Where Is the Phoenix A Black Hole Located?

The Phoenix A black hole sits at the center of the brightest galaxy in the Phoenix cluster, a massive collection of roughly a thousand gravitationally bound galaxies located about 5.8 billion light-years from Earth in the southern constellation Phoenix. Its formal catalog name is SPT-CLJ2344-4243, reflecting its discovery by the South Pole Telescope. What makes this black hole extraordinary is not just its address but the environment surrounding it: a galaxy cluster so luminous in X-rays and so active in star formation that it has become one of the most studied objects in modern astrophysics.

The Phoenix Cluster and Its Place in the Sky

The Phoenix cluster was first detected through a subtle imprint it leaves on the cosmic microwave background, a phenomenon where hot gas in a massive cluster slightly distorts the energy of passing photons. The South Pole Telescope picked up this signal, and follow-up observations confirmed it as an extraordinarily massive galaxy cluster at a redshift of 0.596, meaning the light we see from it has been traveling for roughly 5.8 billion years.¹ In the night sky, the cluster lies in the constellation Phoenix, a relatively faint southern constellation visible from the Southern Hemisphere and lower northern latitudes. It is not something you can see with a backyard telescope; its detection required specialized instruments sensitive to microwave, X-ray, and radio wavelengths.

What sets the Phoenix cluster apart from other galaxy clusters is its extreme X-ray brightness. It is the most X-ray-luminous galaxy cluster known, with an X-ray luminosity measured at roughly 82 × 10⁴⁴ ergs per second.² That intense glow comes from the hot gas filling the space between galaxies in the cluster, heated to tens of millions of degrees. The total mass of the system, including dark matter, hot gas, and all the galaxies, has been estimated at around 2.3 × 10¹⁵ solar masses within a characteristic radius used by astronomers to compare clusters.³ That makes it one of the most massive galaxy clusters discovered.

Zooming In on the Central Galaxy

Galaxy clusters typically have a dominant galaxy sitting near their gravitational center, often called the brightest cluster galaxy. In the Phoenix cluster, that central galaxy is where the black hole resides. The galaxy itself is not exceptionally massive in stars compared to some other giant ellipticals. Recent observations with the James Webb Space Telescope measured its stellar mass at about 2.6 × 10¹⁰ solar masses, roughly a quarter of the Milky Way’s stellar mass.⁴ What makes it remarkable is not the stars already there but the rate at which new ones are forming.

The central galaxy in the Phoenix cluster is producing stars at a rate that dwarfs nearly every other known galaxy. JWST-based measurements put the star formation rate averaged over the past ten million years at about 1,340 solar masses per year.⁴ For perspective, the Milky Way forms stars at a rate of about one to three solar masses per year. Earlier estimates based on optical and submillimeter data placed the rate at around 800 solar masses per year, and modeling of the star formation history suggests the current burst began roughly 26 million years ago, ramping up dramatically from a long-term average of about 400 solar masses per year.⁵ The fuel for all of this comes from the hot gas surrounding the galaxy, which is cooling and condensing at an astonishing pace.

What Is Driving the Extreme Cooling

The hot gas between galaxies in a cluster slowly radiates energy as X-rays. In many clusters, the densest gas near the center cools faster than gas farther out, creating what astronomers call a cool core. The Phoenix cluster has the most extreme cool core ever observed. Temperature measurements show the gas drops from a peak of about 14 keV (roughly 160 million degrees) at large radii down to about 2 keV near the center, with three-dimensional modeling suggesting the innermost gas may be as cool as 1 keV, or about 12 million degrees.⁶ That temperature gradient, spanning a factor of roughly 14 across about 300,000 light-years, is the steepest known in any galaxy cluster.

New JWST observations have gone further, detecting gas at intermediate temperatures that had been extremely difficult to observe before. By mapping emission from neon ions at a temperature of about 300,000 degrees, researchers found this cooling gas spread across a large region that lines up with the coldest spots in the X-ray gas, the molecular gas, and the sites of active star formation. They estimated that a recent episode of rapid cooling is dumping gas at a rate of 5,000 to 23,000 solar masses per year from the hot phase into cooler states.⁷ That is an extraordinary rate, and it means the raw material for star formation is being supplied faster than the galaxy can consume it.

The Black Hole Itself

At the very center of this frenzied galaxy sits the supermassive black hole. Its mass has not been directly measured using the gold-standard techniques applied to closer black holes, like tracking the orbits of stars or mapping gas velocities in the immediate vicinity. Instead, estimates come from indirect methods that relate a black hole’s mass to properties of its host galaxy or cluster. One analysis, using relationships between cluster properties and central black hole mass, suggested the Phoenix A black hole could have a mass on the order of 100 billion solar masses, placing it among the most massive black holes theorized to exist.⁸

That number comes with substantial uncertainty. The indirect methods used to estimate it rely on scaling relationships calibrated on smaller black holes and less extreme environments. And at 5.8 billion light-years away, the Phoenix cluster is too distant for the kind of resolved kinematic measurements that pinned down the masses of closer supermassive black holes. Still, even conservative estimates place it firmly in the “ultramassive” category, well above 10 billion solar masses. Whether it truly reaches 100 billion is one of the open questions that makes this system so interesting to theorists.

