Nonuniversal Interstellar Density Spectra Probed by Pulsars

Abstract

Galactic interstellar turbulence affects density distribution and star formation. We introduce a new method of measuring interstellar turbulent density spectra by using the dispersion measures (DMs) of a large sample of pulsars. Without the need of invoking multiple tracers, we obtain nonuniversal density spectra in the multiphase interstellar medium over different ranges of length scales. By comparing the analytical structure function of DMs with the observationally measured one in different areas of sky, we find a shallow density spectrum arising from the supersonic turbulence in cold interstellar phases, and a Kolmogorov-like density spectrum in the diffuse warm ionized medium (WIM). Both spectra extend up to hundreds of parsecs. On larger scales, we identify, for the first time, a steep density spectrum in the diffuse WIM extending up to several kiloparsecs. Our results show that the DMs of pulsars can provide unique new information on the interstellar turbulence.