On the Ionization of Warm Opaque Interstellar Clouds and the Intercloud Medium

This paper uses a variety of observations to construct an integrated picture of the ionization in the interiors of quiescent warm opaque interstellar clouds and in the intercloud medium (ICM) outside dense H II regions and hot dilute bubbles. Our main conclusion is that within ~1 kpc of the Sun the ionization rate of hydrogen per unit volume both in the interiors of such clouds and in the ICM is independent of the local density of neutral hydrogen and varies with position by less than ~20%. These conclusions strongly favor the decaying neutrino hypothesis for the ionization of the interstellar medium in these regions. Our analysis is based on a variety of observations, of which the most remarkable is the 1993 discovery by Spitzer & Fitzpatrick that, in the four slowly moving clouds along the line of sight to the halo star HD 93521, the column densities of both S II and C II*, which individually range over a factor of ~4, are proportional to the column density of H I to within ~20%. This proportionality is used to show that the free electrons exciting the C II to C II* are located mainly in the interiors of the clouds, rather than in their skins, despite the large opacity of the clouds to Lyman continuum radiation. The same conclusion also follows more unambiguously from the low value of the Hα flux in this direction, which was found by Reynolds in unpublished observations. These results are then used, in conjunction with observations of three pulsar parallaxes and dispersion measures, and with data on He I, N II, and O I line emissions, to constrain the ionization of H, He, N, and O and the flux of Lyman continuum photons from O stars in the ICM. It is argued that the total density of the ICM is kept close to the electron density in the clouds by a regulation mechanism, and a possible such mechanism is suggested. This picture of the ionization of the ICM also favors the decaying neutrino theory.