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

A sample of seven H II galaxies from the survey by Klein, Weiland, & B rinks (1991) was observed with the NRAO-VLA in the B array at frequenc ies of 325 MHz and 1489 MHz. The resulting radio continuum spectra-sup plemented by values from the literature-display a wide range of spectr al shapes. The spectra of two galaxies can be understood as the conven tional combination of thermal and nonthermal power-law emission spectr a. Four galaxies have radio spectra which show a significant flattenin g 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 lo w-frequency flattening of the majority of the sample's spectra is disc ussed in detail invoking a variety of radio emission and absorption me chanisms and energy losses of relativistic electrons. Fits for several of these mechanisms are performed. They are (1a) synchrotron loss spe ctra resulting from a one-time electron injection or (1b) from a conti nuous electron injection which began a few Myr ago; (2) dominance of d ifferent electron loss mechanisms across the frequency range without t ime 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 in jected electrons, spectral index of the radio emission, the emission m easure, the thermal electron density, and the age of the relativistic electrons are derived. The average spectral index of the nonthermal em ission 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 gal axies, contradicting the expectation by Klein, Weiland, & Brinks. The presence of H II regions with thermal electron densities of almost-equ al-to 25 cm-3 and emission measures of a few 10(5) pc cm-6 is needed t o establish a case for free-free absorption. For two galaxies, the app roximate sizes of the H II regions required for free-free absorption c an be verified from Halpha images. If the spectra are interpreted as t ime-dependent synchrotron loss spectra, they show signs of an electron distribution generated by SNRs a few times 10(6) yr ago. Model (2) co uld apply only to Mkn 297. An Halpha image of Mkn 297 shows starformin g activity distribution over a large number of knots which are the lik ely result of a merger. Its radio spectrum can be interpreted as the s um of the emission from the individual knots, the ages of the starburs ts in the knots not being uniform. Observations needed to discriminate between models (1) and (3) are discussed.