How Jets and Bubbles Push Back Against Cooling

A black hole this massive and embedded in such a gas-rich environment does not just passively sit there. Material falling toward it powers jets of magnetized plasma that shoot outward at close to the speed of light. In the Phoenix cluster, these jets have carved out two large cavities in the surrounding hot gas, each about 10,000 parsecs (roughly 33,000 light-years) across and centered about 15,000 parsecs from the black hole.⁹ These cavities show up as dark voids in X-ray images, where the jets have displaced the hot gas.

Radio observations confirm that the cavities are filled with magnetized plasma from the jets. When researchers subtracted a smooth model of the cluster’s X-ray emission and overlaid radio contours, the radio-bright regions lined up precisely with the X-ray cavities, confirming that the jets are inflating the bubbles.¹⁰ In most galaxy clusters with cool cores, this kind of jet-driven feedback is powerful enough to offset the cooling of the hot gas, keeping it from condensing and forming stars. The jets essentially reheat the gas, creating a thermostat-like cycle.

What makes the Phoenix cluster unusual is that the feedback appears to be losing the battle. Despite the jets and cavities, the gas is still cooling at an extraordinary rate and stars are forming far faster than in any other known cluster center. This suggests the cooling is running away, at least temporarily, outpacing the energy the black hole can inject. Researchers have described it as a system caught in a rare transitional moment, where the cooling rate spiked faster than feedback could respond.

Filaments of Cold Gas Wrapping Around the Bubbles

One of the most striking features revealed by observations with the ALMA radio array and earlier optical telescopes is a network of cold molecular gas filaments extending outward from the central galaxy. These filaments stretch for more than 30,000 light-years and are draped around the rims of the jet-inflated X-ray cavities.⁹ Earlier optical spectroscopy had already detected a complex emission-line nebula spanning a similar extent, glowing in multiple wavelengths characteristic of warm ionized gas.¹¹

The positioning of these filaments is not random. They appear to have been swept up or drawn out by the expanding radio bubbles, forming thin threads of cold dense gas along the boundaries where the jet plasma meets the surrounding hot atmosphere. Stars are forming along these filaments, meaning the jets are not just heating the gas but are also, paradoxically, helping to organize some of it into the dense clumps that collapse into stars. This dual role of feedback, simultaneously heating and triggering star formation, is something astronomers have debated in other clusters, but the Phoenix cluster provides one of the clearest laboratories to study it because the effects are so amplified.

How Big Can a Black Hole Get

The estimated mass of the Phoenix A black hole pushes up against theoretical predictions about how large a black hole can grow. Several independent lines of reasoning converge on an upper limit somewhere in the range of 10 to 100 billion solar masses, depending on the assumptions.

One argument focuses on the physics of accretion itself. For a black hole to grow by swallowing gas, that gas typically needs to form a disk. But as the black hole becomes more massive, the point at which the disk begins to break apart under its own gravity moves inward, eventually crossing the boundary at which matter inevitably spirals in. When that happens, a stable accretion disk can no longer form. One calculation places this limit at about 50 billion solar masses under standard conditions, rising to a theoretical absolute ceiling of about 270 billion solar masses for a black hole spinning at the maximum possible rate.¹²

A separate argument comes from considering the host galaxy. When a black hole becomes very massive, the rate at which gas can reach it from the broader galaxy drops because star formation in the galaxy’s outer regions consumes the gas first. At the point where only a trickle of gas arrives, the flow near the black hole changes character, becoming a much hotter, puffier structure that naturally launches powerful outflows and jets. Those jets then shut off further feeding, creating a self-regulating cap. Observations of the most massive black holes support this idea: the largest ones tend to sit in galaxies with strong radio jets, consistent with the prediction that they have reached a point where jet-driven feedback prevents further growth.¹³ A separate analysis using the relationship between black holes and their host galaxies argued for a practical upper limit around 10 billion solar masses.¹⁴

If the Phoenix A black hole truly approaches 100 billion solar masses, it would strain or potentially exceed several of these predicted limits, which is one reason the estimate remains controversial. Pinning down its actual mass would test our understanding of how black holes grow in the most extreme environments the universe has to offer.

