The first cosmological simulations of a dwarf galaxy with magnetic fields, radiative transfer and cosmic rays

The first cosmological simulations of a dwarf galaxy with magnetic fields, radiative transfer and cosmic rays

Amongst many different galaxy types, dwarf galaxies are certainly some of the most mysterious and intriguing systems. They lie at the heart of multiple unanswered questions in galaxy formation and have fuelled a number of controversies, the so-called `small-scale’ challenges to our standard cosmological model, the ΛCDM. These challenges concern, for example, a large disparity between the number of observed dwarfs with respect to the predicted number of dark matter haloes that may be hosting these systems, observed density profiles with central cores rather than the cuspy profiles predicted by CDM haloes and too few massive observed satellites.

These problems have prompted prolific and intense research into what is now known as the near-field cosmology and were motivation for state-of-the-art observational efforts by e.g. DES, VISTA, GAIA, LSST facilities, which aim to give us a more complete and unbiased census on the dwarf properties of our Local Group.

Clearly with such major observational campaigns in place it is mandatory to have a detailed theoretical understanding of the formation and evolution of our Local Group to be able to interpret the observed data and to link the observed luminous matter to the underlying overall matter distribution which can inform us about the cosmology.

In fact, a large body of theoretical work has identified two important processes which may fundamentally affect dwarf properties and largely alleviate, if not solve, all of the aforementioned discrepancies, indicating no departures from ΛCDM are necessarily needed. The first one regards energetic feedback by supernovae (SN) which can dramatically affect the baryon fraction, star formation and even the dark matter distribution especially in low mass haloes. The second one concerns the reionization of the Universe where energetic photons emitted by high redshift sources can photo-heat the gas suppressing the infall into low mass galaxies thus drastically reducing the number of luminous low mass haloes.

Star formation with its associated SN feedback in low mass haloes and reionization are tightly intertwined processes and the upcoming bounds on the faint-end of the galaxy luminosity function at high redshifts by JWST, together with the first observational probes of reionization and its evolving patchiness to be delivered by LOFAR, HERA and SKA will shed light on the progenitors of present-day dwarfs.

Bearing these important physical processes in mind, as a part of the DiRAC dp012 project, we have performed the first simulations where realistic SN feedback in conjunction with state-of-the-art, on-the-fly radiative transfer is included together with constrained transport MHD and cosmic ray feedback accounting for both their streaming and diffusion processes (Martin-Alvarez et al. in prep). Our simulations demonstrate that all these physical processes acting together are needed to reproduce realistic dwarf galaxies. Additionally, we are now able to predict self-consistently the HI component of these systems and to generate detailed radio synchrotron synthetic observations, for next-generation radio facilities such as SKA (see Fig. 1). This work serves as a pathfinder for our upcoming suite of simulations featuring larger cosmological zoom-in volumes that will explore the role played by cosmic rays, radiative transfer and magnetic fields in the formation of galaxies across cosmic time.

Figure 1: Projections of our full-physics simulated dwarf galaxy. (Inset box) Mass projection of the entire simulated box, zooming into the galaxy halo. (Large panel) gas density (blue), gas temperature (orange) and stellar density (yellow). (Right-hand panels) Synthetic observations of the galaxy, artificially positioned at 2Mpc from the Earth. From top to bottom: radio synchrotron as would be observed by VLA and SKA, optical for SDSS, and near-IR for JWST. (Bottom row) From left to right, these four panels are projections of the magnetic field, hydrogen ionisation fraction, cosmic ray energy density, and gas density (grey) separated into inflowing (blue) and outflowing (red) gas. Reproduced from Martin-Alvarez et al. in prep.