Phoenix Cluster

Cooling flow and star formation

The Phoenix Cluster is a cool-core cluster with an unusually luminous X-ray-emitting intracluster medium. A multiwavelength study led by Michael McDonald derived a classical cooling-flow rate of 3820±530 M_☉ per year, among the highest values reported for a galaxy-cluster core.^[4] The same study estimated that the central galaxy was forming stars at a rate of 740±160 M_☉ per year.

Later observations showed that the measured cooling rate depends on the temperature range and method used. An analysis of deep XMM-Newton observations measured a mass-deposition rate of approximately 620 M_☉ per year between 0.3 and 3.0 keV using one set of detectors, while obtaining lower values and upper limits for gas cooling at the lowest X-ray temperatures.^[8] These measurements indicate that only part of the gas represented by the classical cooling-flow estimate is observed to reach the lowest temperatures.

Hubble Space Telescope observations revealed filamentary blue emission extending more than 40 kiloparsecs from Phoenix A. The emission was attributed primarily to young stars rather than scattered light or a large-scale ionised outflow from the active nucleus. The observations produced an extinction-corrected star-formation rate of 798±42 M_☉ per year.^[9]

The central starburst is substantially more active than star formation in the Milky Way or in NGC 1275, the central galaxy of the Perseus Cluster.^[10] Some starburst galaxies at higher redshifts nevertheless have still greater star-formation rates.^[11]

AGN feedback and X-ray cavities

In many cool-core clusters, energy released by a central active galactic nucleus prevents most of the intracluster gas from cooling. In the Phoenix Cluster, this feedback initially appeared insufficient to balance the exceptionally rapid radiative cooling.

Deep observations with the Chandra X-ray Observatory, the Hubble Space Telescope, and ground-based telescopes confirmed deep X-ray cavities within approximately 10 kiloparsecs of the cluster centre. The cavities are associated with radio jets and imply mechanical jet powers of approximately (2–7)×10⁴⁵ ergs per second. Larger, older cavities at distances of about 100 kiloparsecs provide evidence of an earlier episode of radio-mode feedback roughly 100 million years ago.^[12]

Later Chandra, Hubble, and Very Large Array observations showed that cooling in the central region is highly asymmetric. Much of the lowest-entropy gas is concentrated in a filament extending northward from Phoenix A, while cooler gas is distributed around and behind a pair of X-ray cavities inflated by radio jets. The observations suggest that the jets can both heat the intracluster medium and encourage local condensation by lifting low-entropy gas and increasing turbulence along the jet axis.^[13]

Radio mini-halo

Observations with the Giant Metrewave Radio Telescope detected diffuse radio emission extending approximately 400–500 kiloparsecs around the central radio source. The emission was classified as a radio mini-halo, with a measured flux density of 17±5 mJy at 610 MHz and an estimated 1.4-GHz radio power of (10.4±3.5)×10²⁴ watts per hertz.^[14]

The mini-halo may be produced by relativistic particles re-accelerated through turbulence in the cluster core. The non-concentric distribution of the X-ray-emitting gas is consistent with gas sloshing caused by a minor merger, although the origin of radio mini-halos remains uncertain.^[14]

Central galaxy

Quick facts Observation data (J2000.0 epoch), Right ascension ...

Phoenix A
Phoenix A in imaging from the DESI Legacy Imaging Surveys
Observation data (J2000.0 epoch)
Right ascension	23h 44m 43.89s[3]
Declination	−42° 43′ 12.4″[3]
Redshift	0.597[3]
Heliocentric radial velocity	179072 km/s
Apparent magnitude (V)	18.80[3]
Characteristics
Size	110.48 kiloparsecs (360,300 light-years) (diameter; 2MASS K-band total isophote)[3]
Other designations
RBS 2043; 2MASX J23444387−4243124; MRSS 292-067217; 2CXO J234443.9−424312; LEDA 3988894

Close

The cluster's central elliptical cD galaxy, Phoenix A (RBS 2043; 2MASX J23444387−4243124), hosts an active galactic nucleus with both quasar-like and type 2 Seyfert characteristics. The active nucleus is powered by a central supermassive black hole. The galaxy's precise morphological classification remains uncertain.

Based on its total K-band aperture, Phoenix A has an angular diameter of approximately 16.20 arcseconds, corresponding to an isophotal diameter of about 110.48 kiloparsecs (360,300 light-years).^[3]

The surrounding cluster core contains a large reservoir of hot intracluster gas. As in other massive galaxy clusters, the hot gas contains more ordinary matter than the cluster's member galaxies combined. Cooling from this reservoir supplies gas to the central starburst and active nucleus.

Molecular gas and filaments

Observations with the Atacama Large Millimeter/submillimeter Array detected (2.1±0.3)×10¹⁰ M_☉ of cold molecular gas in Phoenix A. The gas fuels both the central starburst and activity associated with the black hole.^[15]

Much of the molecular gas is distributed in filaments approximately 10–20 kiloparsecs long around the outer edges of radio bubbles inflated by the active nucleus. The filaments have smooth velocity gradients and comparatively narrow spectral lines, indicating ordered flows rather than simple gravitational free fall. The gas may have been lifted by the expanding bubbles or may have condensed from low-entropy gas carried upward in their wakes.^[15]

A later analysis using the James Webb Space Telescope measured a molecular-gas mass of 2.2+0.4
−0.1×10¹⁰ M_☉, consistent with the earlier carbon-monoxide measurements. The study derived average star-formation rates of 1340±100 M_☉ per year over the preceding 10 million years and 740±80 M_☉ per year over the preceding 100 million years.^[16]

Supermassive black hole

The central black hole powers the active nucleus of Phoenix A and the relativistic jets that produce cavities in the surrounding intracluster medium. M. Brockamp and colleagues developed a model linking the central stellar-density profile of a galaxy, its inferred feedback power, and the adiabatic growth of its black hole.^[17]

Their calculations identified the nuclear black hole in Phoenix A as a candidate for a mass on the order of 100 billion M_☉.^[17] The value has not been confirmed through direct measurements of stellar or gas dynamics and is therefore substantially more uncertain than dynamically determined black-hole masses. If the estimate is accurate, Phoenix A would contain one of the most massive black holes proposed.

For a non-rotating black hole, the Schwarzschild radius is:

${\displaystyle r_{\rm {s}}={\frac {2GM}{c^{2}}}}$

Using the modelled mass of 100 billion M_☉ as an illustrative value would give:

• a mass approximately 24,000 times that of Sagittarius A*, the black hole at the centre of the Milky Way;
• a mass of roughly twice the estimated total mass of the Triangulum Galaxy, including its dark-matter halo;^[18]
• a Schwarzschild radius of approximately 295.25 billion kilometres (2,000 astronomical units; 0.031 light-years), or about 50 times the average distance between the Sun and Pluto;
• a corresponding event-horizon circumference that light would take approximately 72 days to traverse.

Because the underlying mass is model-dependent, these comparisons are illustrative rather than direct measurements.

A black hole of this mass would fall within the proposed category of stupendously large black holes, or SLABs. The term has been used for hypothetical black holes approaching or exceeding 100 billion M_☉, including objects that may have originated as unusually massive primordial black holes.^[19]