The seven dwarfs illuminated: The impact of radiation on dwarf galaxies and their circumgalactic medium

We present a high-resolution cosmological zoom-in simulation of a group of field dwarf galaxies that includes on-the-fly radiative transfer (RT) and is evolved to z = 0. Emission from stars is included according to age-dependent spectral energy distributions, and a redshift-dependent UV background is modelled by background particles and propagated through the simulation volume. The simulation also incorporates star formation, supernova (SN) feedback, a non-equilibrium primordial chemical network, and metal cooling and diffusion. We compare results from this simulation to previous simulations with the same initial conditions and largely similar galaxy formation physics, but without RT. The inclusion of RT results in the formation of eight additional faint dwarf galaxies with stellar masses of 104 M⊙ to several 105 M⊙ and only old stellar populations, similar to the observed ultra-faint dwarf galaxies. These eight faint dwarf galaxies formed before and during cosmic reionisation and were mostly quenched by z ~ 3–4 when reionisation was complete. The simulated galaxies follow many observed scaling relations such as the stellar mass-halo mass relation, the mass-size relation, and the luminosity-velocity dispersion relation. However, the simulation appears to underpredict the stellar iron abundances for the faintest population with Lv < 106L⊙. For the more massive “classic” dwarf galaxies, radiative feedback suppresses star formation, making it less bursty and reducing explosive outflows. This consequently reduces the dark matter core sizes by a factor of two to three, rendering the core sizes (~1 kpc) more consistent with observations. The distribution of neutral hydrogen H I in the circumgalactic medium (CGM) is ubiquitous, with a covering fraction of unity within Rvir, and in good agreement with observations. It is rather insensitive to radiative or SN feedback at z = 0, but at z > 5, it is much higher in the RT simulation. In contrast, the distribution of low-ionisation species such as Si II is very compact and declines sharply beyond the interstellar medium (ISM) scale. C IV and O VI have a more extended distribution, but their column densities are generally below the detection limit. Radiative feedback leads to smaller column densities of the metal ions, partly due to the reduction of total metal production, and partly because hard photons from the stellar radiation escape the ISM and further ionise the CGM. The abundance of C IV is particularly sensitive to the latter effect.