Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apjs..115..203w&link_type=abstract
Astrophysical Journal Supplement v.115, p.203
Astronomy and Astrophysics
Astronomy
18
Galaxies: Individual Messier Number: M81, Galaxies: Kinematics And Dynamics, Galaxies: Spiral
Scientific paper
A study of the pattern speeds of disk galaxies is under way using integrated forms of the continuity equation that relate the pattern speed to the velocity and surface density of the pattern tracer. Integration removes the unobservable sky-plane velocities and allows averaging to improve accuracy and spatial resolution. This approach assumes the disk is flat and the pattern rigid but makes no assumption about the dynamics. The method relies on the nonaxisymmetric nature of the velocity field and brightness distribution of a spiral. The speeds of nonrigid patterns may be approximated by the same equations in regions where the shear is small. The method is used to determine the speed of the H I spiral pattern in M81 using intensity and velocity maps from the VLA. The mean pattern speed is 23.4 +/- 2.3 km s-1 kpc-1. If the pattern rotates rigidly then corotation is at a radius of 8.4 kpc, the outer Lindblad resonance (OLR) is within the spiral at 12.6 kpc, and the inner Lindblad resonance (ILR) is in the central hydrogen hole and well inside the inner limit of spiral structure. The pattern speed may be lower in the northern half of M81 than the southern, in which case the present high symmetry of M81 is accidental. This lower speed could be explained by the lower H I content in the south arm. Reapplication of the method without the assumption of rigidity shows that the pattern is shearing at a rate of 2.3 km s-1 kpc-2, which is consistent with swing amplification owing to a recent close passage of M82 and NGC 3077. The pattern and the material have nearly the same speed from a radius of about 9 kpc outward. This means there is no OLR in the observed H I galaxy and explains the lack of asymmetric photometric gradients across the arms. The timescale for winding of the pattern and the period of the orbit of NGC 3077 are roughly the same; periodic reamplification of the pattern by a close passage of NGC 3077 can remove the winding dilemma. The rate of shear, the velocity field of the south arm, and the very small amount of interarm H I are inconsistent with a nearly rigid density wave. The models most similar to M81 are the shearing gas models. Stochastic models give similar morphologies but make no prediction about velocities.
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