  We carry out 1D hydrodynamical simulations of the evolution of a spherically
symmetric supernova remnant (SNR) subject to an external radiation field (ERF)
that influences the cooling and heating rates of the gas. We consider
homogeneous media with ambient hydrogen number densities $n_{\rm H,0}$ of $0.1$
and $1$ cm$^{-3}$ permeated by an average radiation field including the cosmic
microwave, extragalactic, and Galactic backgrounds, attenuated by an effective
column density $N_{\rm H,eff}$ from $10^{18}$ to $10^{21}$~cm$^{-2}$. Our
results may be classified into two broad categories: at low $N_{\rm H,eff}$,
the ERF presents little absorption in the ultraviolet (ionising) regime, and
all the 'unshielded' cases feature an equilibrium temperature $T_{eq} \sim
7000$~K below which the ambient gas cannot cool further. In this scenario, the
SNR develops a nearly isothermal shock profile whose shell becomes thicker over
time. At higher $N_{\rm H,eff}$, the ERF is heavily absorbed in the UV range,
yielding a roughly constant heating function for temperatures $\lesssim 10^4$
K. These `shielded' cases develop a thin, cold and dense shell throughout their
evolution. Energy and momentum injection to the medium do not change
significantly between both scenarios, albeit luminosity is higher and more
uniformly distributed over the shell for unshielded SNR.

Astrophysics of Galaxies