🔭 astrophysics

Tidal disruption event rates across cosmic time: forecasts for LSST, Roman, and JWST and their constraints on the supermassive black hole mass function

This paper presents a semi-empirical model forecasting how Tidal Disruption Event rates evolve with redshift under various supermassive black hole mass functions and galaxy-scale processes, demonstrating that upcoming surveys like LSST, Roman, and JWST can effectively constrain the cosmic evolution of low-mass black holes.

Original authors: Mitchell Karmen, Suvi Gezari, Colin Norman, Muryel Guolo

Published 2026-07-16

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Mitchell Karmen, Suvi Gezari, Colin Norman, Muryel Guolo

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 Black Hole Detective Story

Imagine the universe as a giant, bustling city where the most powerful residents are supermassive black holes. These aren't the tiny black holes made from collapsed stars; these are the giants, millions or billions of times heavier than our Sun, sitting in the center of almost every galaxy. For a long time, astronomers have been great at counting the biggest, most famous black holes, but the "middle-class" ones—the smaller, lighter giants—are a bit of a mystery, especially the ones from the early days of the universe. We know they exist, but we don't know exactly how many there are or how they grew up.

To find out, scientists look for a specific type of cosmic drama called a Tidal Disruption Event, or TDE. Think of a TDE as a celestial mugging. If a star wanders too close to a black hole, the black hole's gravity is so strong that it stretches the star like a piece of taffy until it snaps apart. This violent shredding creates a massive, bright flash of light that we can see from Earth. By counting how often these "muggings" happen, astronomers can figure out how many black holes are lurking in the dark. But here's the tricky part: the universe has changed a lot over time. Galaxies have merged, stars have packed tighter, and dust has shifted. To understand the black hole population, we have to untangle all these moving parts to see what's really going on.

The Paper's Big Picture

In this paper, a team of astronomers built a sophisticated "cosmic crystal ball" to predict how often these star-shredding events will happen as we look back in time. They wanted to answer a simple but huge question: How does the number of supermassive black holes change as the universe gets younger? To do this, they didn't just look at the black holes; they built a semi-empirical model, which is a fancy way of saying they combined real observations from today with smart guesses about how the universe has changed.

They started with the local rate of TDEs that we already know from current surveys (like the Zwicky Transient Facility) and then applied a series of "filters" to see how the rate would change at different points in cosmic history. They considered four main factors that act like volume knobs on the TDE rate:

The Black Hole Population: They tested two different theories about how the number of black holes changes over time. One theory suggests the number of smaller black holes drops off quickly as we go back in time, while another suggests they are more common.
Galaxy Mergers: Galaxies crash into each other, and these crashes can stir up the stars, making them more likely to get eaten. This effect is strongest around "cosmic noon" (about 10 billion years ago, or redshift $z \sim 2$ ).
Stellar Density: In the early universe, galaxies were smaller and denser. This means stars were packed tighter near the black holes, increasing the chances of a TDE.
Dust: As we look further back, there is more dust in the universe, which can hide the bright flashes of TDEs from our view.

What They Found

The authors found that the story of TDEs isn't just a straight line going up or down. Instead, it's a rollercoaster. When they combined all these factors, they predicted that the rate of TDEs actually increases as we look back in time, reaching a peak around "cosmic noon" ( $z \sim 1.2$ to $z \sim 2$ ). This happens because the boost from denser galaxies and more frequent mergers temporarily outweighs the fact that there are fewer black holes back then. However, if you look even further back, beyond that peak, the rate starts to drop again. Why? Because at those extreme distances, there simply aren't enough black holes massive enough to disrupt a star but small enough to let the light escape. The black holes are either too rare or too massive to be seen.

Crucially, the paper shows that the exact shape of this rollercoaster depends heavily on which theory of black hole growth you believe. If you use one model (the "Shankar" model), the peak is lower and happens earlier. If you use the other (the "Illustris" simulation), the peak is higher and happens later. The difference between these two models is huge: by redshift $z \sim 3$ , the predicted number of TDEs can differ by a factor of 10. This means that by simply counting TDEs in the future, we can finally figure out which black hole growth theory is correct.

The Future Telescope Forecast

The paper then acts as a fortune teller for three upcoming super-telescopes, predicting exactly how many TDEs they will catch:

LSST (The Vera C. Rubin Observatory): This is the "wide net" fisherman. It will scan a massive area of the sky and is expected to find thousands to tens of thousands of TDEs every year. Because it sees so many, it will be the best tool for pinning down the overall shape of the black hole population, especially for the smaller, lower-mass black holes that are hard to see otherwise.
Roman Space Telescope: This is the "high-definition" camera. It will see fewer TDEs (maybe a few dozen to a hundred a year), but they will be cleaner and clearer. It can see further back in time (up to $z \sim 2.75$ ) and will be able to see the host galaxies in high resolution, helping scientists understand if the TDEs are happening in merging galaxies or dense star clusters.
JWST (James Webb Space Telescope): This is the "deep dive" explorer. It won't find many TDEs by accident in a single snapshot because its viewing area is so small. However, it is the only telescope powerful enough to see TDEs at the very edge of the universe ( $z > 3$ ). The authors suggest that if JWST watches the same patch of sky repeatedly over time, it might finally catch a glimpse of a TDE from the very first generation of black holes.

The Bottom Line

The paper concludes that TDEs are the perfect tool to solve the mystery of the "missing" low-mass black holes. While current telescopes can only see a tiny slice of the universe, the upcoming surveys will give us a massive, statistical sample. By comparing the number of TDEs we find at different distances, we won't just be counting stars being eaten; we will be mapping the growth history of the supermassive black holes that rule our universe. The authors are confident that these upcoming surveys will provide the first direct constraints on how these cosmic giants formed and evolved, turning a theoretical guess into a measured fact.

