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The Radio Spectral Energy Distribution and Star Formation Rate Calibration in Galaxies

Author/s: F. S. Tabatabaei, E. Schinnerer, M. Krause, G. Dumas, S. Meidt, R. Beck, A. Damas-Segovia, E. J. Murphy, D. D. Mulcahy, B. Groves, A. Bolatto, D. Dale, M. Galametz, K. Sandstrom, M. Boquien, D. Calzetti, R. C. Kennicutt, L. K. Hunt, I. De Looze, E. W. Pellegrini

Reference: 2017 ApJ 836 185 | Link

Comparison of the extinction-corrected SFR diagnostics against the 6 cm, 20 cm, and MRC radio SFRs in logarithmic scale for the entire sample. Also shown are the equality line (dashed), the OLS fit and its 95% confidence bounds (solid line/curve), and the bisector fit (dotted line). The MRC leads to a better equality (B=0.95) and smaller scatter (15%) than the monochromatic radio luminosities at 6cm or 20cm.
Comparison of the extinction-corrected SFR diagnostics against the 6 cm, 20 cm, and MRC radio SFRs in logarithmic scale for the entire sample. Also shown are the equality line (dashed), the OLS fit and its 95% confidence bounds (solid line/curve), and the bisector fit (dotted line). The MRC leads to a better equality (B=0.95) and smaller scatter (15%) than the monochromatic radio luminosities at 6cm or 20cm.
Measuring the star formation rate is key to understand the formation and evolution of galaxies. Until now, a variety of observations at different wavelengths have been performed to calculate the star formation rate (SFR), each with its advantages and disadvantages. As the most commonly used SFR tracers, the visible and the ultraviolet emission can be partly absorbed by interstellar dust. This has motivated the use of hybrid tracers, which combine two or more different emissions, including the infrared (IR), which can help to correct this dust absorption. However, the use of these tracers is often uncertain because other sources or mechanisms which are not related to the formation of massive stars can intervene and lead to confusion. Monochromatic radio continuum emission from galaxies has been also used as an extinction-free tracer of star formation based on its observed correlation with the infrared emission. Calibrating the radio SFRs using the radio-IR correlation has been also questioned because of a conspiracy of several other parameters leading to this correlation. This has motivated a detailed study of the origin and energetic of the radio continuum emission from a sample of nearby galaxies covering a wide range in star formation and ISM properties within the project 'the Key Insight in Nearby Galaxies Emitting in Radio (KINGFISHER)'. Thanks to the Effelsberg multi-wavelength observations at 1.4, 4.8, 8.4, and 10.5 GHz, the spectral energy distribution (SED) of the radio continuum emission from the KINGFISHER is studied using a Bayesian Markov Chain Monte Carlo technique. For the first time, the mid-RC (1–10 GHz, MRC) bolometric luminosity of the galaxies is determined and calibration relations versus the monochromatic radio luminosities are presented. The thermal emission is responsible for ~23% of the MRC on average. The extinction-corrected diagnostics of the star-formation rate (SFR) with the thermal and non-thermal radio tracers are compared. It is shown that the MRC radio luminosity provides the most precise measure of the massive star-formation rate in galaxies. Moreover, it is shown that the non-thermal spectral index flattens with increasing SFR surface density in galaxies, indicating the effect of the star-formation feedback on the cosmic-ray electron population in galaxies. Comparing the radio and IR SEDs, we find that the radio-IR correlation could decrease with SFR, in highly star forming galaxies, due to the amplification of the magnetic fields. This particularly implies a decrease in the IR/MRC at high redshifts, where mostly luminous/star-forming galaxies are detected.

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