Dust Dynamics in the Galactic Disk-Halo Vicinity

Abstract

We report on the dynamics of interstellar dust grains above the plane of the Milky Way. Our Galactic model closely matches its large-scale observed properties, namely the luminosity output, the gas content and distribution, as well as the extinction of starlight by dust. The gravitational model is composed of a central bulge, a disk and a dark matter halo. In spite of the fact that the emphasis in the results is for our Galaxy, we also discuss the effects of varying key galactic parameters, such as the total luminosity output and gas distribution. The parameter space of the main dust grain properties is also explored; these include the grain type (graphite and silicate) and size (0.001 - 0.3 micron). The grains were launched at various positions in the Milky Way, but always in the disk-halo connection region; namely at three Galactocentric radii (5, 8 and 11 kpc) and three initial heights (150, 300 and 1000 pc). The grains were subject to radiation pressure, a gravitational force, as well as Coulomb and collisional drag with the gas. Due to the large size of the parameter space, there is a wide variety of possible grain dynamics. Indeed, depending on the physical conditions in which the grains were launched, the grains could either: (1) fall down towards the midplane, (2) stay at the same height, (3) rise at a more or less constant speed, (4) rise and then fall down, (5) rise and then stabilize at some height, and (6) be quickly expelled into the intergalactic medium. In general, graphite grains reach greater heights than silicate grains. The smaller grains (of radius a = 0.01 and 0.001 micron) tend to stay at the same height they started at. The classical grain (a = 0.1 micron) is the most sensitive to radiation pressure and usually reaches the highest heights, even if they are modest. The largest grain we have studied (a = 0.3 micron) also responds well to radiative forces, but its large mass prevents it from going as high as the classical grain, and it even falls down towards the midplane under some circumstances.