Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...287..419g&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 287, no. 2, p. 419-432
Astronomy and Astrophysics
Astrophysics
23
Corotation, Dynamic Response, Galactic Rotation, Gas Dynamics, Gas Streams, Interstellar Gas, Kinematics, Resonance, Spiral Galaxies, Carbon Monoxide, Computerized Simulation, Galactic Structure, Hydrodynamics, Molecular Clouds, Molecular Gases
Scientific paper
We discuss the validity of two different methods used to know the pattern speeds in spiral galaxy disks, applied to the grand design spiral NGC 4321. First, we discuss a method based on the study of the observed kinematics of the disk gas: we are seeking for the detection of the change of sign of the radial streaming motions, as predicted by the theory, when we go beyond the corotation circle (Omega = Omegap). Selected zones of the grand design spiral galaxy NGC 4321 were mapped in the 2-1 and 1-0 transitions of (12)CO. We sampled a region covering the minor axis in order to maximize the effect of radial streaming motions. Although we detected strong streaming motions associated with the stellar bar, the interarm and the crossing of the spiral arms, no change of sign in the radial streaming motions is seen in the observations. Therefore, according to the observed kinematics, we inferred that the whole inner spiral structure, up to a radial distance deprojected onto the plane of the galaxy of RG approximately equal to 180 sec, is located apparently inside corotation. A second method was developed to test the validity of the observational determination of the corotation resonance: we made numerical simulations of the molecular cloud hydrodynamics in M100 and compared the gas response with the spiral structure seen in the optical and CO observations. The fit between the simulated result and the observed galaxy allows to know the pattern speed of the spiral and so to locate the corotation resonance. We obtained the best fit solution for Omegap = 20 km/s/kpc, implying that corotation lies at RG approximately equal to 110 sec. This value is in clear contradiction with the observational determination quoted above that places corotation at the outer disk. A further analysis of the particle orbits in the rotating frame of the spiral wave, showing the wide radial extension of the corotating region and so the predominance of epicyclic orbits within the region where spiral arms develop together with the distorted geometry of the spiral structure in M100 due to the presence of a stellar bar, allows to understand why traditional methods, based on the study of the kinematics of the gas, might be highly uncertain to know the position of corotation in the disk of spiral galaxies as M100. The second method, based on a global fit of the spiral using numerical simulations, gives a much more trusty determination for the corotation.
Combes François
Garcia-Burillo Santiago
Sempere Maria J.
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