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Session 82 - Spirals & Irregulars.
Display session, Friday, January 09
Exhibit Hall,

[82.18] A Comparison of M31 and the Milky Way in Molecular Gas

T. M. Dame (Harvard-Smithsonian CfA), L. Loinard (l'Observatoire de Grenoble), M. H. Heyer (FCRAO), J. Lequeux (DEMIRM, Observatoire de Paris), P. Thaddeus (Harvard-Smithsonian CfA)

M31 has long been thought deficient in molecular gas, the CfA survey of Dame et al. (1993, ApJ, 418, 730) showing that its total CO luminosity is only about one-fourth that of the Milky Way. However, our new CO survey of M31 with the FCRAO 14-m telescope (Loinard et al., previous abstract) has revealed that beyond 9 kpc, the radius of M31's molecular ring, the two galaxies are very similar in terms of molecular content, and GMCs there and here are similar in size and mass. The confinement of GMCs to the spiral arms is also similar in the two galaxies.

The molecular spiral arm S4, which lies near the peak of the Population I ring in M31, is remarkably similar to the Carina Arm of the Milky Way, which also lies at a galactocentric radius of 9 kpc. Both arms contain GMCs several hundred pc in size, spaced roughly every kpc along the arm, and each containing several million solar masses of molecular gas.

Although velocity-integrated CO line intensity (Wco) peaks at a significantly larger radius in M31 ( 9 kpc) than in the Milky Way ( 4 kpc), the ratio of Wco normalized by the atomic column density, Wco/N(HI), is rather similar in the two galaxies, apparently peaking at 2-3 kpc and decreasing steadily outward.

We will also present some preliminary evidence for the existence of anomalous-velocity CO emission in the nuclear region of M31, similar in some respects to that seen in the nuclear disk of our Galaxy.

Program listing for Friday