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Impact of subhalo dynamical friction heating on the formation of the first structures in the universe

This paper utilizes TNG50 simulation data to demonstrate that dynamical friction heating from orbiting subhalos significantly suppresses gas cooling in early minihalos at high redshifts, thereby hindering Population III star formation and promoting the creation of direct-collapse black hole seeds.

Original authors: Zhenyu Wu, Sadegh Khochfar, Muhammad A. Latif, Ben Morton, Britton Smith

Published 2026-03-23

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CC BY 4.0

Original authors: Zhenyu Wu, Sadegh Khochfar, Muhammad A. Latif, Ben Morton, Britton Smith

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: A Cosmic Dance Floor

Imagine the early universe not as empty space, but as a giant, crowded dance floor. The "dancers" are massive clouds of invisible dark matter (the Host Halos). But these aren't empty clouds; they are packed with smaller, invisible dancers (the Subhalos) orbiting around the big ones.

As these smaller dancers spin around the big ones, they don't just glide smoothly. They bump into the "air" of the dance floor (the gas). This friction creates heat. This paper asks a simple question: Is this friction hot enough to stop the gas from cooling down and forming the very first stars?

The Main Characters

The Host Halo (The Big Boss): A massive sphere of dark matter that holds everything together.
The Subhalos (The Orbiting Minions): Smaller clumps of dark matter orbiting inside the Big Boss.
The Gas (The Crowd): The actual matter that can turn into stars. It needs to get cold to collapse and become a star.
Dynamical Friction (The Drag): As the Minions orbit, they drag their feet through the Crowd. This slows the Minions down and heats up the Crowd.

The Problem: Cooling vs. Heating

To form a star, gas needs to cool down. Think of it like a hot cup of coffee; it needs to lose heat to become drinkable (or in this case, to collapse into a star).

Cooling: The gas tries to radiate heat away, mostly using a special molecule called Molecular Hydrogen ( $H_2$ ).
Heating: The orbiting Minions (Subhalos) drag through the gas, creating friction that heats it up.

If the heating is stronger than the cooling, the gas stays hot and puffy. It can't collapse. No collapse = No stars.

The Discovery: Two Different Worlds

The authors found that this friction heating acts differently depending on when in the universe's history you look.

1. The Modern Universe (Low Redshift, $z \approx 0$ )

The Scene: Giant galaxy clusters, like massive cities.
The Result: Here, the friction is very strong. The big bosses have so many orbiting minions that the friction heats the gas enough to stop new stars from forming.
The Analogy: Imagine a giant stadium full of people (gas). If thousands of smaller groups (subhalos) run around the track, their movement stirs up the air so much that the stadium gets too hot for anyone to settle down. This explains why massive galaxies today have stopped making new stars (a process called "quenching").

2. The Early Universe (High Redshift, $z \approx 15$ )

The Scene: Tiny, primitive "minihalos" just after the Big Bang. This is when the very first stars (Population III) were trying to form.
The Result: This is the paper's big surprise. Even in these tiny, early clouds, the friction from the few orbiting subhalos is strong enough to compete with cooling.
The Analogy: Imagine a small campfire (the gas) trying to stay lit. Usually, the wind (cooling) blows the heat away, and the fire dies. But if you have a few people running around the fire (subhalos), their movement stirs up the air just enough to keep the fire hot.
The Consequence: This extra heat makes it much harder for the first stars to form. It raises the "bar" for how much molecular hydrogen is needed to start a star.

The Plot Twist: Direct Collapse Black Holes (DCBHs)

If the friction prevents the gas from cooling enough to form a normal star, what happens?

The Scenario: The gas gets hot enough to avoid fragmenting into many small stars. Instead, it stays as one giant, hot cloud.
The Outcome: This giant cloud eventually collapses all at once into a Direct Collapse Black Hole (DCBH).
Why it matters: We see supermassive black holes in the early universe that are too big to have grown from normal stars. They need a "head start." This paper suggests that subhalo friction is a natural way to give that head start. It acts like a "star suppression field," forcing the gas to skip the "star" phase and go straight to "black hole."

The "Resolution" Problem (The Pixel Count)

The authors also point out a technical issue for scientists running computer simulations of the universe.

The Issue: To see this friction heating, you need to be able to "see" the small subhalos. If your simulation is too blurry (low resolution), it misses the small orbiting minions.
The Rule: You need to be able to resolve subhalos that are at least 5% the mass of the main host. If you can't see them, you miss the heating effect, and your simulation will get the wrong answer about how stars and black holes form.

Summary in a Nutshell

Orbiting clumps of dark matter create friction as they move through gas.
This friction heats the gas, acting like a cosmic heater.
In the early universe, this heater is strong enough to stop the gas from cooling down enough to form the first normal stars.
Instead of stars, this heat helps the gas collapse directly into giant black holes, explaining how the universe got its supermassive black holes so quickly.
Computer simulations need to be high-resolution enough to see these small orbiting clumps, or they will miss this crucial heating mechanism.

The Takeaway: The universe isn't just a quiet place where gas cools and stars form. It's a chaotic dance floor where the movement of invisible dark matter clumps keeps the gas hot, changing the recipe for how the first stars and black holes are born.

1. Problem Statement

The thermal history of baryons within dark matter halos is a primary regulator of star formation. While feedback mechanisms like supernovae, AGN, and photo-ionization are well-studied, the role of gravitational heating via dynamical friction (DF) from orbiting subhalos remains less explored, particularly at high redshifts ( $z > 6$ ).

