Can Black Holes Weigh More Than 10 Billion Suns?

What happens when a cosmic giant becomes so massive that our best measuring tools simply... fail? Picture an object so dense, so unfathomably heavy, that it warps not just space and time—but the very equations we use to understand it.

Welcome to FreeAstroScience, where we break down the universe's most mind-bending discoveries into ideas you can grasp over your morning coffee. Today, we're exploring a frontier that just got a whole lot bigger: ultramassive black holes. These aren't your garden-variety cosmic vacuum cleaners. They're monsters weighing more than 10 billion times our Sun.

A groundbreaking study published in December 2025 has just doubled the number of known ultramassive black holes. And here's the kicker—the discovery forced scientists to rethink how we measure these behemoths in the first place.

If you've ever wondered how astronomers "weigh" something they can't even see, or why the biggest black holes break our favorite cosmic equations, you're in the right place. Stick with us to the end. This story involves detective work, failing formulas, and the strange relationship between black holes and the galaxies they call home.

What Exactly Is an Ultramassive Black Hole?

We throw around big numbers in astronomy. Millions. Billions. After a while, they blur together. So let's put this in perspective.

The black hole at the center of our Milky Way—called Sagittarius A*—weighs about 4 million solar masses. That's 4 million times the mass of our Sun packed into a region smaller than our solar system .

Now consider M87*, the black hole we photographed in 2019. It tips the scales at roughly 6 billion solar masses . That's 1,500 times heavier than our home black hole.

An ultramassive black hole (UMBH) crosses the 10 billion solar mass threshold. Before December 2025, scientists had confirmed just seven of them . That's it. Seven objects in the entire observable universe verified to be this heavy.

Why so few? Because finding them—and confirming their mass—is extraordinarily difficult.

How Do Scientists Weigh Something Invisible?

Here's the challenge: black holes don't emit light. We can't point a telescope at one and measure it directly. When a supermassive black hole is "active"—meaning it's gobbling up gas and shooting out jets—we can study its luminosity and jet behavior to estimate mass .

But when it's quiet? We need indirect methods.

The M-Sigma Relation: Astronomy's Workhorse

For decades, astronomers relied on something called the M-sigma (M-σ) relation. Here's how it works:

Stars near a galaxy's center orbit the black hole
Some stars move toward us (their light gets blue-shifted)
Others move away (their light gets red-shifted)
This creates a statistical spread in the spectrum called "sigma" (σ)
Bigger black holes = faster-orbiting stars = larger sigma

The relationship is elegant. Measure the stellar motions, plug them into an equation, and out pops a black hole mass .

This approach worked brilliantly for intermediate-mass black holes. Researchers like Saglia et al. (2016) refined it into a reliable prediction tool .

But there's a catch.

When the M-Sigma Relation Breaks Down

The M-sigma relation assumes a certain relationship between a galaxy's central black hole and its surrounding stars. For most galaxies, this relationship holds tight.

Not for the biggest ones.

**Brightest Cluster Galaxies (BCGs)**—the massive ellipticals sitting at the centers of galaxy clusters—play by different rules. These giants often show surprisingly low velocity dispersions despite hosting enormous black holes .

Think about it this way: if you used the M-sigma relation on many BCGs, you'd predict black holes under 1 billion solar masses. The actual measurements? Often 10 to 20 times higher.

Why does this happen?

The answer lies in how these galaxies formed. Massive elliptical galaxies grow through "dry mergers"—collisions with other galaxies that involve very little gas. Without gas, there's no new star formation. And without star-forming processes, the velocity dispersion doesn't increase the way it would in gas-rich collisions .

The black holes keep growing. The sigma? It stalls.

"Nearly all BCGs from our sample host overmassive BHs relative to the predictions of the canonical M-sigma relation, and the effect is particularly severe due to the low σ values of most BCGs." — de Nicola et al. (2025)

Eight New Giants: The 2025 Breakthrough

A team led by Stefano de Nicola at the Max Planck Institute for Extraterrestrial Physics didn't just identify the problem. They solved it—and made remarkable discoveries along the way .

