The Infrared and Radio Fluxes Densities of Galactic HII Regions

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 ${\log }_{10}(F_{24micron}/F_{12micron})\ge 0$ and ${\log }_{10}(F_{70micron}/F_{12micron})\ge 1.2$, and ${\log }_{10}(F_{24micron}/F_{12micron})\ge 0$ and ${\log }_{10}(F_{160micron}/F_{70micron})\le 0.67$ 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 ${\log }_{10}(F_{70micron}/F_{12micron})$ 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.

27 pages, 16 figures, 5 tables

References in corpus (6)

• The VLA Galactic Plane Survey
• Dust in starburst nuclei and ULIRGs: SED models for observers
• Understanding Spatial and Spectral Morphologies of Ultracompact H II Regions
• The MIXR sample: AGN activity versus star formation across the cross-correlation of WISE, 3XMM, and FIRST/NVSS
• A self-consistent model of Galactic stellar and dust infrared emission and the abundance of polycyclic aromatic hydrocarbons
• Absolute diffuse calibration of IRAC through mid-infrared and radio study of HII regions