NED Abstract 
Copyright by American Astronomical Society.
Reproduced by permission

1990ApJ...352...30I

   LOW-FREQUENCY RADIO CONTINUUM EVIDENCE FOR COOL IONIZED GAS IN NORMAL
                              SPIRAL GALAXIES

                               F. P. ISRAEL
                    Sterrewacht Leiden, The Netherlands

                                    AND

                               M. J. MAHONEY
           Clark Lake Radio Observatory, University of Maryland
           Received 1988 January 18: accepted 1989 September 18

                                 ABSTRACT

   We used the Clark Lake Radio Observatory TPT array at a frequency of
57.5 MHz to survey a total of 133 galaxies, mostly late-type spirals; 68
galaxies were detected. Observed radio intensities S_obs_ are
systematically lower than intensities S_extr_ extrapolated from measurements
at higher frequencies. The ratio S_obs_/S_extr_ correlates well with the
axial ratio of the observed galaxies and is smallest for edge-on galaxies.
This is interpreted as increasing free-free absorption of nonthermal
emission in galaxy disks with increasing tilt. The small ratio for edge-on
galaxies also indicates that disk-dominated spiral galaxies generally do
not have significant non-thermal halos.
   The implied free-free absorption cannot not be caused by classical
discrete H II regions. If it is to be explained by a smoothly distributed
diffuse ionized gas component, this component should have a filling factor
greater than 0.5, and a very low electron temperature T_e_ ~ 50 K. We
consider the more likely explanation to be the pervasive presence of a
clumpy medium of well-mixed non-thermally emitting and thermally absorbing
gas, with a small filling factor. With an electron temperature
500 K <= T_e_ <= 1000 K and a clump density of order 1 cm^-3^, ionization
could be maintained by the normal OB star content of galaxies. There is at
present no clear observational evidence for the presence of such a gas in
our own Galaxy. If it exists, it  should be located in the "thick disk." In
particular, it should be mostly absent in the Galactic plane, and almost
completely absent in the solar neighborhood.

Subject headings: galaxies: general - interstellar: matter - radio sources:
galaxies - radio sources: spectra