Bulletin of Taras Shevchenko National University of Kyiv. Astronomy, no. 62, p. 12-16 (2020)

Spectral index in the radio continuum 140 MHz – 1.4 GHz emission for compact star-forming galaxies

S. Parnovsky, Dr Hab., Prof.
I. Izotova, PhD

Taras Shevchenko National University of Kyiv


The sample of compact star-forming galaxies (CSFGs) has been identified with sources in the catalogues of radio sources. It includes only galaxies with active star formation without objects with spectral signs of active galactic nuclei. For CSFGs with known flux densities in radiocontinuum at frequencies of 1.4 GHz from the FIRST or the NVSS and in the range of 120–168 MHz from the LOFAR, as well as in hydrogen emission lines from the SDSS, the spectral index and the spectral index of non-thermal emission component for these frequencies have been estimated. Note that obtaining the spectral index of nonthermal emission component is a much more difficult task than determining the spectral index of total emission. It is shown that the approximation by the model dependence using the least squares method can lead to unreliable estimates of the fraction of thermal (free-free) emission. Therefore, the flux densities of the thermal component are derived from the extinction- and aperture corrected fluxes of the Hα emission line. This method of estimation without using a fitting is more reliable. It is shown that the fact that derived spectra in radio range are quite flat is due, in particular, to the influence of selection effects caused by limited sensitivity of radio telescopes. For the same reason, only galaxies with a small fraction of thermal emission were included in the sample. Therefore, the spectral index is not much different from the spectral index of non-thermal emission.

Key words
Star-forming galaxies, continuum radio emission, thermal emission, spectral index.


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DOI: https://doi.org/10.17721/BTSNUA.2020.62.12-16