A Molecular Spiral Arm in the Far Outer Galaxy

TL;DR

This paper reports the discovery of a new molecular spiral arm beyond the Outer Arm in the Milky Way, detected through CO observations, representing the most distant molecular gas observed in our galaxy.

Contribution

It identifies and characterizes a previously unknown molecular spiral arm in the far outer Galaxy, extending our understanding of galactic structure.

Findings

Detected CO in 10 of 220 positions along the new arm

Mapped a large molecular cloud with a radius of 47 pc

Found the arm at a distance of approximately 21 kpc, the most distant molecular gas in the Milky Way

Abstract

We have identified a spiral arm lying beyond the Outer Arm in the first Galactic quadrant ~15 kpc from the Galactic center. After tracing the arm in existing 21 cm surveys, we searched for molecular gas using the CfA 1.2 meter telescope and detected CO at 10 of 220 positions. The detections are distributed along the arm from l = 13 deg, v = -21 km/s to l = 55 deg, v = -84 km/s and coincide with most of the main H I concentrations. One of the detections was fully mapped to reveal a large molecular cloud with a radius of 47 pc and a molecular mass of ~50,000 Mo. At a mean distance of 21 kpc, the molecular gas in this arm is the most distant yet detected in the Milky Way. The new arm appears to be the continuation of the Scutum-Centaurus Arm in the outer Galaxy, as a symmetric counterpart of the nearby Perseus Arm.