue that this is driven by an enhanced clustering of young stars at high gas surface densities. Looking at the H II 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 α between samples of H II regions with a high and low ionization parameter, q, and argue that they are driven by the aging of H II regions.
Highlights
Hα emission is one of the most effective tracers of young stars that formed within the last 10 Myr (e.g. Kennicutt & Evans 2012a; Haydon et al 2020)
Assuming that the aging effect is the main driver behind the luminosity function (LF) slope variations we observe between the H ii region sub-samples with high and low q, we can reasonably expect the magnitude of such variations ∆αq to correlate with the timescales that regulate the star formation process in a given galaxy
We have studied the nebular luminosity functions (LFs; i.e. built using the Hα luminosity of H ii regions) in the star-forming disks of 19 nearby galaxies making up the PHANGS–MUSE sample
Summary
Hα emission is one of the most effective tracers of young stars that formed within the last 10 Myr (e.g. Kennicutt & Evans 2012a; Haydon et al 2020). Variations in the slope of the LF can unveil whether global properties of galaxies, as well as local parameters such as chemical abundance, dust content, gas dynamics (e.g. spiral arm perturbations) influence the star formation process. A handful of studies have attempted to examine the relation between the LF slope and global galaxy parameters mostly finding weak or statistically insignificant correlations (Kennicutt et al 1989; Elmegreen & Salzer 1999; Youngblood & Hunter 1999; van Zee 2000; Thilker et al 2002; Liu et al 2013; Cook et al 2016). This work is focused on the study of nebular LFs and their variations as a function of galaxy global properties (e.g. galaxy morphology, star formation rate, stellar mass, and gas-phase metallicity) and galactic environment (e.g. spiral arm vs interarm) across the entire PHANGS–MUSE sample.
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