WISE Catalog of Galactic HII Regions V3.0

WISE Catalog of Galactic HII Regions Changelog
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Version 3.0 - July 2026
0) Rewrote all catalog creation scripts in python for better stability
1) Updated with 5GHz GDIGS RRL results from Linville et al., 2023
2) Added candidate G002.226+0.057
3) Added 1.3GHz FAST RRL results from Hou et al., 2022
4) Added 5GHz GLOSTAR RRL results from Khan et al., 2024
5) Changed ~2000 radio-quiet candidates to candidates based on radio continuum detections in 1.3 GHz MeerKAT SMGPS (Goedhart et al., 2024) and Galactic center (Heyworth et al., 2021) data
6) Added spectra for HII Region Discovery Survey sources, and also for HII regions within the GDIGS area.
7) Added "complexes" to link members of large star formation regions: SgrB2, G5.9, G9.6, W31, G10.5, W39, W33, G12.8, G29, W43, W49, G45.1, G48.6, W51B, W51A, S99, Cygnus X, W3, W4, S252, S255-S258, Orion, RCW38, G305, G333W, G333E, G348.5, NGC6334, NGC6357. We will add more in coming releases.

Version 2.3 - December 2020
1) Changed 31 radio quiet candidates near l=359° to candidates based on radio continuum emission in MeerKAT 1.28 GHz data: G357.989-00.121, G358.138-00.180, G358.355-00.448, G358.358-00.042, G358.366+00.034, G358.371-00.468, G358.378-00.499, G358.387-00.484, G358.413+00.049, G358.472-00.401, G358.488+00.077, G358.505+00.069, G358.507-00.368, G358.516-00.065, G358.561-00.096, G358.700-00.055, G358.711+00.036, G358.714+00.054, G358.718-00.041, G358.735-00.002, G358.736+00.005, G358.745-00.007, G358.793+00.012, G358.798+00.069, G358.804+00.036, G358.830-00.022, G358.835+00.017, G358.894+00.094, G358.931-00.030, G358.931-00.038, G358.934-00.151
2) Added candidates G358.631-0.479, G358.730+0.052, G358.667-0.044, G358.305+0.074, G358.983+0.037 based on radio continuum emission in MeerKAT 1.28 GHz data.
3) Removed candidates G358.822-0.002, G358.508+0.057 based on disagreementsin positions between IR and based on radio continuum emission in MeerKAT 1.28 GHz data.
4) Changed three radio-quiet regions to candidates based on radio continuum data in Ingallinera et al., (2019): G343.251+0.734, G344.392-0.127, G345.656-0.016
5) Removed known region G026.319-00.014 is it is a probable PN.
6) Added RRL observations from Wenger et al. (2021)
7) Added large radio-loud candidate G028.647+0.198
8) Changed 26 radio quiet candidates to candidates based on radio continuum detections in Wenger et al. (2021): G258.608-01.925, G259.013-01.546, G259.268-02.611, G259.502-02.573, G259.608-02.991, G259.612-02.698, G261.319-03.063, G261.449-01.345, G261.634-02.134, G261.638-02.095, G261.648-01.642, G262.176-01.960, G263.227+01.572, G264.597-00.246, G264.647-01.110, G269.522-01.250, G270.171-01.820, G270.255-01.633, G297.463-00.733, G302.453-00.740, G328.982+00.572, G337.474-01.054, G339.639-01.061, G339.699+00.302, G341.071-01.128, G342.213-01.153