The researchers examined 16 Brightest Cluster Galaxies without previous black hole measurements. Using a sophisticated approach called the triaxial Schwarzschild model, they simulated countless stellar orbits around each galaxy's core.

The triaxial part matters. Real galaxy cores aren't perfectly spherical. They're ellipsoids with three different axes. By accounting for this shape, the model produces much more accurate mass estimates .

The results were stunning:

8 new ultramassive black holes discovered
The known UMBH population doubled in a single study
Black hole masses ranged from 2.8 billion to 24.7 billion solar masses

This wasn't incremental progress. It was a leap.

The Core Size Secret

If the M-sigma relation fails for massive galaxies, what can we use instead?

The answer had been hiding in plain sight: core size.

Massive elliptical galaxies have a distinctive feature. Their surface brightness profiles flatten near the center, creating what astronomers call a "light deficit" or depleted core. This region appears dimmer than you'd expect if you simply extrapolated the outer brightness profile inward .

These cores exist because of black hole binary scouring. When two galaxies merge, their central black holes eventually spiral together. Before they combine, they orbit each other as a binary system. During this phase, they gravitationally eject nearby stars—like cosmic bouncers clearing a dance floor .

The result? A bigger black hole carves out a bigger core.

Why Core Size Works Better

The correlation between black hole mass and core size (M_BH-r_c) turns out to be remarkably tight—far tighter than M-sigma for these massive systems .

The researchers found that core size predicts black hole mass with much smaller scatter. It's a direct physical connection: the gravitational influence of the black hole literally shapes the core around it .

Even better? You don't need complex spectroscopy to measure it. Core size comes from photometry alone—just high-resolution images .

The Mathematics Behind the Mass

Let's look at the actual relationships the team discovered. Don't worry—we'll break down what each equation means.

The M_BH-Core Size Relation

log(M_BH/M_☉) = (0.916 ± 0.081) × log(r_c/kpc) + (10.087 ± 0.053)

Translation: For every factor of 10 increase in core size, black hole mass increases by about a factor of 8.2. The intrinsic scatter is just ε = 0.222—much tighter than the M-sigma relation.

Sphere of Influence vs. Core Size

log(r_SOI/kpc) = (0.960 ± 0.060) × log(r_c/kpc) + (0.028 ± 0.039)

Translation: The sphere of influence (the region where the black hole dominates gravitationally) is essentially identical to the core size. This is direct evidence that black holes shape their environments.

The sphere of influence is defined as the radius where the enclosed stellar mass equals the black hole mass: M_BH = M_∗(r ≤ r_SOI)

The fact that r_SOI ≈ r_c isn't a coincidence. It's physics in action .

Core Density Relation

log(ρ_core/M_☉kpc^-3) = (-2.08 ± 0.11) × log(r_c/kpc) + (9.313 ± 0.068)

Translation: Larger cores have lower densities, following a steep inverse relationship. This suggests progenitor galaxies had remarkably similar central density profiles.

This last equation tells us something profound about galaxy evolution. The progenitors of today's massive ellipticals apparently had similar central structures—a kind of "universal" density profile .

Meet the New Ultramassive Black Holes

Here's the full roster of BCGs studied, along with their key measurements. The highlighted rows are confirmed UMBHs (M_BH > 10 billion solar masses):

Galaxy	Black Hole Mass (10⁹ M_☉)	Core Size (kpc)	Velocity Dispersion (km/s)	UMBH?
A2256	24.7 ± 6.9	2.29	320	✅
A2107	22.4 ± 3.3	—	373	✅
A2147	16.2 ± 0.9	0.80	266	✅
A1775	15.1 ± 3.3	2.13	336	✅
A292	13.8 ± 3.9	1.25	311	✅
A160	13.2 ± 3.7	0.92	302	✅
A1185	11.2 ± 3.1	1.05	302	✅
A1749	10.5 ± 2.9	0.84	287	✅
A2388	9.6 ± 1.0	1.00	234	❌
A240	9.2 ± 2.6	0.68	258	❌
A592	8.6 ± 2.4	0.57	278	❌
A2319	8.0 ± 2.3	0.36	386	❌
A399	7.2 ± 2.0	1.50	289	❌
A634	5.6 ± 1.3	0.25	267	❌
A1314	5.3 ± 3.0	0.62	277	❌
A2506	2.8 ± 1.1	0.36	235	❌

Data source: de Nicola et al. (2025) . Green rows indicate confirmed ultramassive black holes (>10 billion solar masses). Core size measured in kiloparsecs (1 kpc ≈ 3,260 light-years).

