The Infrared and Radio Flux Densities of Galactic H ii regions

Abstract

We derive infrared and radio flux densities of all ∼1000 known Galactic H ii regions in the Galactic longitude range . Our sample comes from the Wide-Field Infrared Survey Explorer ( WISE ) catalog of Galactic H ii regions. We compute flux densities at six wavelengths in the infrared ( Spitzer GLIMPSE 8 μ m, WISE 12 μ m and 22 μ m, Spitzer MIPSGAL 24 μ m, and Herschel Hi-GAL 70 μ m and 160 μ m) and two in the radio (MAGPIS 20 cm and VGPS 21 cm). All H ii region infrared flux densities are strongly correlated with their ∼20 cm flux densities. All H ii 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 and , and and reliably select H ii regions, independent of size. The infrared colors of ∼22% of H ii regions, spanning a large range of physical sizes, satisfy the IRAS color criteria of Wood & Churchwell for H ii regions, after adjusting the criteria to the wavelengths used here. Because these color criteria are commonly thought to select only ultra-compact H ii regions, this result indicates that the true ultra-compact H ii region population is uncertain. Compared to a sample of IR color indices from star-forming galaxies, H ii regions show higher 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.