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 17\buildrel{\circ}\over{.} 5< {\ell }< 65^\circ . 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 μm, WISE 12 μm and 22 μm, Spitzer MIPSGAL 24 μm, and Herschel Hi-GAL 70 μm and 160 μm) 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 {{log}}₁₀({F}_{24μ {{m}}}/{F}_{12μ {{m}}})≥slant 0 and {{log}}₁₀({F}_{70μ {{m}}}/{F}_{12μ {{m}}})≥slant 1.2, and {{log}}₁₀({F}_{24μ {{m}}}/{F}_{12μ {{m}}})≥slant 0 and {{log}}₁₀({F}_{160μ {{m}}}/{F}_{70μ {{m}}})≤slant 0.67 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 {{log}}₁₀({F}_{70μ {{m}}}/{F}_{12μ {{m}}}) 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.