How radiation, magnetic fields, and cosmic rays reshape dwarf galaxies

Sergio Martin-Alvarez, Debora Sijacki, Martin G. Haehnelt, Marion Farcy, Yohan Dubois, Vasily Belokurov, Joakim Rosdahl, Enrique Lopez-Rodriguez

Find the full publication here: https://ui.adsabs.harvard.edu/abs/2023MNRAS.525.3806M/abstract

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What is this about?

Dwarf galaxies are key to unlocking the mysteries of galaxy formation. Because they’re small and gravitationally fragile, the physics that shapes them—especially stellar feedback—can dramatically change their properties. While supernova-driven feedback alone has long been studied, real galaxies host additional forms of physics: stellar radiation, magnetic fields, and cosmic rays. How do these non-thermal processes influence dwarf galaxy properties, such as their gas content, morphology, and dark matter distribution?

That's precisely what we've explored in the first paper of our new Pandora simulation series.

What did we do? We performed cosmological zoom-in simulations of a dwarf galaxy, with multiple simulations of the very same galaxy, varying the physics included:

• A baseline run with just hydrodynamics and supernova feedback.

• Additional runs that systematically add:

  • Magnetic fields (MHD)

  • Stellar radiation (with radiative transfer of multiple photon groups)

  • Cosmic rays (with anisotropic diffusion and realistic transport)

  • And a full-physics run including all the above effects.

We analysed how each physical ingredient affects the galaxy’s baryonic and dark matter properties, focusing on structural changes to the baryonic mass - that is, the standard mass that we see around us and distribution. We focused on the gas, stars and neutral hydrogen of the galaxy. It is also interesting to understand how changes on the normal matter of a galaxy can also affect the distribution of its invisible, yet mass-dominating dark matter. To explore this, we looked into how the central density profile of this dark matter changes: is it centrally concentrated in a mass peak, or more flatly distributed?

Figure: Same galaxy, different physics! In our Pandora simulations, we took one dwarf galaxy and evolved it under various physical scenarios—adding things like radiation (RT), magnetic fields (MHD), cosmic rays (CRs), and supernova feedback (HD). Each panel shows the same galaxy's gas (top rows, with red colours showing underdense, and blue colours showing highly dense) and its starlight (bottom rows, with the black backgrounds, as an approximation of how the galaxy would look when observed through a telescope). Notice how dramatically different the galaxy looks depending on what physics are at play. It's like a cosmic experiment showing how crucial non-thermal physics really is!

Guess what we found out?

Non-thermal physics reshape the evolution and the properties of galaxies, in a non-straight-forward manner: they interplay with each other pushing the physics and the evolution of the galaxy in different directions. For our 'full-physics' models, we found that

Galaxies become more extended and increase their degree of rotation: When radiation, magnetic fields, and cosmic rays are included, the stellar and gaseous distributions of matter become puffier and more extended. As their size gets larger, their support against gravity becomes more dominated by coherent rotation. All of these changes better resemble the appearance of the galaxies we see around us.

Dark matter cores emerge naturally: An especially intriguing finding is that the combination of non-thermal physics significantly transforms the inner structure of the dark matter halo. Instead of the steep, dense cusp seen in the feedback-only run, the full-physics model with all the non-thermal processes develops a more flattened central core. This provides a new feedback pathway to build the cored dark matter profiles that we believe some dwarf galaxies around us may feature.

Why does this matter?

Our work shows that all these non-thermal physics well-known to be at play in our Universe (radiation, cosmic rays, and magnetic fields) are not merely an optional detail: they fundamentally reshape dwarf galaxies. These physics directly influence how much gas galaxies retain, how their stellar and gaseous components distribute themselves, and how dark matter responds dynamically.

By incorporating these realistic physics processes, we take a crucial step towards more accurate simulations—simulations that better reflect the complex reality observed in the Universe.

This was the initial groundwork for the Pandora series, which further explores these processes and their observational consequences in dwarf galaxy evolution.

Where can I read more?

If you want to explore the effect of these physics in more galaxy properties, I'd recommend reading the full paper. There's even more information about structural properties, dynamics, colour-magnitude, magnetic fields, and more!

Here is a link to it:

https://ui.adsabs.harvard.edu/abs/2023MNRAS.525.3806M/abstract

Feel free to contact me with any questions you might have!

Un saludo!