Low Redshift: It is established that DF converts orbital kinetic energy into thermal energy, potentially quenching star formation in massive halos ( $M > 10^{12} M_\odot$ ).
High Redshift Gap: The impact of DF on the formation of the first stars (Population III) and Direct Collapse Black Hole (DCBH) seeds in metal-free minihalos ( $M \sim 10^6 - 10^8 M_\odot$ ) is poorly constrained. Specifically, it is unclear if DF heating can compete with molecular hydrogen ( $H_2$ ) cooling, which is the primary coolant in these early, metal-free environments.

2. Methodology

The authors developed an analytic model to quantify DF heating rates and validated it against cosmological simulation data.

Data Source: The study utilizes the TNG50-12 hydrodynamical simulation (IllustrisTNG project) to derive the Subhalo Mass Function (SHMF) across redshifts $z=0$ to $z=15$ .
Analytic Framework:
- DF Force: Based on Ostriker (1999), calculating the drag force on a subhalo moving through a gaseous medium.
- Heating Rate: Derived by integrating the DF power ( $-\mathbf{F}_{DF} \cdot \mathbf{v}$ ) over the subhalo population.
- SHMF Modeling: The authors fitted the TNG50 subhalo data to a universal fitting formula (power law + exponential decay) to determine the evolution of SHMF parameters ( $A, \alpha, \beta, \omega$ ) as a function of redshift.
- Scatter Analysis: Recognizing that the number of subhalos varies significantly between halos, the authors performed Monte Carlo sampling (500 realizations) assuming Poissonian fluctuations to quantify the scatter in heating rates.
- Corrections: The model incorporates corrections for:
  - Non-linearity: Using Kim & Kim (2009) to adjust for supersonic perturbations where the drag force is reduced due to detached bow shocks.
  - Mach Number: Calculating the average correction factor $\langle I/M \rangle$ based on the velocity distribution of subhalos in TNG50.
Cooling Comparison: Cooling rates were calculated using the Grackle chemistry and cooling library, considering metal-free conditions ( $Z=0$ ) and varying $H_2$ fractions.

3. Key Contributions

Redshift-Dependent SHMF: Provided a new, redshift-evolved analytic fit for the SHMF from $z=15$ to $z=0$ , showing that high-redshift subhalo populations are closer to "unevolved" distributions (less tidal stripping) compared to $z=0$ .
Quantification of Scatter: Demonstrated that the DF heating rate for a fixed host halo mass can vary by two orders of magnitude due to the intrinsic scatter in the number of subhalos (Poissonian fluctuations).
Resolution Criteria: Established that to capture DF heating accurately in simulations, subhalos with a mass ratio $\psi = m_{sub}/M_{host} \gtrsim 0.05$ must be resolved.
High-Redshift Impact: First systematic evaluation of DF heating against $H_2$ cooling in minihalos, showing it can significantly alter the conditions for Pop III and DCBH formation.

4. Key Results

A. Low Redshift ( $z < 6$ ) Massive Halos

Quenching Mechanism: In massive halos ( $M \gtrsim 10^{13} M_\odot$ ), the mean DF heating rate exceeds the cooling rate, consistent with previous findings that gravitational heating contributes to galaxy quenching.
Scatter Importance: While the mean heating is significant, the large scatter implies that in halos with fewer subhalos, DF heating alone may be insufficient to quench star formation.
Comparison to DB07: The authors' mean heating rates are 1–2 orders of magnitude lower than the upper limits estimated by Dekel & Birnboim (2007), likely because the latter ignored tidal stripping of subhalos.

B. High Redshift ( $z \sim 15$ ) Minihalos

Competition with Cooling: In metal-free minihalos ( $10^6 - 10^8 M_\odot$ ), DF heating rates can be comparable to molecular hydrogen cooling rates when the $H_2$ fraction is low ( $f_{H_2} \sim 10^{-5}$ ).
Suppression of Pop III Stars: DF heating increases the critical $H_2$ fraction ( $f_{crit}$ ) required for gas collapse and Pop III star formation by up to one order of magnitude. This makes Pop III formation more difficult, as standard cosmological $H_2$ fractions often fall below this new, higher threshold.
DCBH Formation:
- DF heating favors the formation of Direct Collapse Black Hole (DCBH) seeds.
- Even in the presence of a Lyman-Werner (LW) background that suppresses $H_2$ to $f_{H_2} \sim 10^{-5}$ , DF heating can prevent the remaining gas from cooling and fragmenting into stars.
- This allows the gas to remain in an atomic cooling regime ( $T \sim 8000$ K) and collapse isothermally into a massive seed, even without extreme LW radiation bursts.

5. Significance and Implications

DCBH Seeding: The study suggests that DF heating is a continuous, naturally occurring mechanism in the $\Lambda$ CDM framework that aids DCBH formation. Unlike scenarios requiring rare, intense bursts of LW radiation or specific merger histories, DF heating operates whenever subhalos orbit within a host.
Simulation Requirements: The findings impose strict resolution requirements on cosmological simulations. To correctly model the thermal history of the early universe and the formation of the first black holes, simulations must resolve subhalos down to $\psi \gtrsim 0.05$ . Current simulations lacking this resolution may underestimate heating and overestimate Pop III formation.
Semi-Analytic Models: The authors argue that semi-analytic models for black hole seeding must include DF heating as a standard component, as it significantly alters the critical conditions for the first structures.

Conclusion

Wu et al. demonstrate that dynamical friction from subhalos is a non-negligible heating mechanism in the early universe. By converting gravitational potential energy into thermal energy, DF heating can suppress molecular cooling in minihalos, raising the barrier for Population III star formation and creating favorable conditions for the formation of massive Direct Collapse Black Hole seeds. This mechanism acts continuously alongside other feedback processes, necessitating its inclusion in models of early structure formation.