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Rates of tidal disruption events from constrained cosmological simulations of the local Universe: population properties and implications for transient surveys

Using constrained cosmological simulations of the local Universe, this study estimates tidal disruption event rates to reveal that the total TDE budget is overwhelmingly dominated by cuspy satellite galaxies in extended cluster halos rather than central cores, with yields driven by black hole demographics and spatial concentration rather than total cluster mass.

Original authors: Julian S. Sommer, Ildar Khabibullin, Klaus Dolag, Luca Sala, Benjamin Seidel, Jenny G. Sorce, Alice Damiano

Published 2026-08-03

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CC BY 4.0

Original authors: Julian S. Sommer, Ildar Khabibullin, Klaus Dolag, Luca Sala, Benjamin Seidel, Jenny G. Sorce, Alice Damiano

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 Cosmic Dance of Stars and Monsters

Imagine the universe as a vast, bustling city where the most powerful residents are Supermassive Black Holes (SMBHs). These aren't just dark holes; they are cosmic monsters sitting in the centers of almost every galaxy, with gravity so strong that not even light can escape once it gets too close. Usually, these monsters are quiet, sleeping giants that don't eat much because there's nothing nearby to snack on. But sometimes, a star wanders too close. If it gets within a certain "danger zone," the monster's gravity pulls harder on the side of the star facing it than on the far side. This difference in force stretches the star like a piece of taffy until it rips apart. This spectacular event is called a Tidal Disruption Event (TDE). It's like a cosmic fireworks show, flaring up with bright light for months or years, giving astronomers a rare chance to see these otherwise invisible monsters.

For a long time, scientists tried to guess how often these cosmic snacks happen using simple math on flat, two-dimensional maps. They assumed all galaxies looked the same and that stars moved in predictable, calm circles. But the universe is messy, three-dimensional, and full of surprises. With new telescopes ready to scan the sky and powerful supercomputers simulating the entire local universe, we can now ask a better question: How often do these events actually happen in the real, complex neighborhoods of our cosmic city, and does the type of neighborhood matter?

The Paper's Story: Simulating the Cosmic Snack Bar

In this study, a team of astronomers used a massive, high-resolution simulation called "SLOW" (Simulation of the LOcal Web) to recreate the local universe, focusing on six specific cosmic neighborhoods: five superclusters (like the Coma, Hercules, Shapley, Virgo, and Perseus) and one isolated galaxy cluster (Fornax). Instead of guessing, they built a digital universe where they could watch how black holes and stars interact over billions of years. Their goal was to count how many TDEs should happen in these regions and see if the old, simple math matched the complex reality.

The researchers didn't just count black holes; they looked closely at the "kitchen" around each one. They checked if the stars around the black hole were packed tightly in a sharp spike (a "cusp") or spread out in a flat, fluffy cloud (a "core"). They also checked if the black hole was spinning fast or slow, because a spinning monster has a different "eating radius" than a stationary one. They applied strict filters to make sure they were only counting black holes that were sitting still in stable galactic centers, ignoring those that were being tossed around by violent mergers or stripped of their stars.

What they found:
The team calculated that across these six environments, the average rate of TDEs is about 600 events per cubic gigaparsec per year (Gpc⁻³ yr⁻¹). When they looked at it per individual black hole, the rate is roughly 4.5 × 10⁻⁵ per year. This means that for any single black hole, there is about a 1 in 22,000 chance of eating a star in any given year.

This number is actually quite close to what we see in real observations, which is great news. However, the where and why are very different from what old theories predicted. The paper explicitly rules out the idea that the center of a galaxy cluster is the main place where these events happen. In the simulations, the dense cores of massive clusters are actually "starved" of TDEs. Why? Because the black holes there are often too massive (too big to rip a star apart without swallowing it whole) and the stars have been cleared out by the chaotic history of the cluster.

