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

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 Sobs are systematically lower than intensities Sextr extrapolated from measurements at higher frequencies. The ratio Sobs/Sextr 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 nonthermal 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 Te ≈ 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 ≤ Te ≤ 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.