Version 2.2 - February 2019
1) Removed regions G049.048-00.886 and G049.775-00.951 near W51 as they are probably caused by escaping radiation from the larger region, and are thus not distinct.
2) Added SHRDS sources from Wenger et al. (2019).
3) Changed radio-quiet candidates to candidates based on radio continuum observations from VLA project AW0261: G358.894+00.094, G358.858-00.022, G358.793+00.012, G358.735-00.002, G358.736+00.005, G358.711+00.036, G358.714+00.054, G358.798+00.069, G358.505+00.069, G358.561-00.096, G359.278-00.062, G359.246-00.046, G359.237-00.034
4) Added radio-quiet sources G358.452-0.023, G358.460+0.020, G358.478+0.013, G358.472+0.046, G358.503+0.120 G358.616+0.081 and candidate G358.593+0.099.
5) Removed radio quiet sources G359.759-0.285, G359.811-0.383, G359.698-0.374, and candidate G359.685-0.263, replaced with G359.756-0.351, which is S16.
6) Removed duplicate G358.634+0.063.
7) Added G000.021+0.263 as part of S17.
8) Added G000.321-0.215 as the larger part of S20.

Version 2.1 - October 2017
1) Added candidate G355.681+0.144
2) Updated distance errors using the Monte Carlo code from Wenger et al. (2018).

Version 2.0 - July 2017
1) Added radio and infrared flux densities from Makai et al. (2017)

Version 1.5 - June 2016
1) Changed some Galactic center radio quiet sources into candidates based on VLA 6cm and Hubble Paschen alpha identifications provided by Cornelia Lang and Dominic Ludovici.
2) Changed some first-quadrant radio quiet sources into candidates based on VLA THOR 21cm continuum data.
3) Downloaded new MGPS/SUMSS data (Updated January 2015) where previous holes in coverage were filled in. Classified remaining 108 sources previously lacking radio continuum data.
4) Separated RCW60B into its component HII regions.
5) Added radio quiet sources G336.481-1.081, G336.524-1.465, G336.284-1.222, and G336.289-1.248
6) Removed duplicate sources G019.146+00.355, G031.307-00.106, G032.749-00.065, G015.176+00.044, G021.596-00.161
7) Removed candidates G312.037+00.084 and G311.972+00.071
8) Removed group sources G319.481-00.103, G317.237-00.186, G316.689-00.064, G317.300-00.211
9) Added diffuse HRDS sources from Anderson et al. (2017)
10) Removed G029.895-0.042, G030.690-0.258, G034.045+0.053, G034.916-0.016, G037.9036-0.275271, G049.491,-0.369, G49.495-0.347, G049.583-0.383, G057.546-0.274 because they were not distinct
11) Added G063.142+0.423, G063.159+0.426, G063.123+0.434, G030.255+0.05270, G032.792+0.187, G032.799+0.194, G033.884+0.058, G044.423+0.534, G053.058+0.190
12) Added known region G020.479+00.165, which was confused with a SNR.
13) Added known region G045.689-0.235, which was somehow missed.
14) Added candidate G028.435-0.008
15) Changed 358.753+00.0532, G358.463+00.0636, and G358.482+00.0608 to candidates based on VLA data from project AY137

Version 1.4 - August 2015
1) Removed G341.503-01.109 (PN)
2) Removed G345.052-01.856 (OH/IR star)
3) Grouped G350.594+01.149 with NGC6334
4) Removed duplicate regions G304.206+01.351 and G304.207+01.352
5) Removed G261.513-01.225 (star)
6) Removed G261.618-01.974 (star)
7) Removed G330.176+01.006 (not distinct)
8) Added small radio quiet sources G296.272-00.388, G297.324-00.008, 297.455-00.010, 297.519-00.031, G299.610+00.169, G299.770-00.008, G298.903+00.218, G300.051-00.399, G301.841-00.880, G302.020+00.254, G308.202-01.020, G335.763+02.594, G336.577+2.829
9) G030.425+00.463 was incorrectly associated with S66. Changed to a "candidate".
10) G303.137-0.887, G303.073-0.929, and G303.232-0.890 were incorrectly associated with G303.115-00.947. Changed to "candidates"
11) Added observations by Law et al. (2009) of GC lobe G359.555-0.0405.
12) Associated G291.070-00.794 with G291.059-00.770.
13) Removed duplicate candidates G019.146+00.355 and G021.596-00.160
14) Changed radio-quiet source 179.682-0.563 to candidate due to GB6 emission.

