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R. A. Benjamin, D. P. Cox (U. Wisconsin-Madison)
Stellar and gaseous rotation curves have proven to be an important tool for understanding the mass distribution of galaxies. Up until recently, this tool has had limited value in exploring the three-dimensional mass distribution of galaxies, since the rotating material is restricted to a thin plane. Recent detection of interstellar matter up to 5 kiloparsecs above the plane of our Galaxy and NGC 891 could help extend our knowledge of the mass distribution of these galaxies, provided that the motion of this gas is dominated by rotation. Assuming that gas is hydrostatically supported in the vertical direction and rotationally supported in the radial direction, the predicted drop-off in rotation velocity is much steeper than is observed. At R=5 kpc in NGC 891, Rand (1997) finds a decrease of 30 km/s over a vertical distance of 5 kpc as compared to an expected 80 km/s decrease. Including gas pressure gradients makes the disagreement worse, and no reasonable mass model (flattening the dark matter halo, for example) can reconcile such a model with the available data. We examine two alternatives: The first possibility is that the interstellar disk "drags" the interstellar halo. This requires a viscosity of order 100 km/s kpc, and we explore the consequences for the evolution of the interstellar disk. The second possibility is that the gas is dynamically de-coupled from the disk. We present the expected velocities and velocity dispersion for a range of reasonable flows in order to motivate future observational work on this subject.
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