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 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.
