Linking Stellar Populations to H <scp>ii</scp> Regions across Nearby Galaxies. II. Infrared Reprocessed and UV Direct Radiation Pressure in H <scp>ii</scp> Regions

Abstract Radiation pressure is a key mechanism by which stellar feedback disrupts molecular clouds and drives H ii 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 the Very Large Telecope Multi-Unit Spectroscopic Explorer (VLT-MUSE), and the Hubble Space Telescope (HST) H α -based region sizes to estimate the strength of radiation pressure in ≈18,000 H ii 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">F2100W</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mi>α</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">corr</mml:mi> </mml:mrow> </mml:msubsup> <mml:mo>≳</mml:mo> <mml:mn>75</mml:mn> </mml:math> . 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.