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J. A. Collins (University of Colorado), R. A. Benjamin (University of Wisconsin), R. J. Rand (University of New Mexico)
In order to better understand the kinematics and rotation of diffuse ionized gas (DIG) halos in spiral galaxies, we have developed a model in which the gas is assumed to be concentrated in clouds which cycle ballistically through the halo. We present characteristics of this model, including a sample of individual cloud orbits which indicate a significant outward radial migration of gas. The calculated mass flux associated with this cycling is higher than expected for normal spirals indicating possible shortcomings of a purely ballistic model. In addition, we have generated synthetic velocity profiles in z (height above the midplane) for the purpose of comparing with velocity centroid data from previously obtained long-slit spectra of the edge-on spirals NGC 891 and NGC 5775. In each case, a purely ballistic model fails to explain the observed DIG kinematics. In the case of NGC 891, the fall-off in rotation velocity versus z is not as steep as predicted by the model, suggesting a possible coupling between disk and halo rotation, or alternatively, the presence of an outwardly directed pressure gradient. The ballistic model more successfully predicts the DIG kinematics observed in NGC 5775, though it cannot explain the observed trend of high-z velocities nearly reaching the systemic value. Such a discrepancy indicates that the high-z gas is subjected to an inwardly directed pressure gradient or that the unusual kinematics is a manifestation of a tidal interaction with the companion galaxy, NGC 5774. This work was partially supported by NSF grant AST-9986113 and NASA Theory grant NAG 5-8417.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.