A Radio Mini-Halo Surrounding the Core

Beyond the jets themselves, the center of the Phoenix cluster hosts a diffuse radio structure known as a mini-halo. This is a cloud of radio-emitting particles that fills the inner region of the cluster, extending well beyond the jets and cavities. Radio observations at 1.52 GHz measured the mini-halo’s total brightness and found a steep spectral index between 610 MHz and 1.52 GHz, indicating that the electrons producing the emission are losing energy rapidly.¹

Mini-halos are found in other cool-core clusters, but their origin remains debated. One possibility is that turbulence in the gas re-accelerates old electrons left behind by past episodes of jet activity. Another is that collisions between cosmic ray protons and the dense cluster gas produce fresh radio-emitting particles. In the Phoenix cluster, the connection between the mini-halo and the extreme cooling and jet activity makes it a valuable test case for distinguishing between these scenarios. The mini-halo is another piece of the puzzle showing how deeply the central black hole’s activity shapes its surroundings across enormous scales.

Strong Lensing and Mapping the Cluster’s Mass

Because the Phoenix cluster is so massive, it bends light from more distant galaxies behind it, an effect called gravitational lensing. Researchers identified nine strongly lensed background sources whose distorted images provided constraints on the cluster’s mass distribution, particularly in the inner core region. Several of these lensed images extend nearly to the very center of the cluster, which is especially useful for measuring how steeply the mass is concentrated around the black hole’s host galaxy.¹⁵

This mass profile matters for understanding the cooling flow because the rate at which gas falls inward depends on both how fast it cools and how strong the gravitational pull is at each radius. By combining the lensing-derived mass profile with X-ray temperature and density measurements, researchers can calculate how quickly gas should be collapsing toward the center under gravity alone, compared to how quickly it is actually cooling. In the Phoenix cluster, the two timescales are close to each other across a wide range of radii, which is part of why the cooling is so runaway: gravity is pulling gas inward nearly as fast as it can cool, leaving feedback little room to intervene.

Why the Phoenix Cluster Keeps Attracting New Observations

Since its discovery, the Phoenix cluster has been observed with nearly every major telescope available: the Chandra X-ray Observatory, XMM-Newton, the Hubble Space Telescope, ALMA, the Very Large Array, the Gemini telescope, and most recently the James Webb Space Telescope. Each new instrument has revealed a different layer of the system, from the X-ray glow of the hot gas to the infrared light of dust and molecular hydrogen to the radio emission from jets and the mini-halo. The JWST observations in particular opened a new window by detecting mid-infrared emission lines from gas at intermediate temperatures, filling in a critical gap between the million-degree X-ray gas and the few-hundred-degree molecular gas that fuels star formation.⁷

Future observations are likely to focus on two outstanding questions. The first is nailing down the black hole’s mass more precisely, which may eventually become possible with next-generation extremely large telescopes capable of resolving gas motions closer to the center. The second is tracking how the feedback cycle evolves over time. If the current cooling spike is truly a short-lived event, repeated observations over coming decades could catch the jets ramping up to eventually quench the starburst, providing a real-time view of the feedback loop that governs how the largest galaxies in the universe grow.

1
The Astrophysical Journal. Probing the Origin of Diffuse Radio Emission in the Cool Core of the Phoenix Galaxy Cluster
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IOPscience / The Astrophysical Journal. Probing the Origin of Diffuse Radio Emission in the Cool Core of the Phoenix Galaxy Cluster
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Astronomy & Astrophysics. New XMM-Newton observation of the Phoenix cluster: properties of the cool core
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The Astrophysical Journal. Cold Gas and Star Formation in the Phoenix Cluster with JWST
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arXiv. Cold Gas and Star Formation in the Phoenix Cluster with JWST
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The Astrophysical Journal. Anatomy of a Cooling Flow: The Feedback Response to Pure Cooling in the Core of the Phoenix Cluster
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arXiv / Nature. Directly Imaging the Cooling Flow in the Phoenix Cluster
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Astronomy & Astrophysics. Unveiling Gargantua: A new search strategy for the most massive central cluster black holes
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The Astrophysical Journal. Alma Observations of Massive Molecular Gas Filaments Encasing Radio Bubbles in the Phoenix Cluster
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Astronomy & Astrophysics. Very Large Array observations of the mini-halo and AGN feedback in the Phoenix cluster
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The Astrophysical Journal. THE STATE OF THE WARM AND COLD GAS IN THE EXTREME STARBURST AT THE CORE OF THE PHOENIX GALAXY CLUSTER (SPT-CLJ2344-4243)
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Monthly Notices of the Royal Astronomical Society: Letters. How big can a black hole grow?
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The Astrophysical Journal Letters. Quenching of Supermassive Black Hole Growth around the Apparent Maximum Mass
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Monthly Notices of the Royal Astronomical Society. Is there an upper limit to black hole masses?
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The Astrophysical Journal. Anatomy of a Cooling Flow: The Feedback Response to Pure Cooling in the Core of the Phoenix Cluster – Section: 2.1 Optical: HST