Is the Phoenix Black Hole Real?

The supermassive black hole at the center of the Phoenix galaxy cluster is real, confirmed by X-ray observations, radio jet detections, and the signatures of intense accretion. What is far less settled is its mass. Popular accounts often cite a figure around 100 billion times the mass of the Sun, which would make it the heaviest black hole ever recorded, but that number rests on indirect scaling methods with known uncertainties. The black hole’s existence is beyond serious doubt; its place in the record books is not.

How We Know the Black Hole Is There

The Phoenix cluster (catalogued as SPT-CLJ2344-4243) sits at a redshift of about 0.596, meaning its light has traveled roughly six billion years to reach us. At the heart of its brightest cluster galaxy lies an active galactic nucleus, a region where matter spirals into a supermassive black hole and releases enormous amounts of energy. X-ray observations with the Suzaku and Chandra space telescopes detected a heavily absorbed power-law X-ray spectrum coming from the core, with an absorption column density of about 3.2 × 10²³ cm⁻² and an unobscured luminosity of roughly 4.7 × 10⁴⁵ erg per second in the 2–10 keV band. A neutral iron emission line was also discovered, and the combination of these properties led researchers to classify the central engine as a type 2 quasi-stellar object, an extremely luminous but dust-shrouded quasar.¹

Radio observations added another layer. Researchers spatially resolved bipolar and bar-shaped radio structures extending roughly 10 to 20 kiloparsecs from the central galaxy’s nucleus, aligned with cavities visible in X-ray images. These are almost certainly radio jets and lobes inflated by the black hole, with age estimates on the order of 10 million years. A second, younger pair of compact jets near the very center appears to be only about a million years old, suggesting the black hole has gone through multiple outburst episodes.² Deep Chandra X-ray imaging further showed that cool gas in the cluster core is draped around and behind a pair of X-ray cavities, reinforcing the picture of jet-inflated bubbles pushing through the surrounding hot gas.³

So there is no real controversy about the black hole’s existence. Multiple independent lines of evidence, from X-ray spectroscopy to radio imaging to the behavior of surrounding gas, all point to a very active supermassive black hole at the center of the Phoenix cluster. The debate is about how heavy it is.

Where the Mass Estimate Comes From

You cannot simply put a black hole on a scale. In most cases, astronomers infer black hole masses from their surroundings. The most common methods involve either watching how stars and gas move near the black hole (dynamical measurements) or using relationships between the black hole’s mass and the properties of its host galaxy, like the velocity dispersion of stars in the galaxy’s central bulge. These scaling relations, especially the one linking black hole mass to stellar velocity dispersion (the M–sigma relation), have been calibrated using nearby galaxies where direct dynamical measurements are possible.

The Phoenix cluster’s central galaxy is far away, making direct dynamical measurements extremely difficult. The mass estimates that circulate for “Phoenix A” are largely derived from these scaling relations. The problem is that scaling relations were built from a sample of galaxies much less extreme than the Phoenix system, and extrapolating them to a galaxy sitting at the center of one of the most massive, most violently star-forming galaxy clusters in the universe introduces real uncertainty.

Research into how virial black hole mass estimates behave for extreme systems illustrates the issue vividly. A study of a luminous, starburst-hosting quasar at redshift 3 recovered black hole mass estimates spanning more than an order of magnitude, with individual constraints ranging between about one billion and roughly 15 billion solar masses, depending on which emission line was used and what assumptions were made about the geometry of the gas orbiting the black hole. The spread was attributed to measurement uncertainties, potential departures from equilibrium, and the exotic nature of the target, including high accretion rates, powerful outflows, and a dusty center.⁴ The Phoenix cluster’s central black hole lives in a similarly extreme environment, so the same cautions apply.

Selection Bias and Inflated Masses

There is a more structural problem with the catalog of “ultramassive” black holes. To measure a black hole’s mass dynamically, you need to resolve its gravitational sphere of influence, the region where the black hole’s gravity dominates over the gravity of surrounding stars. This means that at any given distance, only the most massive black holes can be measured. Lighter black holes in the same galaxies simply do not leave a large enough gravitational footprint to detect with current instruments.

Monte Carlo simulations of this selection effect show that it artificially boosts the apparent normalization of the M–sigma relation by a factor of at least three. The bias is even larger for the relationship between black hole mass and total stellar mass.⁵ In plain terms, the black holes we can measure tend to be heavier than average for their host galaxies, and building scaling relations from that biased sample means we overpredict masses when we apply those relations to galaxies we have not measured directly. For a system like the Phoenix cluster, where the mass estimate already pushes into extreme territory, a factor-of-three inflation matters enormously.

