A black hole is an object in space where gravity is so intense that nothing, not even light, can escape. This extreme gravitational pull results from matter compressed into a tiny volume. The boundary beyond which escape is impossible is called the event horizon. Supermassive Black Holes (SMBHs) are the largest class, residing at the centers of nearly all large galaxies. The black hole in the Phoenix Cluster, informally called Phoenix A, is a contender for the universe’s largest known black hole.
The Scale of Phoenix A and Its Current Status
Phoenix A is recognized as one of the most massive black holes ever found, with scientific estimates placing its mass in an extraordinary range. While the exact figure is challenging to measure directly, models suggest a mass of approximately 100 billion times that of our Sun. This size places it in a theoretical category known as “ultramassive” black holes.
To grasp this scale, consider the size of its event horizon, the point of no return. Based on the 100 billion solar mass estimate, the event horizon of Phoenix A would span roughly 590 billion kilometers (3,900 astronomical units). If placed at the center of our solar system, its event horizon would extend far beyond the orbit of Pluto, engulfing all the planets and dwarf planets. This size was inferred through dynamical models that analyze the energy output and the surrounding hot gas in the galaxy cluster, not by direct imaging.
Location in the Phoenix Cluster
Phoenix A resides at the heart of the Phoenix Cluster, formally cataloged as SPT-CL J2344-4243. This cluster is situated in the southern constellation Phoenix and is one of the most massive galaxy clusters known in the observable universe. The cluster is approximately 5.7 billion light-years from Earth.
The central galaxy hosting Phoenix A is a massive, bright elliptical galaxy. The cluster’s environment is characterized by an enormous reservoir of superheated gas that emits X-rays. This gas, known as the intracluster medium, is the fuel source that drives the black hole’s growth and star formation within the central galaxy.
Astrophysical Mechanisms of Supermassive Black Hole Growth
Supermassive black holes reach their colossal sizes through two primary mechanisms: accretion (the steady intake of surrounding matter) and mergers with other black holes. Accretion involves gas and dust spiraling inward via an accretion disk, converting gravitational potential energy into light and heat. This efficient process allows the black hole to increase its mass over billions of years.
In the Phoenix Cluster, accretion is occurring at a high rate, which is the main reason for Phoenix A’s growth. The cluster is notable for a phenomenon called a “runaway cooling flow,” where the hot gas in the core is cooling rapidly. In most clusters, the central black hole produces jets that heat the surrounding gas, preventing cooling and starving the black hole of fuel.
In the Phoenix Cluster, the cooling gas filaments are feeding the central galaxy’s star formation and the black hole simultaneously. This cooling flow delivers vast amounts of matter directly to the accretion disk, fueling its rapid expansion. Astronomers estimate the black hole is accreting material at a rate equivalent to about 60 solar masses per year, a growth spurt that is rare in the nearby universe. Another element is that the growth of ultramassive black holes is accelerated by gravitational mergers, where the SMBH consumes smaller black holes or merges with the SMBH of another galaxy during a cluster collision. The Phoenix Cluster’s position as a massive galactic hub makes it an ideal environment for such merger events.
Ranking Phoenix A Against Other Known Giants
While Phoenix A is often cited as the largest, its 100 billion solar mass estimate is based on indirect modeling of the cluster’s dynamics and energy output. This differs from the dynamically measured masses of other giants, which are considered more secure, leading to periodic changes in the ranking. The quasar TON 618, for instance, is a contender whose mass is estimated to be around 66 billion solar masses.
Another dynamically measured black hole is the one in the galaxy Holmberg 15A, with an estimated mass of 40 billion solar masses. The difficulty in obtaining precise measurements for these incredibly distant objects means the current record holder is often debated. Phoenix A’s estimated mass places it in the upper echelon of known black holes. The existence of Phoenix A at this scale suggests that the mechanisms allowing black holes to reach such extreme sizes are still active and are more effective in environments like the Phoenix Cluster.