Instead, the paper suggests that the real "snack bars" are the smaller, satellite galaxies floating in the outer edges of these clusters. These outer galaxies often have "cuspy" centers (tightly packed stars) and lighter black holes that are just the right size to rip stars apart efficiently. The study found that the total number of TDEs is driven more by how many of these small, efficient galaxies exist and how they are spread out, rather than just the total mass of the giant cluster in the middle.

The "Hercules" and "Fornax" Twist:
The paper highlights two interesting examples. The Hercules supercluster is a massive, chaotic place that is still actively building itself. Because it's so busy with mergers, its black holes have grown too big and are spinning in ways that make them less efficient at creating TDEs. Even though it's huge, it produces fewer TDEs per black hole than expected. On the other hand, the Fornax cluster is smaller and quieter. It has a higher fraction of small, unmerged black holes that haven't grown too big yet. This makes Fornax surprisingly efficient at producing TDEs per black hole, even though it's a smaller place.

The Bottom Line:
The authors conclude that the old way of estimating TDE rates—assuming a simple, uniform distribution of stars—overestimates what happens in the crowded centers of clusters and underestimates the importance of the outer edges. The true budget of these cosmic events is dominated by the "suburbs" of the universe: the cuspy, low-mass satellite galaxies where the black holes are just the right size to throw a stellar party. While the simulations suggest a theoretical upper limit of 4.5 × 10⁻⁵ yr⁻¹ per black hole, this aligns well with what telescopes are starting to see, giving us a clearer picture of where to look for the next great cosmic flare.

Technical Summary: Rates of Tidal Disruption Events from Constrained Cosmological Simulations of the Local Universe

Problem Statement
Tidal disruption events (TDEs) serve as critical probes for studying quiescent supermassive black hole (SMBH) populations. However, a significant discrepancy exists between theoretical predictions and observational measurements. Idealized two-body relaxation models often predict rates of $\gtrsim 10^{-4} \text{ yr}^{-1} \text{ gal}^{-1}$ , whereas observational constraints from optical, X-ray, and mid-infrared surveys typically yield rates an order of magnitude lower ( $\sim 10^{-5} \text{ yr}^{-1} \text{ gal}^{-1}$ ). Furthermore, traditional analytical approaches often rely on 2D extrapolations and simplified assumptions regarding galactic density profiles (e.g., singular isothermal spheres), failing to account for the complex interplay of cosmological structure formation, merger histories, and the specific morphological diversity (cuspy vs. cored) of host galaxies. As the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) approaches, refined, spatially resolved estimates of TDE rates are required to interpret upcoming transient data accurately.

Methodology
The authors utilize the "Simulation of the LOcal Web" (SLOW), a state-of-the-art constrained cosmological simulation of the local Universe, specifically focusing on six high-resolution "zoom-in" regions: the Coma, Hercules, Shapley, Virgo, and Perseus superclusters, and the Fornax galaxy cluster. These regions span radial extents of 55–92 Mpc, covering environments from group scales to massive superclusters ( $M_{500} \approx 3.5 \times 10^{13} - 1.7 \times 10^{15} M_\odot$ ).

The methodology proceeds through several rigorous filtering and calculation steps:

Black Hole Extraction: SMBHs are extracted from the simulation volumes, with their masses ( $M_\bullet$ ) and dimensionless spin parameters ( $\tilde{a}$ ) recorded.
Main-Sequence Filtering: To ensure the validity of loss-cone dynamics calculations, a strict filter scheme isolates dynamically stable "main-sequence" black holes. This excludes unbound, rotation-dominated, or tidally stripped systems via four criteria:
- Offset Cut: BHs must be within 1.5 kpc of their subhalo center.
- Velocity Cut: Relative velocity must be lower than the local stellar velocity dispersion.
- Fractional Anisotropy (FA) Cut: The local stellar environment must exhibit $FA < 0.4$, ensuring an approximately isotropic, pressure-supported sphere required for standard loss-cone formalisms.
- Dark Matter Ratio Cut: A dark matter to stellar mass ratio $> 1$ is required to exclude bare, stripped nuclei.
Morphological Classification: Host galaxies are classified as "cuspy" ( $g \ge 2$ ) or "cored" ( $g \le 1$ ) based on the 3D stellar density slope measured within 1 kpc. This radius is chosen to avoid numerical softening artifacts while serving as a proxy for unresolved nuclear structure.
Relativistic Efficiency Correction: The study applies a relativistic efficiency factor ( $\eta$ ) derived from Kesden (2012). This factor quantifies the fraction of the loss cone that results in an observable TDE rather than a direct capture, accounting for the black hole's spin and the Hills mass limit.
Rate Calculation: Volumetric TDE rates ( $\Gamma_{vol}$ ) are calculated using the Stone & Metzger (2016) framework, which applies distinct baseline rates for cusp and core environments, weighted by the relativistic efficiency $\eta$ .