Separate analyses of the local black hole mass function, built by tracing how quasars have accumulated mass over cosmic time, also hint that ultramassive black holes above about 10 billion solar masses should actually be more common than current observations suggest, independent of what efficiency is assumed for their growth.⁶ That does not directly confirm or deny the Phoenix black hole’s mass, but it does suggest the landscape of the heaviest black holes is still poorly mapped.

Theoretical Ceilings on Black Hole Mass

Physicists have also approached the question from the opposite direction: given how accretion discs work, is there a maximum mass a black hole can reach? The answer appears to be yes, although the exact ceiling depends on assumptions.

One analysis considered the point at which a black hole’s own gravity becomes so strong that a stable accretion disc can no longer form around it. Beyond that threshold, the black hole effectively chokes off its own food supply. The resulting physical upper limit is on the order of 50 billion solar masses for any black hole still actively accreting through a disc.⁷ A separate study took a different angle, noting that when the gas supply rate from galactic scales exceeds about a thousand solar masses per year, most of that gas converts to stars in a ring tens of parsecs from the black hole, long before the gas can actually fall in. This effectively caps the accretion rate at the subparsec scale to a few solar masses per year at most, which limits black hole growth to roughly 5 to 10 billion solar masses over the age of the universe.⁸

If you take the stricter limit of 5 to 10 billion solar masses seriously, a 100-billion-solar-mass black hole would be impossible. The more generous limit of 50 billion solar masses would still leave the headline figure for Phoenix A looking implausible. But these theoretical ceilings assume growth through gas accretion. Mergers between galaxies can deliver additional mass to a black hole by combining two already-massive black holes into one, potentially bypassing the accretion bottleneck. How much mergers can push the total mass upward remains an active area of study.

How the Heaviest Black Holes Grow

For typical supermassive black holes, gas accretion dominates the growth budget. Gas falls inward through a disc, radiates energy, and gradually adds mass to the black hole. But for the very heaviest black holes, the story changes. Simulations tracking black hole assembly across cosmic time find that, on average, supermassive black holes grow through a mix of gas inflow triggered by galaxy disc instabilities and merger-driven accretion. For the most massive black holes at the present epoch, however, mergers between black holes contribute the majority of the total mass, accounting for roughly 60 percent of the final mass budget.⁹

This fits with a broader picture in which the heaviest black holes sit inside galaxies that have undergone multiple major mergers. Each merger brings together two large black holes. As the pair spirals inward, gravitational slingshot interactions eject nearby stars, carving out a depleted core in the host galaxy. That process shapes the relationship between the black hole mass, the size of the depleted core, and the velocity dispersion of the remaining stars.¹⁰ The Phoenix cluster’s central galaxy, sitting at the bottom of the gravitational potential of an enormously massive cluster, is exactly the kind of environment where repeated mergers would be expected. Whether that merger history has pushed the black hole to 20 billion, 50 billion, or 100 billion solar masses is what remains unclear.

For context, one of the few black holes with a well-constrained mass in this extreme range is the one in Holm 15A, the central galaxy of the Abell 85 cluster, estimated at about 40 billion solar masses through direct stellar-dynamical modeling.¹¹ That measurement, while still subject to some uncertainty, is far better constrained than the Phoenix A figure because Holm 15A is much closer and more amenable to resolved kinematic studies.

The Most Extreme Starburst and Cooling Flow in Any Known Cluster

Even if the precise black hole mass remains fuzzy, the environment around it is staggering by any measure. The Phoenix cluster hosts what appears to be the most intense cooling flow and associated starburst of any known galaxy cluster. Hubble Space Telescope imaging revealed complex, filamentary blue emission extending more than 40 kiloparsecs from the central galaxy, consistent with a massive population of young stars rather than light from the central quasar. The extinction-corrected star formation rate was estimated at roughly 800 solar masses per year.¹² Follow-up spectroscopy confirmed a massive population of young stars, about two billion solar masses’ worth, with an age of roughly 4.5 million years, consistent with a sustained star formation rate of about 610 solar masses per year.¹³

ALMA radio observations mapped cold molecular gas filaments, each billions of solar masses heavy, running along the edges of the radio bubbles blown by the black hole’s jets. That cold gas is simultaneously fueling the starburst and feeding the black hole.¹⁴ Recent JWST observations have pushed the picture further, detecting emission from neon gas at a temperature that traces the transition zone where the hot cluster atmosphere is rapidly cooling into the cold phase. The implied cooling rate during a recent episode was between 5,000 and 23,000 solar masses per year, far exceeding the star formation rate and suggesting that the cooling is episodic, with dramatic short-lived spikes.¹⁵

Updated JWST measurements pin the total molecular gas reservoir at about 21 billion solar masses and revise the star formation rate to between roughly 740 and 1,340 solar masses per year, depending on the timescale considered. Even the extreme starburst represents only 10 to 20 percent of the classical cooling rate, compared to the typical one percent seen in other cool-core clusters.¹⁶ In other words, even at its current extraordinary pace of star formation, the Phoenix cluster’s central galaxy is converting only a small fraction of the gas that is cooling out of the hot atmosphere.

