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A high resolution HI image of M 81 taken with the VLA is used to study characteristics of spiral density waves in the disk. The two main spiral arms show different properties, consistent with previous surveys. The northeast arm shows strong signs of a classical density wave: a high concentration of gas along the inner (downstream) part of the arm (average arm-to-interarm intensity contrasts of up to 4:1), strong streaming motions (up to 30 km/sec) consistent with density-wave models; and velocity dispersions of up to 20 km/sec aligned with the intensity peaks. The southwest arm shows only minor signs of the presence of a density wave: the highest concentration of gas is aligned along the middle of the arm (contrasts up to 3:1); while streaming motions are present, they are inconsistent with density wave models; and velocity dispersions lower than in the northeast arm (8--12 km/sec). These inconsistencies lead us to believe that the density wave in the southwest arm may have been disrupted by an interaction with one of M81's numerous companions.
The velocity profiles are not uniform as one moves along an arm; they show a high degree of small-scale structure. Much of this structure corresponds to features in the intensity maps - HI knots, clumps, and holes. While the density waves do a good job gathering the gas clouds into the spiral arm regions, accounting for the large-scale spiral structure in the disk, they cannot explain the small scale structure and anomalies in the velocity profiles. One must take into account mechanisms such as cloud-cloud interactions, star formation, and supernova events to explain the small-scale structure of the gas in the galaxy.
The integrated intensity along the spiral arms is seen to fluctuate as a function of galactocentric radius. The peaks along the arms are in rough agreement with those seen in the optical work of Elmegreen etal (1989, ApJ, 343, 602). These fluctuations are consistent with modal density wave theory, which describes these variations as regions where waves cross as they move inward and outward between the resonances in the galaxy.
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