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We investigate the connection between the turnover frequency in the radio spectrum, $\nu_{\rm TO}$, and the rate of ionising ultra-violet photons, $Q_{\rm HI}$, in extragalactic sources. From a large, optically selected, sample we find $\nu_{\rm TO}$ to be correlated with $Q_{\rm HI}$ in sources which 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 $\nu_{\rm TO}$--$Q_{\rm HI}$ correlation is purely empirical, we use the ionising 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 ionising photon rate. Furthermore, for a given gas mass, we find that the turnover frequency is anti-correlated with the scale-factor of the gas density. While other mechanisms, such as ageing electrons or sy

Author

Curran, S. J.

Published

2024

Language

EN