Impact of Anomalous Microwave Emission (AME) on Radio Spectral Energy Distributions: SKA Observations of Galaxies Near and Far

Abstract: Radio continuum emission powered by the thermal bremsstrahlung process is a clean, dust-free tracer of star formation in galaxies. However, the existence of anomalous microwave emission (AME) that is also prominent in a similar frequency range may challenge the use of thermal radio continuum emission to measure the star formation rates of galaxies. So while the nature of AME and the ISM conditions that lead to strong observable AME are still not well understood, the impact of AME on the radio emission from galaxies needs to be investigated for the SKA, which will be sensitive to large numbers of faint, high-redshift galaxies. In this chapter, we compute the observable flux density of free-free emission and AME and investigate the impact of AME on the galaxy radio spectral energy distribution for given observing frequencies and redshifts. Our conclusion is that (1) significance of AME is determined by the size of the AME region relative to the observing beam and ISM hydrogen column density, (2) thermal free-free emission dominates the radio continuum emission for distant galaxies at $\approx 10$ GHz frequency with negligible contribution from AME, (3) high-angular resolution observation of nearby galaxies resolving individual star forming region may need multi-frequency observations to avoid a potential bias from AME in the measure of star formation rate from single frequency observation.