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For refcode 1990ApJ...352...30I:
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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
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