Using data from the all-sky Wide-Field Infrared Survey Explorer (WISE) satellite, we made a catalog of over 8000 Galactic HII regions and HII region candidates by searching for their characteristic mid-infrared (MIR) morphology. WISE has sufficient sensitivity to detect the MIR emission from HII regions located anywhere in the Galactic disk. We believe this is the most complete catalog yet of regions forming massive stars in the Milky Way. Of the ~8000 cataloged sources, ~1500 have measured radio recombination line (RRL) or H-alpha emission, and are thus known to be HII regions. This sample improves on previous efforts by resolving HII region complexes into multiple sources and by removing duplicate entries. There are ~2500 candidate HII regions in the catalog that are spatially coincident with radio continuum emission. Our group's previous RRL studies show that ~95% of such targets are HII regions. We find that ~500 of these candidates are also positionally associated with known HII region complexes, so the probability of their being bona fide HII regions is even higher. At the sensitivity limits of existing surveys, ~4000 catalog sources show no radio continuum emission. Using data from the literature, we find distances for ~1500 catalog sources, and molecular velocities for ~1500 HII region candidates.
The WISE Catalog of Galactic HII Regions contains ~2000 HII region candidates lacking ionized gas spectroscopic observations. All candidates have the characteristic HII region mid-infrared morphology of WISE 12µm emission surrounding 22µm emission, and additionally have detected radio continuum emission. We here report Green Bank Telescope hydrogen radio recombination line and radio continuum detections in the X-band (9 GHz; 3 cm) of 302 WISE HII region candidates (out of 324 targets observed) in the zone 225° ≥ l ≥ -20°, |b| ≤ 6° Here we extend the sky coverage of our HII region Discovery Survey, which now contains nearly 800 HII regions distributed across the entire northern sky. We provide LSR velocities for the 302 detections and kinematic distances for 131 of these. Of the 302 new detections, 5 have (l,b,v) coordinates consistent with the Outer Scutum-Centaurus Arm (OSC), the most distant molecular spiral arm of the Milky Way. Due to the Galactic warp, these nebulae are found at Galactic latitudes >1° in the first Galactic quadrant, and therefore were missed in previous surveys of the Galactic plane. One additional region has a longitude and velocity consistent with the OSC but lies at a negative Galactic latitude (G039.183-01.422; -54.9 km/s). With Heliocentric distances >22 kpc and Galactocentric distances >16 kpc, the OSC HII regions are the most distant known in the Galaxy. We detect an additional three HII regions near l=150° whose LSR velocities place them at Galactocentric radii >19 kpc. If their distances are correct, these nebulae may represent the limit to Galactic massive star formation.
We derive infrared and radio flux densities of all ~1000 known Galactic HII regions in the Galactic longitude range 17.5°< l <65° Our sample comes from the Wide-Field Infrared Survey Explorer (WISE) catalog of Galactic HII regions. We compute flux densities at six wavelengths in the infrared (Spitzer GLIMPSE 8µm, WISE 12µm and 22µm, Spitzer MIPSGAL 24µm, and Herschel Hi-GAL 70µm and 160µm) and two in the radio (MAGPIS 20cm and VGPS 21cm). All HII region infrared flux densities are strongly correlated with their ~20cm 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 < 1pc), have slightly elevated IR to radio ratios. The colors log10(F24µm/F12µm)≥ 0 and log10(F70µm/F12µm)≥ 1.2, and log10(F24µm/F12µm)≥ 0 and log10(F160µm/F70µm)≤ 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 Wood & Churchwell for HII regions, after adjusting the criteria to the wavelengths used here. Because 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. Compared to a sample of IR color indices from star-forming galaxies, HII regions show higher log10(F70µm/F12µm) ratios. We find a weak trend of decreasing infrared to ~20cm flux density ratios with increasing Rgal, in agreement with previous extragalactic results, possibly indicating a decreased dust abundance in the outer Galaxy.
As part of our ongoing HII Region Discovery Survey (HRDS), we report the Green Bank Telescope detection of 148 new angularly-large Galactic HII regions in radio recombination line (RRL) emission. Our targets are located at a declination greater than -45°, which corresponds to 266° > l > -20° at b = 0° All sources were selected from the WISE Catalog of Galactic HII Regions, and have infrared angular diameters >260''. The Galactic distribution of these "large" HII regions is similar to that of the previously-known sample of Galactic HII regions. The large HII region RRL line width and peak line intensity distributions are skewed toward lower values compared with that of previous HRDS surveys. We discover 7 sources with extremely narrow RRLs <10 km/s. If half the line width is due to turbulence, these 7 sources have thermal plasma temperatures <1100 K. These temperatures are lower than any measured for Galactic HII regions, and the narrow line components may arise instead from partially ionized zones in the HII region photo-dissociation regions. We discover G039.515+00.511, one of the most luminous HII regions in the Galaxy. We also detect the RRL emission from three HII regions with diameters >100 pc, making them some of the physically largest known HII regions in the Galaxy. This survey completes the HRDS HII region census in the Northern sky, where we have discovered 887 HII regions and more than doubled the previously-known census of Galactic HII regions.