A Distant Extended Spiral Arm in the Fourth Quadrant of the Milky Way

Using data from the Southern Galactic Plane Survey, we present a possible distant spiral arm in the fourth quadrant of the Milky Way. The very distinct and cohesive feature can be traced for over 70° as the most extreme positive velocity feature in the longitude-velocity diagram. The feature is at a Galactic radius between 18 and 24 kpc and appears to be the last major structure before the end of the H I disk. We compare the feature with a Galactic spiral model and show that it is well reproduced by a spiral arm of pitch angle i ~ 9°. The arm is quite well confined to the Galactic plane, dropping at most 1 kpc below the Galactic equator. Over most of its length, the arm is 1-2 kpc thick.