Linking stellar populations to HII regions across nearby galaxies. II. Infrared Reprocessed and UV Direct Radiation Pressure in HII Regions

TL;DR

This study quantifies the contributions of infrared reprocessed and UV direct radiation pressure in approximately 18,000 HII regions across 19 nearby galaxies, revealing that IR pressure is generally a small fraction of UV pressure but can dominate in certain regions.

Contribution

Introduces a new method combining JWST, VLT-MUSE, and HST data to directly estimate radiation pressure components in a large sample of HII regions beyond the Local Group.

Findings

IR radiation pressure is about 5% of UV pressure in galaxy disks

IR pressure rises to 10% in galaxy centers

Reprocessed IR pressure can dominate in regions with high IR to Hα luminosity ratio

Abstract

Radiation pressure is a key mechanism by which stellar feedback disrupts molecular clouds and drives HII region expansion. This includes direct radiation pressure exerted by UV photons on dust grains, pressure associated with photoionization, and infrared (IR) radiation pressure on grains due to dust-reprocessed IR photons. We present a new method that combines high resolution mid-IR luminosities from JWST-MIRI, optical attenuation and nebular line measurements from VLT-MUSE, and HST H$\alpha$-based region sizes to estimate the strength of radiation pressure in $\approx 18,000$ HII regions across 19 nearby star-forming galaxies. This is the most extensive and direct estimate of these terms beyond the Local Group to date. In the disks of galaxies, we find that the total reprocessed IR pressure is on average 5% of the direct UV radiation pressure. This fraction rises to 10% in galaxy centers. We expect reprocessed IR radiation pressure to dominate over UV radiation pressure in regions where $L_{\rm F2100W}/L_{\rm H\alpha}^{\rm corr} \gtrsim 75$. Radiation pressure due to H ionizations is lower than pressure on dust in our sample, but appears likely to dominate the radiation pressure budget in dwarf galaxies similar to the Small Magellanic Cloud. The contribution from all radiation pressure terms appears to be subdominant compared to thermal pressure from ionized gas, reinforcing the view that radiation pressure is most important in compact, heavily embedded, and young regions.