2012ApJ...759...96B

We report the detection of radio recombination line (RRL) emission using the Arecibo Observatory at X band (9 GHz, 3 cm) from 37 previously unknown H II regions in the Galactic zone 66° ≥ ℓ ≥ 31° and| b| ≤ 1°. This Arecibo H II 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 µm 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 H II 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 local standard of rest 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 H II region complex that has an associated H I and ¹³CO shell. The ∼90 pc diameter of the G52L nebula in this complex may be the largest Galactic H II region known, and yet it has escaped previous detection.