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For refcode 1993A&A...274..148G:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1993A&A...274..148G CO in Messier 51 II. Molecular cloud dynamics S. Garcia-Burillo, F. Combes, and M. Gerin IRAM, 300 rue de la piscine, F-38406 S Martin d'Heres, France Centro Astronomico de Yebes, IGN, Apartado 148, E-l9080 Guadalajara, Spain Ecole Normale Superieure, Paris, and Observatoire de Meudon, F-92190 Meudon, France Received February 21, accepted December 7,1992 Abstract. We analyse in detail the molecular gas dynamics in M 51, by comparing the IRAM CO data obtained with 12" resolution (0.5 kpc) with the result of Monte-Carlo simulations of the cloud hydrodynamics, during the encounter with NGC 5195. Simulations are 3D and we do not consider gas self-gravity. The fit between the simulated result and the observed galaxy allows us to determine the pattern speed of the spiral, and to locate the principal resonances in the disk: the corotation, the ILR and the OLR. We can then compare closely the arm-interarm contrasts and the kinematics of the molecular gas between the model and the observations. We have succeeded in reproducing the high arm-interarm contrasts (3- 5) observed in the ^12^CO map simply by orbit crowding. The contrast is slightly higher in the simulation, but we find evidence of macroscopic opacity effects in the observations that could, by saturation, weaken the contrast. As expected from density wave theory, the arm-interarm contrast is everywhere higher in the near-infrared map than in the CO map. We find no evidence for a galactic shock either in the simulations, or in the observations. The simulated and observed velocity fields are remarkably similar. In particular, the strong streaming motions observed both in the arm and in the interarm regions (60-75km/s in the plane of the galaxy with respect to purely circular motion) are reproduced by the model. The simulated and observed velocity-dispersion maps reveal maxima in the spiral arms. But the amount of velocity dispersion in the arms is higher in the observations. We discuss two possible explanations for the observed line-widths, accounting for such discrepancy. First, we study the fueling of large-scale turbulence as a result of the dissipation of the kinetic energy provided by differential rotation in the disk, under the action of viscous torques. Although it seems possible to account locally for the observed line-widths in the case of M 51, the associated lifetime for the generated turbulence would be surprisingly short: after one rotation period the molecular gas disk would collapse towards the center. An alternative explanation envisages the occurrence of "macroscopic" opacity effects; a substantial fraction of the CO emission comes from giant molecular cloud envelopes or "haloes", sub-thermally excited, at about T_ex_ = 5-7K. In the central spiral arms, the surface filling factor of these "haloes" is larger than one, even taking into account their velocity spread. This is supported by the low CO(2-1)/CO(1-0) ratio all over the galaxy, the narrower line-width in ^13^CO and the high surface filling factor observed in the simulations. We find evidence for GMA (giant molecular associations that are only an unbound random superposition of molecular clouds. Since the width of a typical spiral arm is a few times the diameter of a GMC, these GMA look like beads on a string. Key words: galaxies: barred - evolution of - general - kinematics and dynamics of - interstellar medium: clouds: general - radio lines: molecular
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