The Infrared and Radio Fluxes Densities of Galactic HII Regions

TL;DR

This study compiles infrared and radio flux densities for about 1000 Galactic HII regions, revealing correlations, color criteria for identification, and potential variations with galactic radius, enhancing understanding of HII region properties.

Contribution

It provides a comprehensive analysis of infrared and radio fluxes for a large sample of Galactic HII regions, establishing new color criteria for their identification and examining their properties across different sizes and galactic locations.

Findings

Strong correlation between infrared and radio flux densities.

Infrared colors reliably identify HII regions regardless of size.

Potential decrease in dust abundance with increasing galactic radius.

Abstract

We derive infrared and radio flux densities of all ~1000 known Galactic HII regions in the Galactic longitude range 17.5 < l < 65 degree. Our sample comes from the Wide-Field Infrared Survey Explorer (WISE) catalog of Galactic \hii regions \citep{anderson2014}. We compute flux densities at six wavelengths in the infrared (GLIMPSE 8 microns, WISE 12 microns and 22 microns, MIPSGAL 24 microns, and Hi-GAL 70 microns and 160 microns) and two in the radio (MAGPIS 20 cm and VGPS 21 cm). All HII region infrared flux densities are strongly correlated with their ~20 cm flux densities. All HII regions used here, regardless of physical size or Galactocentric radius, have similar infrared to radio flux density ratios and similar infrared colors, although the smallest regions ($r<1\,$pc), have slightly elevated IR to radio ratios. The colors $\log_{10}(F_{24 micron}/F_{12 micron}) \ge 0$ and $\log_{10}(F_{70 micron}/F_{12 micron}) \ge 1.2$, and $\log_{10}(F_{24 micron}/F_{12 micron}) \ge 0$ and $\log_{10}(F_{160 micron}/F_{70 micron}) \le 0.67$ reliably select HII regions, independent of size. The infrared colors of ~22$\%$ of HII regions, spanning a large range of physical sizes, satisfy the IRAS color criteria of \citet{wood1989} for HII regions, after adjusting the criteria to the wavelengths used here. Since these color criteria are commonly thought to select only ultra-compact HII regions, this result indicates that the true ultra-compact HII region population is uncertain. Comparing with a sample of IR color indices from star-forming galaxies, HII regions show higher $\log_{10}(F_{70 micron}/F_{12 micron})$ ratios. We find a weak trend of decreasing infrared to ~20 cm flux density ratios with increasing $R_{gal}$, in agreement with previous extragalactic results, possibly indicating a decreased dust abundance in the outer Galaxy.