Technical Summary: Tidal Disruption Event Rates Across Cosmic Time

Problem Statement
Measuring the mass distribution of supermassive black holes (SMBHs) over cosmic time remains a significant challenge, particularly for the low-mass population ( $M_\bullet \lesssim 10^8 M_\odot$ ) at redshifts $z > 1$ . This mass range is critical for understanding SMBH seeding and early growth models but is poorly constrained by traditional quasar surveys, which are biased toward high-mass, high-luminosity objects. While Tidal Disruption Events (TDEs) offer a unique probe for these lower-mass SMBHs, current samples are limited to very low redshifts ( $z \lesssim 0.5$ ). Consequently, it is currently impossible to distinguish between different redshift-evolution models of the SMBH mass function (BHMF). As wide-field and deep surveys like the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST), the Roman Space Telescope, and JWST come online, there is an urgent need for robust theoretical forecasts to interpret the upcoming influx of high-redshift TDE data.

Methodology
The authors construct a semi-empirical model to predict the redshift-dependent volumetric TDE rate ( $\dot{N}_{TDE}$ ). Rather than deriving rates from first principles of stellar dynamics, the model normalizes to the local, empirically measured TDE rate from the Zwicky Transient Facility (ZTF; Yao et al. 2023) and applies redshift-dependent correction factors to account for cosmic evolution. The total rate is calculated as:

$\dot{N}_{TDE} = \int_0^{z_{Ly}} \epsilon(z) F(z) N_{BH}(z) \dot{N}_0(z, \lambda) O(z) dz$

Where:

$\dot{N}_0(z, \lambda)$ is the local empirical rate derived from the ZTF g-band luminosity function, scaled by survey area and sensitivity.
$\epsilon(z)$ accounts for survey efficiency and time-dilation effects.
$N_{BH}(z)$ represents the evolution of the SMBH mass function, specifically the volume density of SMBHs capable of disrupting main-sequence stars ( $10^5 M_\odot \lesssim M_\bullet \lesssim 10^8 M_\odot$ ). The authors test two contrasting models: a semi-empirical model calibrated to AGN (Shankar et al. 2009) and the Illustris cosmological simulation.
$F(z)$ $F (z)$ encapsulates galaxy-scale processes enhancing the rate, defined as $F(z) = M(z)I(z)D(z)$ $F (z) = M (z) I (z) D (z)$ :
- $M(z)$ (Mergers): The evolving major merger rate, which enhances TDE rates in post-merger galaxies.
- $D(z)$ (Nuclear Density): The evolution of central stellar densities, parameterized based on observed compactness of high-redshift galaxies.
- $I(z)$ (IMF): The evolution of the Initial Mass Function, testing the hypothesis of a top-heavy IMF at early cosmic times.
$O(z)$ accounts for dust obscuration, modeled as an evolving fraction based on AGN obscuration trends.

Uncertainties are propagated using Monte Carlo sampling across all parameters.

Key Contributions and Results

Volumetric Rate Evolution: The authors forecast that the volumetric TDE rate does not simply decline with redshift. Instead, galaxy-scale enhancements (increasing nuclear densities and merger rates) drive the rate up until "cosmic noon" ( $z \sim 1-2$ ), after which the rate declines as the population of low-mass SMBHs capable of disrupting stars becomes scarce. The turnover point depends on the specific BHMF model used ( $z \sim 1.2$ for Shankar et al. 2009; $z \sim 2$ for Illustris).
Survey Forecasts:
- LSST: Predicted to detect thousands to tens of thousands of TDEs per year in a flux-limited sample. While the absolute number is high, the sample is sensitive to the low-mass end of the BHMF. The authors estimate that LSST can constrain the redshift evolution slope ( $\gamma$ ) of the BHMF to within $\pm 0.6$ using total yield and median redshift alone, and to $\pm 0.06$ when combining yield with median redshift.
- Roman HLTDS: Expected to detect $\sim 30-100$ TDEs per year. Despite lower numbers, Roman's high-resolution imaging and NIR filters allow for a cleaner sample with better host-galaxy characterization and extended redshift reach ( $z \sim 2.75$ ), making it a powerful discriminator between BHMF models via statistical tests (e.g., Kolmogorov-Smirnov).
- JWST (COSMOS-Web): Serendipitous single-epoch detections are predicted to be rare ( $< 2$ events), with a median redshift of $z \sim 1.2$ . However, multi-epoch monitoring of deep fields is identified as the primary path to discovering the highest-redshift TDEs ( $z > 3$ ), which would directly probe SMBH seeds.
Distinguishability: The current ZTF sample is insufficient to distinguish between BHMF models (differing by only $\sim 5\%$ in rate). However, the models diverge significantly at higher redshifts, where the differences in predicted TDE yields become substantial (up to an order of magnitude by $z \sim 3$ ).

Significance
The paper establishes that TDEs serve as a critical, complementary probe to AGN studies for constraining the low-mass end of the SMBH mass function at high redshifts. By integrating galaxy evolution effects (mergers, density, dust) with BHMF evolution, the authors provide a framework to interpret the upcoming flood of TDE data. The work demonstrates that upcoming surveys, particularly LSST and Roman, will not only increase the statistical sample size but will also enable the first population-level constraints on the origin and growth of supermassive black holes in the early universe. The authors emphasize that while individual high-redshift detections by JWST will be rare, they offer unique constraints on the extreme tail of the redshift distribution and the nature of early SMBH seeding.

The Cosmic Black Hole Detective Story

The Paper's Big Picture

What They Found

The Future Telescope Forecast

The Bottom Line

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