Models of Vertical Disturbances in the Interstellar Medium

This paper describes some interesting properties of waves in, and oscillations of, the interstellar medium (ISM) in the direction normal to the plane of the Galaxy. Our purpose is to examine possible reasons for four observed phenomena: the falling sky in the northern hemisphere; the apparent presence of clouds in absorption spectra when a sight line is occupied primarily only by warm intercloud gas; the peculiar structuring of spiral arms involving clumps, spurs, and feathering; and the existence of an abundance of high-stage ions far off the plane of the Galaxy. We explored the reaction of the interstellar medium—in the vertical direction only—to large imposed disturbances (initial displacements, expansive velocities, and compressions) and to the introduction of small-amplitude waves via oscillation of the midplane. Our findings included (1) the anticipated growth in amplitude of high-frequency waves with height; (2) the four lowest normal modes for the oscillation of the atmosphere as a whole, as functions of the height of the outer boundary; (3) the time for material to "bounce" from one unusually dense state to the next as a function of height; and (4) the tendency for the disk to develop a hot outer halo, either after the passage of a single shock from a large event or in response to a continuous stream of small-amplitude waves. We discovered that three of the four observed phenomena targeted are likely to be closely connected. Following a large expansion, material near the plane falls back first, with material at higher z then falling in upon it. This provides precisely the sort of velocity segregation observed in the northern sky, at about 50 Myr after the event. In addition, this bounce time (and/or the period of the subsequent smaller oscillations, which is about twice the bounce time) may be linked to structure in the spiral arms, with vertical oscillations having been provoked by initial compressions in the arms. Oscillations of the fundamental symmetric (breathing) mode of the ISM also produce substantial disturbances in the outer atmosphere. This can result in the production of an extensive layer of hot gas overlying the cooler disk material, i.e., a hot Galactic halo with a significant population of high-stage ions. Hence, three of the four phenomena may be natural results of the simple existence of strong local compressions at the spiral arms and the associated vertical motions in a thick Galactic disk. Finally, the somewhat mysterious appearance of clouds in some absorption spectra can be produced by small-amplitude waves in the ISM. Under the right conditions, clouds will seem to appear through "velocity crowding," when in fact there are no density concentrations in space.