The relationship between the turnover frequency and photo-ionisation in radio sources

TL;DR

This study explores the empirical link between the radio spectrum turnover frequency and ionising photon rate in extragalactic sources, proposing a free-free absorption model to explain observed correlations and spectral features.

Contribution

It establishes an empirical correlation between turnover frequency and ionising photon rate and models this relationship using free-free absorption across different gas distributions.

Findings

Turnover frequency correlates with ionising photon rate in sources with spectral turnover.

Free-free absorption models reproduce the turnover-size correlation in radio sources.

Turnover frequency is anti-correlated with the gas density scale-factor for a given gas mass.

Abstract

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.