  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 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.
