CO in the Disk of the Barred Spiral Galaxy M83: CO (1-0), CO (2-1), and Neutral Gas

We present "on-the-fly" maps of CO (1–0) and CO (2–1) in the barred spiral galaxy M83. The entire optical disk is filled with CO emission in both lines, including interarm regions. For a "standard" conversion factor the total molecular mass is 2.5 × 109 M⊙, 4% of the total dynamical mass. Molecular gas constitutes 80% of total gas mass within the inner 10' and 30% overall. CO (1–0) emission and 21 cm continuum emission are extremely well correlated spatially in regions where H2 dominates the gas surface density. The spatial correlation between CO (1–0) and 100 μm emission is not as good. While the kinematics of the outer H I disk is severely nonaxisymmetrically perturbed, the kinematics of the CO disk and inner H I disk is well behaved and circular. The molecular disk appears to have a distinct hard edge at 5'. The hard edge is accompanied by a steep decline in the total gas surface density at that radius, although the H I does continue outward at a lower level. This edge is coincident with the onset of strong warping of the outer disk. The mean ratio of CO (2–1)/CO (1–0) emission is ~1, high by Galactic standards. Peaks in this ratio in interarm regions suggest that the CO emission is optically thin there, among other possibilities. Thus, M83 is unusual in that it has a relatively axisymmetric, almost entirely molecular, inner 10' gas disk surrounded by a warped, entirely atomic, outer gas disk. A sharp discontinuity separates the inner and outer gas disks. This kind of gas distribution is not seen in other late-type, gas-rich spirals.

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