Why the Phoenix Cluster Keeps Breaking the Pattern

In most galaxy clusters, the black hole at the center acts as a thermostat. When hot gas in the cluster core cools and starts to fall inward, some of it reaches the black hole, triggering jets and outflows that reheat the surrounding gas and shut down further cooling. This feedback cycle keeps the cooling rate low and limits star formation in the central galaxy to modest levels. The Phoenix cluster appears to be caught in a moment where the cooling has overwhelmed the feedback. The jets are there, the X-ray cavities are there, but the cooling rate is simply too high for the current generation of outbursts to fully counteract.³

The cluster itself is massive, with a total mass within a standard overdensity radius estimated at about 1.25 × 10¹⁵ solar masses.¹⁷ That enormous gravitational potential well means there is a tremendous reservoir of hot gas pressing inward, and the stronger the inflow, the harder it is for any single generation of jets to stem the tide. Researchers have used strong gravitational lensing of background galaxies, with 31 lensed images from nine separate sources, to map the mass profile of the cluster core. The resulting model helps disentangle the complex web of filamentary emission near the center from background lensed sources, sharpening the picture of the cooling and star formation happening in situ.³

What Would It Take to Pin Down the Mass

The honest summary is that nobody has made a direct dynamical measurement of the Phoenix black hole’s mass. What exists are scaling-relation estimates and the indirect inference that something very massive must be powering the observed quasar-level luminosity and jet activity. To get a more reliable number, astronomers would need to resolve the motions of stars or gas very close to the black hole. At a redshift of 0.6, even the finest angular resolution available with current instruments barely scratches the relevant spatial scales.

JWST has already improved our picture of the surrounding gas and star formation. Future extremely large ground-based telescopes, with mirrors 25 to 39 meters across, could in principle push angular resolution fine enough to study stellar kinematics in the central galaxy at a level closer to what has been done for nearer systems like Holm 15A. Even then, the extreme star formation and dust in the Phoenix cluster’s core will make clean measurements challenging. The black hole at the center of the Phoenix cluster is unquestionably real, unquestionably massive, and unquestionably surrounded by one of the most violent environments in the observed universe. Whether it truly holds the record for mass remains one of the more tantalizing open questions in extragalactic astronomy.

1
The Astrophysical Journal. Suzaku Observations of the Type 2 QSO in the Central Galaxy of the Phoenix Cluster
2
Publications of the Astronomical Society of Japan. Discovery of Radio Jets in Phoenix Galaxy Cluster Center
3
The Astrophysical Journal. Anatomy of a Cooling Flow: The Feedback Response to Pure Cooling in the Core of the Phoenix Cluster
4
Astronomy & Astrophysics. JWST ERS Program Q3D: The pitfalls of virial black hole mass constraints shown for a z ∼ 3 quasar with an ultramassive host
5
Monthly Notices of the Royal Astronomical Society. Selection bias in dynamically measured supermassive black hole samples: its consequences and the quest for the most fundamental relation
6
Monthly Notices of the Royal Astronomical Society. Is there an upper limit to black hole masses?
7
Monthly Notices of the Royal Astronomical Society: Letters. How big can a black hole grow?
8
The Astrophysical Journal. IS THERE A MAXIMUM MASS FOR BLACK HOLES IN GALACTIC NUCLEI?
9
The Astrophysical Journal. Tracking the Assembly of Supermassive Black Holes: A Comparison of Diverse Models across Cosmic Time
10
Monthly Notices of the Royal Astronomical Society. Ultramassive black holes and the three M–sigma relations
11
Monthly Notices of the Royal Astronomical Society. Hidden cooling flows in clusters of galaxies – III. Accretion on to the central black hole
12
The Astrophysical Journal Letters. AN HST/WFC3-UVIS VIEW OF THE STARBURST IN THE COOL CORE OF THE PHOENIX CLUSTER
13
The Astrophysical Journal. DEEP CHANDRA, HST-COS, AND MEGACAM OBSERVATIONS OF THE PHOENIX CLUSTER: EXTREME STAR FORMATION AND AGN FEEDBACK ON HUNDRED KILOPARSEC SCALES
14
The Astrophysical Journal. Alma Observations of Massive Molecular Gas Filaments Encasing Radio Bubbles in the Phoenix Cluster
15
Nature. Directly imaging the cooling flow in the Phoenix cluster
16
IOP Publishing. Cold Gas and Star Formation in the Phoenix Cluster with JWST
17
The Astrophysical Journal. COOL CORE BIAS IN SUNYAEV–ZEL’DOVICH GALAXY CLUSTER SURVEYS