Version 1.3 - March 2015
1) Updated catalog based on multiple velocity source measurements to remove line measurements from diffuse ionized gas (Anderson et al., 2015a)
2) Added WISE HRDS observations of over 300 sources from Anderson et al. (2015b), together with kinematic distances
3) Added radio quiet source at 53.14+0.07 based on infrared appearance.
4) Updated G30.795-0.275 with observations from Anderson et al. (2011).
5) Removed duplicate G039.389-0.142.
6) Fixed S148 so it is correctly associated with line observations
7) Changed Zeta Oph to "candidate" since no line observations in catalog.
8) Fixed NGC6334, which was not being correctly associated with Quireza et al. (2006) line observations.
9) Changed G018.426+01.922 from "group" to "known" based on new line observations.
10) Removed line observations for G028.438+00.014. New observations of nearby G028.451+00.001 cast doubt on what was actually measured.
11) Updated maser parallax distances with results from Burns et al. (2014), Chibueze et al. (2014), and Kamezaki et al (2014).

Version 1.2 - June 2014
1) Added Zeta Oph
2) Changed status of G22.791-0.347 from group to known based on observations in Anderson et al. (2011)

Version 1.1 - March 2014
1) Updated with RRL results from Quireza et al. (2006b) and Balser et al. (2011).
2) Combined G080.925-00.214 and G080.931-00.205 into a single region because of previous catalog duplication.
3) Updated maser parallax distances using Reid et al. (2014) compilation
4) S235 and S236 aliases were reversed and have now been corrected.
5) For sources with maser parallax distances, the KDAR is now listed in parentheses. This KDAR is not used in the computation of distances or Galactocentric radii, but can be used for kinematic and maser parallax distance comparison studies.

Version 1.0 - January 2014
Published in ApJS

General
Surveys
Other Data Sources

The WISE Catalog of Galactic HII Regions is a catalog of HII regions and candidates, compiled from Widefield Infrared Survey Explorer (WISE) 12µm and 22µm data. It was created by Loren Anderson, Thomas Bania, Dana Balser, Virginia Cunningham, Trey Wenger, Brittany Johnstone, and William Armentrout, with help from numerous students at West Virginia University. This website is updated as we continue research on the catalog contents.

This website uses the Google Maps API to display the contents graphically, and the Google Tables visualization to display the contents in the table. We combined individual FITS files into mosaics, and cut the mosaic files into tiles for the Google Maps interface with GDAL2Tiles.

Work on the Catalog, including this web site, was supported by NASA ADAP grant NNX12AI59GADAP to Loren Anderson.

The map view can be set directly via URL parameters, e.g. index.html?survey=9&v=40&l=30&b=0&zoom=6:
l, b – Galactic longitude/latitude (degrees) to center the map on.
zoom – initial zoom level (default 5).
survey – initial background layer: 0=Spitzer MIR, 1=GLIMPSE360 MIR, 2=WISE MIR, 3=IGPS 1.4GHz, 4=SMGPS 1.3GHz, 5=CfA ¹²CO, 6=GRS ¹³CO, 7=ATLASGAL 870µm, 8=Hi-GAL FIR, 9=GDIGS RRL, 10=SEDIGISM ¹³CO, 11=WHAM Hα (integrated intensity), 12=SHASSA Hα, 13=BOLOCAM 1.1mm, 14=FAST RRL, 15=WHAM Hα (5 km/s channel maps).
v – for the six velocity-cube surveys (GDIGS RRL, FAST RRL, WHAM Hα channels, CfA ¹²CO, GRS ¹³CO, SEDIGISM ¹³CO), the LSR velocity in km/s of the channel to show (e.g. v=40 selects the 40 to 45 km/s bin).

Background image layers, selectable from the dropdown in the upper-left of the map. For the six velocity-cube surveys (GDIGS RRL, FAST RRL, WHAM Hα channels, GRS 13CO, CfA 12CO, SEDIGISM 13CO), each slider position is a moment-0 (velocity-integrated) map over that bin, in K km/s. All layers use an asinh stretch.

