Finding Distant Galactic HII Regions

TL;DR

This study reports the discovery of distant Galactic HII regions using radio observations, extending the known reach of star-forming regions to the outermost parts of the Milky Way, including the most distant known HII regions.

Contribution

It presents new hydrogen radio recombination line detections of 302 HII region candidates, expanding the Galactic HII region catalog and identifying the most distant known HII regions in the Galaxy.

Findings

302 new HII region detections with velocities and distances

Identification of HII regions in the Outer Scutum-Centaurus Arm

Discovery of the most distant HII regions in the Milky Way

Abstract

The WISE Catalog of Galactic HII Regions contains $\sim2000$ HII region candidates lacking ionized gas spectroscopic observations. All candidates have the characteristic HII region mid-infrared morphology of WISE $12\,\,\mu\,m$ emission surrounding $22\,\mu\,m$ emission, and additionally have detected radio continuum emission. We here report Green Bank Telescope (GBT) hydrogen radio recombination line (RRL) and radio continuum detections at X-band (9GHz; 3cm) of 302 WISE HII region candidates (out of 324 targets observed) in the zone $225^{\circ} > l > -20^{\circ}$, $|b| \le 6^{\circ}$. Here we extend the sky coverage of our HII region Discovery Survey (HRDS), 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, five 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 $>1^{\circ}$ 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 kms). 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 $l \simeq 150^{\circ}$ 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.