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. Subject headings: ISM: structure — Galaxy: structure, kinematics and dynamics