THE EASTERN ARM OF M83 REVISITED: HIGH-RESOLUTION MAPPING OF ~12CO l-0 EMISSION

摘要

We have used the Owens Valley Millimeter Array to map 12CO (J = l--0) along a 3.5 kpc segment of the eastern spiral arm of M83 at resolutions of 6".5 x 3".5, 10", and l6". The CO emission in most of this segment lies along the sharp dust lane demarcating the inner edge of the spira1 arm, but beyond a certain point along the arm the emission shifts downstream from the dust lane to become better aligned with the young stars seen in blue and HP images. This morphology resembles that of the western arm of Ml00. Three possibilities, none of which is wholly satisfactory, are considered to explain the deviation of the CO arm from the dust lane : heating of the CO by UV radiation from young stars, heating by low- energy cosmic rays, and a molecular medium consisting of two (diffuse and dense) components that react differently to the density wave. Regardless, the question of what CO emission traces along this spiral arm is a complicated one. Masses based on CO emission and the virile theorem for l0 emission features 1 roughly agree and are in the range 1.5--l6 x l0~6 Mo. These are lower than the masses of giant molecu- lar associations in M5l, but the discrepancy is probably due to the much higher linear resolution of these observations. Despite the uncertainty in what CO emission is tracing, we do not require a conver- sion factor of CO brightness to H, column density much different from the standard Galactic value if ' these structures are bound. Surprisingly, for the two fields where we can compare with single-dish data, only 2--5 of the single-dish flux is seen in our observat