A Molecular Spiral Arm in the Far Outer Galaxy

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 m telescope and detected CO at 10 of 220 positions. The detections are distributed along the arm from l = 13°, v = −21 km s −1 to l = 55°, v = −84 km s −1 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 M ☉ . 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.