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We present a high resolution VLA HI image of the spiral galaxy M81. The B, C, and D arrays of the VLA were used to image two fields on the galaxy, resulting in a map with a synthesized beamwidth of 12\arcsec (natural weighting), velocity channels of 2.6 km/sec, and a sensitivity of better than 1 mJy/beam in an individual channel. The map covers a velocity range of 475 km/sec. We are now able to resolve the shock in velocity with unprecedented resolution and spatially separate the arm and interarm regions.
The high sensitivity allows the detection of the interarm regions in the intensity map. We find that the interarm regions do not exhibit a uniform appearance; rather, there is a feathery, fragmented appearance to the gas distribution. There also appear to be ``bubbles'' in the interarm regions, possibly indicating the presence of stochastic star formation processes. While the spiral arm regions clearly show the pileup of gas indicating the presence of a spiral density wave, the arms are also not smooth and uniform. Knots and bubbles appear along the arms, occasionally at fairly uniform spacings. We conclude that while the spiral density wave causes the pileup of gas along the arms and contributes to the GLOBAL appearance of the galaxy, one must take into account LOCAL stochastic effects when describing the small scale structure of the gas in the galaxy.
The high velocity resolution allows a detailed study of the kinematical response of the gas to the density-wave shock. It also allows us to study the bubble regions to determine if these features are expanding shells indicative of supernova events. The velocity dispersion distribution across the face of the galaxy will also shed some light on the star formation process.
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