We have identified a spiral arm lying beyond the Outer Arm in the first Galactic quadrant \sim 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 ^ { \circ } , v = -21 km s ^ { -1 } to l = 55 ^ { \circ } , 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 \sim 50,000 M _ { \odot } .
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 .