Bursty dwarfs and well-behaved winds: radiation, magnetism, and cosmic rays regulate star formation and outflows in small galaxies

Sergio Martin-Alvarez, Debora Sijacki, Martin G. Haehnelt, Alice Concas, Yuxuan Yuan, Roberto Maiolino, Risa H. Wechsler, Francisco Rodríguez Montero, Marion Farcy, Mahsa Sanati, Yohan Dubois, Joki Rosdahl, Enrique Lopez-Rodriguez, Susan E. Clark

Find the full publication here:

https://ui.adsabs.harvard.edu/abs/2025arXiv250603245M/abstract

What is this about?

Dwarf galaxies are small but mighty. Because of their shallow gravitational wells, they’re extremely sensitive to how feedback from stars shapes their gas and star formation. And yet, despite years of effort, most of our simulations still get them wrong. In particular, it’s been hard to reproduce two aspects from observations: bursty star formation that comes and goes in cycles (especially as JWST unveils in the early Universe), and outflows of warm, temperate gas — both ionized and neutral — that can carry away many times more gas mass than stars being formed.

That got us thinking: maybe traditional “feedback-only” simulations are missing something fundamental. What happens when you add the non-thermal physics that we know are present in galaxies — things like radiation, magnetic fields, and cosmic rays?

That’s exactly what we set out to test.

Figure - a closer look at galactic outflows: Each column shows the same dwarf galaxy simulated with different physics — from standard feedback (left) to boosted explosions (second column), to radiation + magnetism (third column) and our full-physics model with radiation, magnetic fields, and cosmic rays (right). The top panels highlight stars (gold) and gas (blue), the middle ones show gas temperature, and the bottom panels trace inflows (blue) and outflows (orange and red) by velocity. Simpler models produce hotter, more jagged outflows. Radiation softens them, while cosmic rays generate smoother, cooler winds with richer structure — including in the metal distribution.

What did we do?

We built a suite of simulations — the Pandora project (see this other post: How radiation, magnetic fields, and cosmic rays reshape dwarf galaxies) — of a dwarf galaxy forming in a realistic cosmological environment. Each simulation includes different combinations of physics, from supernova feedback, to magnetic fields (MHD), radiation from stars (high energy photon groups), and cosmic ray energy + transport physics.

By changing the physics but keeping everything else fixed, we isolate how each ingredient affects the galaxy’s star formation, outflows, and gas phases. And we pushed for high resolution so we could actually resolve the relevant structure in the interstellar medium.

Figure - how stars form, and how gas escapes: The top panels show the star formation rate (SFR) over time for different simulations — from simpler models with just feedback (left) to more complex ones that include radiation, magnetic fields, and cosmic rays (right). Radiation tends to smooth things out, while adding cosmic rays brings back the bursts. The bottom panels show how much gas is being thrown out of the galaxy (the mass-loading factor, η). Interestingly, both our full-physics model and a version with boosted feedback drive the strongest outflows. Meanwhile, radiation alone produces gentler winds, closer to the baseline feedback case.

What do we find?

So here is the exciting part. We find important changes to the patterns of star formation, from a messier, less well-defined structure in feedback-only (stellar explosions only) runs to a more episodic structure with radiation, CRs, and magnetic fields. These produce intense, rapid starbursts followed by long lulls. The outflows from the galaxy, the gas that is ejected by these explosions, also becomes colder, heavier, and more realistic. These so called mass-loading factors (the amount of gas expelled per "stellar explosion") match what’s seen in real galaxies, especially when considering the ionized / neutral separation. This is due to cosmic rays and radiation making it easier for outflows to grab onto neutral gas, and their ability to push on it without over-heating it. This is the potential explaination for outflows in dwarfs being so multiphase and extended.

You may be familiar with this inspiring concept that the elements that surround us are crafted inside stars. A side-effect of being formed inside stars is that, during their violent explosions, a large proportion of these metals are lost into intergalactic space, reducing the available budget of metals inside galaxies. However, with these non-thermal physics, we find that a higher proportion of these "metals" is preserved in galaxies, replaced with some of the more comparatively "metal-poor" neutral gas entrained in winds. This also leads to clear predictions for the proportion of these elements in our simulated galaxies, which are in better agreement with observations!

Why does this matter?

Our results suggest that if you want to model dwarf galaxies properly, you can’t just include supernovae and call it a day. Radiation, cosmic rays, and magnetic fields dramatically change how stars form and how gas cycles in and out of galaxies. And they’re key to producing outflows that look anything like what telescopes are actually seeing.

This is especially important in the era of highly accurate observatories, where we’ll be seeing more low-mass galaxies — and their winds — in incredible detail.

This is also just the start. The Pandora simulations are a stepping stone toward larger projects like Azahar (https://www.martin-alvarez.com/azahar), where we’ll explore how non-thermal physics shapes galaxies across cosmic time.

Where can I read more?

If you're curious to dive deeper, the full paper includes even more information: star formation histories, images showcasing the visual appearance of the galaxies, comparisons of ionized and neutral gas outflow properties vs observations, and the mass - metallicity relation).

Here’s the link again: https://ui.adsabs.harvard.edu/abs/2025arXiv250603245M/abstract

Feel also free to reach out with questions or comments!

Un saludo!