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Dark-Matter-Deficient Galaxies from Collisions: A New Probe of Bursty Feedback and Dark Matter Physics

Hydrodynamical simulations demonstrate that high-velocity collisions between gas-rich ultra-diffuse galaxies can produce dark-matter-deficient galaxies, with bursty feedback significantly enhancing their mass by reducing baryonic binding energy, a mechanism that yields distinct observational signatures compared to self-interacting dark matter models.

Original authors: Yi-Ying Wang, Daneng Yang, Keyu Lu, Yue-Lin Sming Tsai, Yi-Zhong Fan

Published 2026-01-30

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CC BY 4.0

Original authors: Yi-Ying Wang, Daneng Yang, Keyu Lu, Yue-Lin Sming Tsai, Yi-Zhong Fan

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Finding Galaxies Without Dark Matter

Imagine the universe is a giant neighborhood where almost every house (galaxy) is built on a massive, invisible foundation called Dark Matter. Usually, this foundation is huge—about 100 times heavier than the house itself.

But astronomers have found some strange "houses" (galaxies) that seem to be missing their foundations entirely. These are called Dark-Matter-Deficient Galaxies (DMDGs). The big question is: How do you knock a house off its foundation without destroying the house?

This paper suggests a new answer: High-speed crashes.

The Main Idea: The "Jelly vs. Rock" Collision

The researchers used supercomputer simulations to crash two gas-rich "ultra-diffuse" galaxies (which are like fluffy, spread-out clouds of stars and gas) into each other at very high speeds.

They discovered that the outcome depends entirely on how "loosely packed" the galaxy is before the crash. They call this baryonic binding energy. Think of it this way:

Tightly Packed (High Binding): The galaxy is like a rock. If you smash two rocks together, they might chip, but they stay mostly solid.
Loosely Packed (Low Binding): The galaxy is like a bowl of jelly. If you smash two bowls of jelly together, the jelly splashes everywhere.

The Secret Ingredient: "Bursty" Feedback

The paper argues that some galaxies become "loose jelly" because of bursty stellar feedback.

The Analogy: Imagine a star forming inside a galaxy. When it explodes as a supernova, it acts like a giant blowtorch. If these explosions happen in rapid, chaotic bursts, they shake the galaxy's gravity.
The Result: This shaking heats up the dark matter and gas, making the galaxy's "gravity well" shallower. The galaxy becomes less tightly bound.
The Crash: When this "loose jelly" galaxy collides with another, the gas and stars get flung out easily, leaving behind a new, small galaxy that has almost no dark matter left.

The Comparison: Feedback vs. Self-Interacting Dark Matter (SIDM)

Scientists have two main theories for why galaxies might have "cores" (loose centers) instead of dense centers:

Bursty Feedback (The Blowtorch): As described above, stars shake the galaxy loose.
SIDM (The Bouncy Ball): This theory suggests dark matter particles bounce off each other like bouncy balls, creating a soft core.

The Paper's Discovery:
The researchers found that these two theories produce very different results when galaxies crash:

The Blowtorch (Feedback): Creates a "loose" galaxy. When it crashes, it produces fewer, but much heavier dark-matter-deficient galaxies. It's like splashing a big bowl of jelly; you get big puddles.
The Bouncy Ball (SIDM): Creates a "soft" core, but the galaxy stays heavy and tight. When it crashes, it produces a mix of heavy and light galaxies, but nothing as massive as the "jelly" scenario.

Why this matters: If we find a lot of massive, dark-matter-free galaxies in the empty spaces between galaxy clusters, it points to the "Bursty Feedback" theory. If we find them only near massive galaxies, it might point to the "Bouncy Ball" theory.

The "Gas" Clue

The simulations also found a second clue to tell these theories apart: Gas.

In the "Bursty Feedback" crash scenario, the gas gets used up quickly to make new stars. So, the resulting dark-matter-free galaxies are gas-poor.
This gives astronomers a way to test the theory: Look for these galaxies. If they have very little gas left, it supports the "bursty feedback" idea.

The Conclusion

The paper concludes that high-speed collisions are a promising way to create galaxies without dark matter. However, the type of galaxy created depends on the internal physics of the original galaxies.

If the original galaxy was shaken loose by star explosions, the crash creates big, heavy, gas-poor dark-matter-free galaxies.
If the original galaxy was soft because of bouncy dark matter, the crash creates a different, less dramatic mix.

Future telescopes (like the ones mentioned in the paper: CSST, LSST, FAST) will look for these specific types of galaxies to see which "recipe" the universe is actually using.

