NEW 325-MHZ OBSERVATIONS OF H-II GALAXIES - THE MECHANISMS THAT SHAPE THE UNUSUAL RADIO SPECTRA

A sample of seven H II galaxies from the survey by Klein, Weiland, & Brinks (1991) was observed with the NRAO-VLA in the B array at frequencies of 325 MHz and 1489 MHz. The resulting radio continuum spectra-supplemented by values from the literature-display a wide range of spectral shapes. The spectra of two galaxies can be understood as the conventional combination of thermal and nonthermal power-law emission spectra. Four galaxies have radio spectra which show a significant flattening toward lower frequencies. The spectrum of II Zw 40 does not fit well in any category and seems to be dominated by thermal emission. The low-frequency flattening of the majority of the sample's spectra is discussed in detail invoking a variety of radio emission and absorption mechanisms and energy losses of relativistic electrons. Fits for several of these mechanisms are performed. They are (1a) synchrotron loss spectra resulting from a one-time electron injection or (1b) from a continuous electron injection which began a few Myr ago; (2) dominance of different electron loss mechanisms across the frequency range without time dependence; and (3) free-free absorption of long-wavelength radio emission by thermal electrons. The suitability of these mechanisms is discussed and model-dependent parameters like the spectral index of injected electrons, spectral index of the radio emission, the emission measure, the thermal electron density, and the age of the relativistic electrons are derived. The average spectral index of the nonthermal emission is alpha = 0.7 +/- 0.2 (S(nu) is similar to nu(-alpha)) for the galaxies with flattened spectra. Only one galaxy in the sample has a very steep nonthermal spectra index of alpha = 1.2 +/- 0.4. We do not find that steep nonthermal spectral indices are prevailing in H II galaxies, contradicting the expectation by Klein, Weiland, & Brinks. The presence of H II regions with thermal electron densities of almost-equal-to 25 cm-3 and emission measures of a few 10(5) pc cm-6 is needed to establish a case for free-free absorption. For two galaxies, the approximate sizes of the H II regions required for free-free absorption can be verified from Halpha images. If the spectra are interpreted as time-dependent synchrotron loss spectra, they show signs of an electron distribution generated by SNRs a few times 10(6) yr ago. Model (2) could apply only to Mkn 297. An Halpha image of Mkn 297 shows starforming activity distribution over a large number of knots which are the likely result of a merger. Its radio spectrum can be interpreted as the sum of the emission from the individual knots, the ages of the starbursts in the knots not being uniform. Observations needed to discriminate between models (1) and (3) are discussed.