Carbon monoxide and the 3-kpc arm feature

Abstract

First isolated using the 21-cm HI spectral line, the 3-kpc arm exhibits the largest non-circular motion of any large-scale gaseous structure in the Galaxy (see e.g. Rougoor, 1964). Carbon monoxide observations best delineate the 3-kpc arm because the CO features are both narrower in velocity and also less confused with emission from more local gas than are HI observations. Because CO is a tracer of molecular hydrogen, the HI and CO data can in principle provide a good estimate of the total gaseous mass of the 3-kpc arm. A previous CO study of the inner Galaxy (Bania, 1977) suggested that, unless it is significantly tilted with respect to b=0°, the 3-kpc arm cannot be a continuous ring structure because emission at extreme positive velocities which should be produced by ring segments lying farther than 10 kpc from the Sun is absent. Consequently, the latitude distribution of CO in the 3-kpc arm has been studied by surveying 12CO emission over the region 350°<ℓ<25° at b=0° and ±0°.33. The survey positions are separated by Δℓ=1°. This coarse angular sampling resolution is deemed sufficient because the typical 3-kpc arm CO cloud has a linear dimension of 100 pc (Bania, 1977). Such clouds will subtend ∼0°.4 at the maximum line-of-sight distances expected for the extreme positive velocity 3-kpc arm emission (∼14 kpc).