Key Contributions and Results

Volumetric and Per-BH Rates: The study derives an average volumetric TDE rate of $\approx 600 \text{ Gpc}^{-3} \text{ yr}^{-1}$ across all six environments. The average TDE rate per black hole is found to be stable at $\approx 4.5 \times 10^{-5} \text{ yr}^{-1}$ , a value consistent with current observational constraints but significantly lower than idealized theoretical predictions.
Spatial Distribution vs. Total Mass: The paper demonstrates that TDE yields are driven by black hole demographics and spatial concentration rather than total cluster mass. While the absolute TDE budget is overwhelmingly dominated by cuspy satellite galaxies in extended cluster halos, the volumetric rate ( $\Gamma_{vol}$ ) is heavily influenced by the number density of black holes ( $n_\bullet$ ).
Core Depletion and Efficiency: Central core rates are heavily reduced due to dynamical depletion (mergers destroying low-mass subhalos) and direct capture constraints (Hills limit). Consequently, the total TDE budget is not dominated by the massive central galaxies but by the population of lower-mass, cuspy satellites in the cluster outskirts.
Environmental Dependence:
- Hercules: Despite being a massive, actively assembling supercluster with a high black hole number density, it exhibits the lowest per-black-hole efficiency ( $\tilde{\Gamma}$ ) due to merger-driven black hole mass growth, which pushes many BHs toward the Hills mass limit where $\eta \to 0$ .
- Fornax: This low-mass environment is the most efficient per black hole ( $\tilde{\Gamma} \approx 1.07$ ) because it hosts a disproportionate fraction of low-mass, unmerged black holes that avoid the Hills cutoff, maintaining high loss-cone efficiencies. Crucially, Fornax exhibits the lowest black hole number density ( $n_\bullet$ ) and the largest half-number radius ( $R_{1/2,\bullet} = 31.26$ Mpc) among the six environments, indicating the most spatially dispersed distribution of black holes.
Discrepancy with 2D Extrapolations: The study highlights that traditional 2D analytical extrapolations overestimate central surface densities. The simulated 2D surface rate densities are approximately one order of magnitude lower than literature values derived from 2D projections, as the true TDE budget is distributed across the extended halo rather than concentrated in the core.

Significance
The paper claims to bridge the gap between theoretical predictions and observational measurements by utilizing a fully 3D cosmological framework that resolves local stellar environments. By moving beyond simplified 2D extrapolations and idealized density profiles, the authors provide a more realistic "TDE budget" that accounts for the complex assembly history of the local Universe.

The results suggest that the observed TDE rate of $\sim 3 \times 10^{-5} \text{ yr}^{-1} \text{ gal}^{-1}$ is robustly supported by cosmological simulations when accounting for dynamical core depletion and relativistic capture limits. The derived rate of $4.5 \times 10^{-5} \text{ yr}^{-1}$ is presented as a robust theoretical upper limit for upcoming transient surveys, acknowledging that the simulation's subgrid physics may slightly overproduce low-mass dwarf galaxies which dominate the absolute yield. The study emphasizes that TDE rates are not universal constants but are fundamentally dictated by the macroscopic properties of the cosmic web, specifically the spatial concentration of black holes and the merger history of their host environments.

The Cosmic Dance of Stars and Monsters

The Paper's Story: Simulating the Cosmic Snack Bar

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