Radio wave scattering in the galactic disk

Abstract

Electron density turbulence in the Galaxy causes radio sources to be broadened angularly by amounts that depend strongly on the direction of the source. Recent scattering measurements using very long baseline interferometry (VLBI) and interplanetary scintillations (IPS) of extragalactic sources, and interstellar scintillations (ISS) of pulsars are used here to constrain the distribution of scattering material in the Galaxy. Although ubiquitous, scattering material appears to be highly concentrated in a thin disk of -100 pc thickness and in clumps of 1-10 pc size probably associated with H II regions, stellar wind bubbles, and/or supernova shocks. Typical transverse velocities of scattering material are much smaller than typical pulsar velocities. We predict the scattering angle as a function of galactic latitude. The results imply a maximum observable surface brightness for incoherent sources of electron synchrotron radiation.