Spitzer MIR – Spitzer Space Telescope GLIMPSE (Benjamin et al. 2003, PASP, 115, 953) and MIPSGAL (Carey et al. 2009, PASP, 121, 76) surveys (Red: MIPS 24µm, green/blue: IRAC 8.0/3.6µm). Data merged and scaled by survey teams. Spatial resolution (PSF FWHM): 6.0″ (24µm), 2.0″ (8.0µm), 1.7″ (3.6µm).
GLIMPSE360 MIR – the Spitzer GLIMPSE360 extension (Meade et al. 2014, GLIMPSE360 Data Description v1.5; no separate refereed survey paper), 4.5µm/3.6µm. Data merged and scaled by survey team. Spatial resolution (PSF FWHM): ~1.7″ (both bands).
WISE MIR – Wide-field Infrared Survey Explorer (Wright et al. 2010, AJ, 140, 1868) 12µm (W3, green) and 22µm (W4, red). Background-subtracted. Spatial resolution (PSF FWHM): 7.4/12.0″.
SMGPS 1.3GHz – SARAO MeerKAT Galactic Plane Survey (Goedhart et al. 2024, MNRAS, 531, 649), 1.3 GHz radio continuum. Spatial resolution: 8″.
IGPS 1.4GHz – mosaic of three 1.4 GHz radio continuum surveys: VGPS (Stil et al. 2006, AJ, 132, 1158), CGPS (Taylor et al. 2003, AJ, 125, 3145), and SGPS (McClure-Griffiths et al. 2005, ApJS, 158, 178). Spatial resolution 1′ for VGPS/CGPS and 100′ for SGPS. Because VGPS/CGPS and SGPS have very different typical brightnesses, each gets its own scaling.
Dame+ 12CO – the composite CO survey of Dame, Hartmann & Thaddeus (2001), ApJ, 547, 792, from the CfA 1.2m telescope. Beam smoothed (FWHM=8.5′) Spectral resolution: 1.3 km/s native; displayed in 10 km/s channels from −170 to +170 km/s. The red outline traces regions where we interpolated due to sparse sampling.
GRS 13CO – Boston University–FCRAO Galactic Ring Survey (Jackson et al. 2006, ApJS, 163, 145), ¹³CO J=1→0. Beam smoothed (FWHM=46&Prime). Spectral resolution: 0.21 km/s native; displayed in 5 km/s channels from −5 to 135 km/s.
SEDIGISM ¹³CO – SEDIGISM survey (Schuller et al. 2017, A&A, 601, A124; Data Release 1: Schuller et al. 2021, MNRAS, 500, 3064), ¹³CO J=2→1 from the APEX 12m telescope. Beam smoothed (FWHM=30.1″). Spectral resolution: 0.25 km/s native; displayed in 5 km/s channels from −200 to +200 km/s.
ATLASGAL 870um – APEX Telescope Large Area Survey of the Galaxy (Schuller et al. 2009, A&A, 504, 415), 870µm dust continuum (LABOCA). Spatial resolution: 19.2″. Data merged and scaled by survey team.
GDIGS RRL – GBT Diffuse Ionized Gas Survey (Anderson et al. 2021, ApJS, 254, 28), stacked α hydrogen radio recombination line (RRL) emission. Beam smoothing (FWHM=2.65′). Spectral resolution: 0.5 km/s native; displayed in 5 km/s channels from −200 to 140 km/s.
WHAM Hα – Wisconsin H-Alpha Mapper all-sky survey (WHAM-SS DR1; Haffner et al. 2003, ApJS, 149, 405), Hα intensity in Rayleighs with a 1° beam. The slider shows 5 km/s channel maps (moment-0 maps from the survey's velocity cube, 2.0 km/s channels) from −100 to 100 km/s, and its last position shows the intensity integrated over all velocities (from the HEALPix map distributed by CADE, which we thank). Inferno color scale, asinh stretch. Zoom is limited to the survey's resolution.
FAST RRL – hydrogen radio recombination line cube from the FAST Galactic Plane Pulsar Snapshot (GPPS) survey (Han et al. 2021, RAA, 21, 107), 1.4 GHz, covering 33°<l<55°, |b|<2° at an angular resolution of about 3′. Spectral resolution: 2.2 km/s native; displayed in 5 km/s channels from −200 to 140 km/s (each bin integrated over exactly 5 km/s). Hydrogen emission lies at roughly −40 to 140 km/s; the bins below that show the carbon and helium recombination lines of the brightest sources.
Hi-GAL FIR – Herschel infrared Galactic Plane Survey (Molinari et al. 2010, PASP, 122, 314), shown as a three-color composite: red = SPIRE 250µm, green = PACS 160µm, blue = PACS 70µm, each with an asinh stretch. Only the area covered by all three bands is shown. Built from the HEALPix (NSIDE=16384, 12.9″ pixel) maps.
SHASSA Hα – Southern H-Alpha Sky Survey Atlas (Gaustad et al. 2001, PASP, 113, 1326), Hα intensity with a 1′ beam, covering the sky south of about +15° declination, shown with an asinh stretch (0–4000 R) in the inferno color scale. Built from the HEALPix (NSIDE=8192) map distributed by CADE, which we thank. Residuals from bright-star subtraction remain visible as dark and bright spots.
BOLOCAM 1.1mm – Bolocam Galactic Plane Survey (Aguirre et al. 2011, ApJS, 192, 4), version 2.1, 1.1 mm dust continuum from the Caltech Submillimeter Observatory with a 33″ beam, shown with an asinh stretch above a 0.05 Jy/beam noise floor in the nipy_spectral color scale. Coverage is patchy: the inner Galaxy and selected fields only. Built from the HEALPix (NSIDE=16384) map distributed by CADE, which we thank.

