THE COLLISION ORIGIN OF COHENS STREAM

High spatial and spectral resolution H I observations of the high-velocity cloud in Cetus called ''Cohen's stream'' reveal 21 cm features at velocities of -280, -120, -37, and -3 km s(-1). These new observations show in detail that the high- (-120 km s(-1)) and very high-velocity (-280 km s(-1)) gas are anticorrelated in position on the sky. These observations cover only a small portion of H I clouds involved, but the anticorrelation persists on the larger scales mapped by Cohen. The obvious explanation is that the high-velocity gas is the remnant of a collision between the very high-velocity gas and a low-velocity cloud. A simple mass and momentum conserving simulation demonstrates the elegance of this model. The asymmetric velocity distribution of the individual pixels in the observed remnant is a natural consequence of the different sizes of the parent clouds. Reasonable column density fluctuations added to the simulated parent clouds result in a remnant whose velocity distribution is much broader than observed. Some form of damping or momentum redistribution is needed to reduce the velocity dispersion. The redistribution of momentum by the enhanced magnetic field in the compressed postshock gas appears to be the most promising damping mechanism. The magnetic tension in the field lines accelerates the slower moving components while decelerating the faster moving ones. Recently shocked remnants are expected to have strong asymmetries in their velocity distributions. Older remnants will have more symmetric distributions, owing to the longer time over which the magnetic field has been allowed to damp the disparate velocities produced in the collision. This asymmetry could be used to detect recently shocked clouds even if their average velocity is not anomalously large.