Relativistic jet feedback II: Relationship to gigahertz peak spectrum
  and compact steep spectrum radio galaxies
  We propose that Gigahertz Peak Spectrum (GPS) and Compact Steep Spectrum
(CSS) radio sources are the signposts of relativistic jet feedback in evolving
galaxies. Our simulations of relativistic jets interacting with a warm,
inhomogeneous medium, utilize cloud densities and velocity dispersions in the
range derived from optical observations, show that free-free absorption can
account for the $\sim \rm GHz$ peak frequencies and low frequency power laws
inferred from the radio observations. These new computational models replace
the power-law model for the free-free optical depth in the \citep{bicknell97a}
model by a more fundamental model involving disrupted log-normal distributions
of warm gas. One feature of our new models is that at early stages, the low
frequency spectrum is steep but progressively flattens as a result of a broader
distribution of optical depths, suggesting that the steep low frequency spectra
discovered by \citet{callingham17a} may possibly be attributed to young
sources. We also investigate the inverse correlation between peak frequency and
size and find that the initial location on this correlation is determined by
the average density of the warm ISM. The simulated sources track this
correlation initially but eventually fall below it, indicating the need for a
more extended ISM than presently modelled. GPS and CSS sources can potentially
provide new insights into the phenomenon of AGN feedback since their peak
frequencies and spectra are indicative of the density, turbulent structure and
distribution of gas in the host galaxy.
