The Arecibo HII Region Discovery Survey

TL;DR

This study reports the discovery of 37 new faint HII regions in the Milky Way using Arecibo's sensitive radio observations, expanding knowledge of Galactic star-forming regions and their distribution.

Contribution

It presents the first detection of faint HII regions with Arecibo, demonstrating its capability to identify smaller and more distant nebulae missed by previous surveys.

Findings

Detected 37 new HII regions with Arecibo

Derived distances for 23 regions, including the largest known Galactic HII region

Identified a large diffuse HII complex with associated HI and 13CO shell

Abstract

We report the detection of radio recombination line emission (RRL) using the Arecibo Observatory at X-band (9GHz, 3cm) from 37 previously unknown HII regions in the Galactic zone 66 deg. > l > 31 deg. and |b| < 1 deg. This Arecibo HII Region Discovery Survey (Arecibo HRDS) is a continuation of the Green Bank Telescope (GBT) HRDS. The targets for the Arecibo HRDS have spatially coincident 24 micron and 20 cm emission of a similar angular morphology and extent. To take advantage of Arecibo's sensitivity and small beam size, sources in this sample are fainter, smaller in angle, or in more crowded fields compared to those of the GBT HRDS. These Arecibo nebulae are some of the faintest HII regions ever detected in RRL emission. Our detection rate is 58%, which is low compared to the 95% detection rate for GBT HRDS targets. We derive kinematic distances to 23 of the Arecibo HRDS detections. Four nebulae have negative LSR velocities and are thus unambiguously in the outer Galaxy. The remaining sources are at the tangent point distance or farther. We identify a large, diffuse HII region complex that has an associated HI and 13CO shell. The ~90 pc diameter of the G52L nebula in this complex may be the largest Galactic HII region known, and yet it has escaped previous detection.