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Date and Time of the Query: 2019-05-26 T16:34:41 PDT
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For refcode 2007ApJ...654..226R:
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Copyright by American Astronomical Society. Reproduced by permission
2007ApJ...654..226R Thermal Emission from H II Galaxies: Discovering the Youngest Systems D. Rosa-Gonzalez, H. R. Schmitt, E. Terlevich, and R. Terlevich Abstract. We studied the properties of very young massive star-forming regions in H II galaxies, with the aim of detecting signs of evolution where the first supernovae (SNe) start to appear. Our sample consists of 31 H II galaxies, characterized by strong hydrogen emission lines, for which low-resolution VLA 3.5 and 6 cm observations were obtained. We found that the radio spectral energy distribution (SED) has a range of behaviors: galaxies where the SED is characterized by a synchrotron-type slope; galaxies with a thermal slope; and galaxies with possible free-free absorption at long wavelengths. The latter represent a signature of heavily embedded massive star clusters. Comparing the different star formation rates (SFRs), we find that SFR(H{alpha}) is on average significantly higher than SFR(1.4 GHz). We confirm this tendency by comparing the ratio of the observed flux at 20 cm to the expected one, calculated based on the SFR H{alpha}, both for the galaxies in our sample and for normal ones. We show that this ratio is a factor of 2 smaller in our galaxies than in normal ones, indicating that they do not follow the FIR/radio correlation (q-parameter). These results suggest that the emission of these galaxies is dominated by a recent star-forming event in which the first SNe started to explode, consistent with the radio emission being dominated by free-free continuum. We propose an evolutionary scenario to explain the observed trends and conclude that the systematic lack of synchrotron emission in those systems with the largest equivalent width of H{beta} can only be explained if those are young starbursts of less than 3.5 Myr of age, i.e., before the first Type II SNe start to explode. Key words: Galaxies: Evolution, Radio Continuum: Galaxies
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