# Ionization Profiles of Galactic HII Regions

## TL;DR

This study uses radio recombination line data from the Green Bank Telescope to investigate how radiation leaking from various Galactic HII regions contributes to ionizing the surrounding interstellar medium, revealing size-dependent effects and ionization dynamics.

## Contribution

It provides new insights into the role of leaking radiation from HII regions in maintaining interstellar ionization, highlighting size-dependent intensity profiles and helium ionization variations.

## Key findings

- Hydrogen RRL intensity decreases as a power-law with distance from HII regions.
- Smaller HII regions show steeper intensity decline, implying larger regions have greater ionization influence.
- Helium ionization decreases with distance, indicating absorption within the HII regions.

## Abstract

Using Green Bank Telescope radio recombination line (RRL) data, we analyze the role of leaking radiation from HII regions in maintaining the ionization of the interstellar medium. We observed a sample of eight Galactic HII regions of various sizes, morphologies, and luminosities. For each region the hydrogen RRL intensity decreases roughly as a power-law with distance from the center of the region. This suggests that radiation leaking from the HII region is responsible for the majority of surrounding ionized gas producing RRL emission. Our results further indicate that the hydrogen RRL intensity appears to be fundamentally related to the HII region sizes traced by their photodissociation regions, such that physically smaller HII regions show a steeper decrease in intensity with increasing distance from the region centers. As a result, giant HII regions may have a much larger effect in maintaining the ionization of the interstellar medium. For six of the eight observed HII regions we find a decrease in the 4He+/H+ abundance ratio with increasing distance, indicating that He-ionizing photons are being absorbed within the ionization front of the HII region. There is enhanced carbon RRL emission toward directions with strong continuum background, suggesting that the carbon emission is amplified by stimulated emission.