Several external catalogs can be overlaid on the map:

ATNF Pulsars – Manchester et al. (2005), AJ, 129, 1993, the ATNF Pulsar Catalogue v2.8.1 – 3534 sources; accessed 2026 September.
CHIME FRBs – CHIME/FRB Collaboration (2026), ApJS, arXiv:2601.09399, the Second CHIME/FRB Catalog of Fast Radio Bursts – 4509 sources; accessed 2026 September.
Red MSX Sources – Lumsden et al. (2013), ApJS, 208, 11, the Red MSX Source (RMS) survey of massive young stellar objects – 2798 sources; accessed 2026 September.
GotC+ [CII] – Pineda et al. (2013), A&A, 554, A103, the Herschel GOT C+ [CII] Galactic plane survey – 471 positions; accessed 2026 September.
Messier – Charles Messier's 1781 catalogue, positions and NGC/IC cross-identifications from Sinnott (1988, ed.), Sky Publishing Corp., "NGC 2000.0" – 110 sources; accessed 2026 September. M24's size (120′ major axis) is from SIMBAD (Wenger et al. 2000, A&AS, 143, 9); M40 and M45 have no size here and are shown as dots.
Westerhout – Westerhout (1958), Bull. Astron. Inst. Netherlands, 14, 215, a survey of discrete Galactic radio sources at 1390 MHz – 107 sources; accessed 2026 September. Common names, where known (10 sources), are from SIMBAD (Wenger et al. 2000, A&AS, 143, 9), accepted only where SIMBAD's position is within 15′ of Westerhout's.
Galactic O Stars – O-type systems from the Galactic O-Star Spectroscopic Survey (GOSSS), the basis of the Galactic O-Star Catalog (GOSC): Sota et al. (2011), ApJS, 193, 24 and Sota et al. (2014), ApJS, 211, 10 (448 systems), plus Maíz Apellániz et al. (2016), ApJS, 224, 4 (142 more) – 590 sources; accessed 2026 October from the VizieR copies of those tables. The online GOSC lists a few more stars from later survey releases.

