Is there dust in galactic haloes?

Abstract

The ubiquitous presence of dust within the disks of spiral galaxies is well established. Dust is present in large quantities over enormous extents in the Universe; for example, Burton¹ concludes that, in our Galaxy, dust is distributed very similarly to the H I distribution, with an exponential radius in the plane of 8 kpc and an exponential scale height perpendicular to the plane of 120 pc. Simple dynamical arguments about the motion of interstellar grains out of the galactic disk show the possibility of confining a significant quantity of dust in a potential well high above the galactic plane. Although the possibility of dust being repelled away from the galaxy by the radiation pressure of the stars in the disk has earlier been considered², we have noted that the functional difference between the gravitational and radiation forces, as the dust recedes, may lead to the existence of stable regions outside the galaxy reminiscent of the libration points for interplanetary dust produced by competing gravitational forces alone (refs 3, 4 ; F.F., J.M .G., B.B. and S.A., in preparation). Here we predict that the presence of dust in these regions may be revealed in bright edge-on galaxies, especially by using the polarization of the scattered light from the symmetric lanes. The detection of scattered light above the galactic plane may be an indicator that the parent galaxy has not suffered close encounters with other galaxies at least within the timescale required to establish the dust layers.