2024MNRAS.530.5177C

We investigate the connection between the turnover frequency in the radio spectrum, ν_TO, and the rate of ionizing ultraviolet photons, $Q_{\rm {H{\small I}}}$, in extragalactic sources. From a large, optically selected sample, we find ν_TO to be correlated with $Q_{\rm {H\,{\small I}}}$ in sources that exhibit a turnover. The significance of the correlation decreases when we include power-law radio sources as limits, by assuming that the turnover frequency occurs below the lowest value observed. However, the power-law fit sources are less well sampled across the band and so these may just be contributing noise to the data. Given that the observed ν_TO-$Q_{\rm {H{\small I}}}$ correlation is purely empirical, we use the ionizing photon rate to obtain the electron density in a free-free absorption model. For each of the constant, exponential, constant plus exponential (Milky Way), and spherical models of the gas distribution, there is also an increase in the turnover frequency with ionizing photon rate. Furthermore, for a given gas mass, we find that the turnover frequency is anticorrelated with the scale-factor of the gas density. While other mechanisms, such as ageing electrons or synchrotron self-absorption, may be required to reproduce the spectral indices, for an exponential scale-factor similar to the linear size, this simple free-free absorption model reproduces the turnover-size correlation seen in radio sources.