Known SNRs (v1.0, September 2026) merges 310 sources from D.A. Green's Catalog (Green 2025, JApA, 46, 14) with 378 sources from SNRcat (Ferrand & Safi-Harb 2012, Adv. Space Res., 49, 1313. Both accessed 2026 September. We removed 13 objects that Dokara et al. (2021), A&A, 651, A86, Anderson et al. (2017), A&A, 605, A58, or Anderson et al. (2025), A&A, 693, A247 argued are probably not genuine SNRs, leaving 382 known SNRs. Where Dokara et al. (2021), Anderson et al. (2017), or Anderson et al. (2025) remeasured a known SNR's position and size, we use their values.

SNR Candidates (v1.0, September 2026) is compiled from thirteen literature sources:
1) Dokara et al. (2021), A&A, 651, A86 – GLOSTAR, 2) Anderson et al. (2017), A&A, 605, A58 – THOR, 3) Ball et al. (2023), MNRAS, 524, 1396 – ASKAP EMU/POSSUM pilot field, 4) Hurley-Walker et al. (2019), PASA, 36, e045 & e048 – GLEAM, 5) Filipović et al. (2023), AJ, 166, 149 – "Ancora," G288.8−6.3, 6) Smeaton et al. (2024a), RNAAS, 8, 158 – "Unicycle," G312.65+2.87, 7) Lazarević et al. (2024), RNAAS, 8, 107 – "Raspberry," G308.73+1.38, 8) Smeaton et al. (2024b), MNRAS, 534, 2918 – "Perun," G329.9−0.5, 9) Tsalapatas et al. (2024), A&A, 690, A247 – LOFAR LoTSS, 10) Anderson et al. (2025), A&A, 693, A247 – SMGPS, 11) Filipović et al. (2025), PASA, 42, e104 – "Teleios," G305.4−2.2, 12) Ball et al. (2025), ApJ, 988, 75 – ASKAP EMU/POSSUM, Vela region, 13) Arias et al. (2026), A&A, in press – flat-spectrum shells & new candidates

The WISE Catalog of Galactic HII Regions ADS pdf Abstract

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.

Finding Distant Galactic HII Regions ADS pdf Abstract

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.

The Infrared and Radio Flux Densities of Galactic HII regions ADS pdf Abstract

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(F_24µm/F_12µm)≥ 0 and log₁₀(F_70µm/F_12µm)≥ 1.2, and log₁₀(F_24µm/F_12µm)≥ 0 and log₁₀(F_160µm/F_70µm)≤ 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(F_70µm/F_12µm) ratios. We find a weak trend of decreasing infrared to ~20cm flux density ratios with increasing R_gal, in agreement with previous extragalactic results, possibly indicating a decreased dust abundance in the outer Galaxy.

A Green Bank Telescope Survey of Large Galactic HII Regions ADS pdf Abstract

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.

The Southern HII Region Discovery Survey (SHRDS): Pilot Survey ADS pdf Abstract

The Southern HII Region Discovery Survey is a survey of the third and fourth quadrants of the Galactic plane that will detect radio recombination line (RRL) and continuum emission at cm-wavelengths from several hundred HII region candidates using the Australia Telescope Compact Array. The targets for this survey come from the WISE Catalog of Galactic HII Regions and were identified based on mid-infrared and radio continuum emission. In this pilot project, two different configurations of the Compact Array Broad Band receiver and spectrometer system were used for short test observations. The pilot surveys detected RRL emission from 36 of 53 HII region candidates, as well as seven known HII regions that were included for calibration. These 36 recombination line detections confirm that the candidates are true HII regions and allow us to estimate their distances.

