AAS journals follow standard practice in capitalizing generic terms that form a part of a proper name of a distinct, unique geographical or celestial region. "Belt" is capitalized because it is part of the name of the Kuiper Belt, a distinct region in the solar system, in the same way that "Valley" is part of the place name "Red River Valley." However, although Galactic is capitalized when it refers to the Milky Way, the bar is (in the opinion of the Editors of the ApJ ) not as yet a distinct mappable named region, but rather a general descriptive term, and so it is not capitalized.