Notice anything strange? Look at A2107. It has the second-highest black hole mass (22.4 billion solar masses) but no measurable core at all. More on that mystery in a moment.

Can Ultramassive Black Holes Hide in Plain Sight?

Here's a twist that complicates everything: ultramassive black holes don't always come with depleted cores.

The prevailing model said massive black holes create cores through binary scouring. No core? Shouldn't have a giant black hole. Simple.

Except nature doesn't always cooperate.

A2107 hosts a 22.4-billion-solar-mass black hole yet shows no resolved core . It's not alone—a 2023 study reported a 30-billion-solar-mass black hole in another core-less galaxy .

How is this possible?

The researchers suggest a few scenarios:

Late wet mergers: Most massive ellipticals grow through gas-free (dry) mergers. But some may have experienced late gas-rich (wet) mergers that reformed a cuspy central profile, erasing evidence of earlier scouring .
Cooling-flow gas: Gas falling in from the surrounding cluster environment could fuel new central star formation, rebuilding what the black hole binary destroyed.
Different formation history: Not every galaxy follows the standard evolutionary path.

Why This Matters

If UMBHs can hide in core-less galaxies, we've been undercounting them. A census of massive black holes based solely on core galaxies would miss an unknown fraction of the population .

This has real implications for gravitational wave astronomy. Pulsar timing arrays detect the background rumble of merging supermassive black holes across the universe. To predict what that signal should look like, we need accurate demographics of the heaviest black holes—including the ones without obvious signatures .

The M_BH-M_∗ relation (black hole mass vs. galaxy stellar mass) still works for these systems, offering a path forward for identifying hidden giants .

What This Discovery Means for Our Cosmic Understanding

Let's step back and appreciate what just happened.

A single study doubled our inventory of ultramassive black holes—objects so massive they strain our imagination. Along the way, the researchers showed that our go-to method for weighing black holes fails at the extreme high end. They identified a better approach using core sizes. And they complicated our picture by finding that some ultramassive black holes don't follow the expected rules at all.

Here's what we've learned:

The M-sigma relation breaks down for the most massive galaxies. Velocity dispersions saturate in dry-merger-dominated systems .
Core size is a better predictor of black hole mass at the high end. The correlation is tighter and has a direct physical explanation .
UMBHs can exist in core-less galaxies, meaning our understanding of massive black hole demographics remains incomplete .
The data support the black hole binary model for core formation. The tight correlations between core size, sphere of influence, and core density all point to scouring during mergers .

The team plans to push these observations to higher redshifts using data from the Euclid space telescope. We may soon learn whether these relationships evolved over cosmic time—or have remained constant since the universe's earliest massive galaxies formed .

A Note on Potential Biases

We should acknowledge uncertainty here. The sample of 16 BCGs represents galaxies selected because they looked like promising UMBH hosts—large cores or high masses. This selection effect means we can't directly estimate how common UMBHs are across all massive galaxies. The existence of UMBHs in core-less galaxies suggests the true population could be larger than current samples indicate.

The measurement uncertainties on individual black hole masses are substantial—often 25-30%. While the correlations are statistically significant, any single measurement could shift with future observations.

Keep Your Mind Active

At FreeAstroScience.com, we believe in presenting the universe's complexity in terms you can actually understand. We didn't water down the science here—we just made it accessible.

The sleep of reason breeds monsters. Keep questioning. Keep learning. Keep looking up.

When you're ready for your next cosmic journey, come back to FreeAstroScience. There's always more universe to explore.

Sources

de Nicola, S., Thomas, J., Saglia, R. P., Kluge, M., Snigula, J., & Bender, R. (2025). Eight New Ultramassive Black Hole Masses confirm Best Correlation with Galaxy Core Sizes. arXiv preprint arXiv:2512.04178.