A Galactic Plane Defined by the Milky Way HII Region Distribution ADS pdf Abstract

We develop a framework for a new definition of the Galactic midplane, allowing for tilt (θ_tilt) rotation about Galactic azimuth 90°) and roll (θ_roll) rotation about Galactic azimuth 0°) of the midplane with respect to the current definition. Derivation of the tilt and roll angles also determines the solar height above the midplane. Here we use nebulae from the Wide-field Infrared Survey Explorer (WISE) Catalog of Galactic HII Regions to define the Galactic high-mass star formation (HMSF) midplane. We analyze various subsamples of the WISE catalog and find that all have Galactic latitude scale heights near 0.30° and z-distribution scale heights near 30 pc. The vertical distribution for small (presumably young) HII regions is narrower than that of larger (presumably old) HII regions (~25 pc versus ~40 pc), implying that the larger regions have migrated further from their birth sites. For all HII region subsamples and for a variety of fitting methodologies, we find that the HMSF midplane is not significantly tilted or rolled with respect to the currently defined midplane, and, therefore, the Sun is near to the HMSF midplane. These results are consistent with other studies of HMSF, but are inconsistent with many stellar studies, perhaps because of asymmetries in the stellar distribution near the Sun. Our results are sensitive to latitude restrictions and also to the completeness of the sample, indicating that similar analyses cannot be done accurately with less complete samples. The midplane framework we develop can be used for any future sample of Galactic objects to redefine the midplane.

The Southern HII Region Discovery Survey. I. The Bright Catalog ADS pdf Abstract

The census of Galactic HII regions is vastly incomplete in the southern sky. We use the Australia Telescope Compact Array to observe 4-10 GHz radio continuum and hydrogen radio recombination line (RRL) emission from candidate HII regions in the Galactic zone 259°< l < 344°,|b| < 4°. In this first data release, we target the brightest HII region candidates and observe 282 fields in the direction of at least one previously known or candidate HII region. We detect radio continuum emission and RRL emission in 275 (97.5%) and 258 (91.5%) of these fields, respectively. We catalog the ~7 GHz radio continuum peak flux densities and positions of 80 previously known and 298 candidate HII regions. After averaging ~18 RRL transitions, we detect 77 RRL velocity components toward 76 previously known HII regions and 267 RRL velocity components toward 256 HII region candidates. The discovery of RRL emission from these nebulae increases the number of known Galactic HII regions in the surveyed zone by 82% to 568 nebulae. In the fourth quadrant, we discover 50 RRLs with positive velocities, placing those sources outside the solar circle. Including the pilot survey, the Southern H ɪɪ Region Discovery Survey has now discovered 295 Galactic HII regions. In the next data release, we expect to add ~200 fainter and more distant nebulae.

The Southern HII Region Discovery Survey. II. The Full Catalog ADS pdf Abstract

The Southern HII Region Discovery Survey (SHRDS) is a 900 hr Australia Telescope Compact Array 4-10 GHz radio continuum and radio recombination line (RRL) survey of Galactic HII regions and infrared-identified HII region candidates in the southern sky. For this data release, we reprocess all previously published SHRDS data and include an additional ~450 hr of observations. The search for new HII regions is now complete over the range 259° < ℓ < 346°, ∣b∣ < 4° for HII region candidates with predicted 6 GHz continuum peak brightnesses ≥30 mJy beam^-1. We detect radio continuum emission toward 730 targets altogether including previously known nebulae and HII region candidates. By averaging ~18 RRL transitions, we detect RRL emission toward 206 previously known HII regions and 436 HII region candidates. Including the northern sky surveys, over the last decade the HII Region Discovery Surveys have more than doubled the number of known Galactic HII regioNS. The census of HII regions in the Wide-field Infrared Survey Explorer (WISE) Catalog of Galactic HII Regions is now complete for nebulae with 9 GHz continuum flux densities ≥250 mJy. We compare the RRL properties of the newly discovered SHRDS nebulae with those of all previously known HII regions. The median RRL FWHM line width of the entire WISE Catalog HII region population is 23.9 km s^-1 and is consistent between Galactic quadrants. The observed Galactic longitude-velocity asymmetry in the population of HII regions probably reflects underlying spiral structure in the Milky Way.