Technical Summary: Dark-Matter-Deficient Galaxies from Collisions

Problem Statement
The observation of Dark-Matter-Deficient Galaxies (DMDGs), such as NGC 1052–DF2 and DF4, challenges the standard $\Lambda$ CDM framework, which predicts that dwarf galaxies should reside in massive dark matter halos with baryon-to-dark-matter ratios significantly lower than unity. While proposed formation channels include tidal stripping in cored halos and high-velocity collisions, the specific physical mechanisms controlling the yield and mass of DMDGs remain unclear. Existing simulations often lack the resolution to resolve the hydrodynamics of gas-rich ultra-diffuse galaxy (UDG) collisions or the large volumes necessary to find such rare events. Furthermore, distinguishing between core formation driven by baryonic feedback and that driven by Self-Interacting Dark Matter (SIDM) remains a significant theoretical challenge.

Methodology
The authors employ controlled hydrodynamical simulations using the Gadget-4 code to investigate the formation of DMDGs via high-velocity collisions between gas-rich UDGs.

Initial Conditions: The study constructs paired simulations with identical total halo masses ( $1-5 \times 10^{10} M_\odot$ $1 - 5 \times 1 0^{10} M_{⊙}$ ) and concentrations but varying inner density profiles. Three scenarios are tested:
1. Cuspy ( $\gamma=1$ ): Standard NFW profiles.
2. Weakened Binding ( $\gamma=0.1$ ): Profiles with a reduced inner slope, simulating the effect of energy injection (e.g., bursty feedback) that flattens the halo and reduces the absolute baryonic binding energy ( $|E_{bind}|$ ).
3. SIDM: Simulations using a cross-section $\sigma/m = 20 \text{ cm}^2/\text{g}$ (for collision runs) or $5 \text{ cm}^2/\text{g}$ (for tidal stripping runs) to generate cores via dark matter self-interactions.
Collision Setup: Progenitors are initialized as gas-only disks (Toomre $Q > 1.5$ ) within dark matter halos. They collide with relative velocities ( $v_r$ ) ranging from 280 to 500 km/s. The study analyzes 15 paired simulation runs.
Tidal Stripping Analysis: A separate set of simulations places progenitors on orbits within a massive host halo ( $M_{200} = 1.1 \times 10^{13} M_\odot$ ) to compare DMDG formation via tidal stripping in the three scenarios.
Diagnostics: The authors quantify the tidal tensor eigenvalues ( $\lambda_{max}$ ) to analyze the compressive vs. repulsive nature of tidal forces on gas debris and track the evolution of bound gas and stellar masses.

Key Contributions and Results

Binding Energy as a Control Parameter: The study identifies the progenitors' baryonic binding energy ( $|E_{bind}|$ ) as the critical control parameter for DMDG formation. Repeated potential fluctuations (simulating bursty feedback) reduce $|E_{bind}|$ by approximately 15% (from $\gamma=1$ to $\gamma=0.1$ ).
Enhanced DMDG Mass in Weakened Binding: In 13 out of 15 paired collision runs, the weakened-binding case ( $\gamma=0.1$ ) produced DMDGs with higher masses than the cuspy case ( $\gamma=1$ ). About two-thirds of these runs exhibited mass enhancements exceeding 100%. The most massive DMDG formed had a stellar mass of $2.60 \times 10^9 M_\odot$ .
Mechanism of Enhancement: The reduction in binding energy creates a shallower central potential, leading to a more harmonic potential well. This suppresses stretching tidal forces and promotes compressive tides, allowing expelled gas to coalesce into fewer, more massive clumps rather than fragmenting into low-mass condensations.
SIDM Distinction: Unlike bursty feedback, SIDM produces cores of comparable size without significantly lowering $|E_{bind}|$ . In collision scenarios, SIDM results in DMDG masses that are either slightly higher or lower than the cuspy case with no clear enhancement trend. Crucially, in host halo environments, SIDM cores enhance dark matter tidal stripping while keeping baryons compact, whereas weakened binding facilitates the stripping of both components.
Observational Signatures: The simulations predict that DMDGs formed via the weakened-binding collision channel will have systematically lower gas fractions compared to their cuspy counterparts due to sustained post-collision star formation. This provides a clean observational signature.

Significance
The paper proposes a new observational window to distinguish between core formation mechanisms. The contrasting roles of SIDM and baryonic feedback in DMDG formation—specifically, the ability of weakened binding to boost DMDG mass in collisions versus the lack thereof in SIDM—suggests that the population statistics of DMDGs in the field (collision-formed) versus those in host halos (tidally stripped) can differentiate between these physical models. The authors argue that upcoming wide-field imaging (CSST, LSST), HI surveys (FAST), and kinematic follow-ups are crucial to test this scenario, as the systematic differences in gas fraction and mass distribution offer a means to probe the underlying physics of dark matter and galaxy formation.