A black hole is a region of spacetime where gravity is so intense that nothing, not even light, can escape its pull. This dominance arises from an immense amount of matter compressed into an incredibly small space. These enigmatic objects exist in a vast range of sizes, from a few times the mass of our Sun to colossal giants at the hearts of galaxies. Understanding the nature of the largest of these cosmic behemoths provides insight into how galaxies form and evolve. This article focuses on the current record holder, representing the upper limit of black hole mass discovered so far.
The Current Record Holder
The largest black hole currently known is located at the heart of a hyperluminous quasar named TON 618. This object is a galaxy’s active nucleus, where the central supermassive black hole is vigorously feeding on surrounding matter. Its mass has been estimated at a staggering 66 billion times the mass of the Sun.
To put this immense size into perspective, the event horizon—the boundary beyond which nothing can escape—spans a diameter of approximately 1,300 astronomical units (AU). Its physical radius is more than 40 times the distance from the Sun to Neptune. The quasar itself is one of the brightest objects observed in the distant universe, shining with the luminosity of over 140 trillion Suns.
The light we observe from TON 618 has traveled for about 10.4 billion years to reach Earth, placing it at a vast distance in the constellation Canes Venatici. Due to the expansion of the universe, its current proper distance is around 18.2 billion light-years. Its existence in the early universe, when the cosmos was only a few billion years old, presents a profound challenge to models of black hole growth, requiring mechanisms that can rapidly accumulate such immense mass.
Categories of Black Holes by Mass
Astronomers classify black holes into categories based on their mass, providing a framework for understanding their formation and scale. The smallest are stellar-mass black holes, which are the remnants of massive stars that collapsed after exhausting their nuclear fuel. These typically range from 3 to 50 times the mass of the Sun. They are scattered throughout galaxies, often observed in binary systems where they pull material from a companion star.
The next proposed group is intermediate-mass black holes (IMBHs), a category that bridges the gap between stellar-mass objects and the giants at galactic centers. These are thought to range from a few hundred to about 100,000 solar masses, but their existence is difficult to confirm and they remain elusive. They may form through the runaway merging of multiple stellar-mass black holes in dense star clusters.
The largest category is the supermassive black holes (SMBHs), which reside at the center of nearly every large galaxy. These objects span from millions to billions of solar masses. The most extreme examples, like TON 618, are sometimes referred to as ultramassive black holes, representing a class with masses exceeding 10 billion solar masses.
Determining the Size of Distant Black Holes
Because black holes do not emit light, their masses must be determined by observing their powerful gravitational influence on surrounding matter. For relatively nearby galaxies, astronomers use Kepler’s laws of motion to track the orbits of individual stars closest to the galactic center. By measuring the speed and path of these stars, scientists can calculate the central mass required to keep them in orbit.
For black holes that are billions of light-years away, like TON 618, a technique called reverberation mapping is employed. This method uses the quasar’s highly luminous accretion disk—a swirling disk of superheated gas and dust falling into the black hole. The radiation from the black hole’s immediate vicinity causes the surrounding gas clouds to flare up after a time delay.
By measuring the time delay between the flare from the disk and the subsequent brightening of the gas clouds, astronomers determine the size of the region. This size, combined with the Doppler shift measurement of the gas’s velocity, allows the use of the virial theorem to estimate the black hole’s mass. The estimate for TON 618 relies on analyzing the broadening of specific emission lines, such as the H-beta line, in the quasar’s spectrum, which is indicative of the gas’s high orbital speed.
How Supermassive Black Holes Grow
Supermassive black holes reach their colossal sizes through two primary mechanisms: accretion and mergers. Accretion involves the gradual consumption of gas, dust, and stars from the surrounding galactic environment. This process is most visible when the black hole is actively feeding, forming a bright accretion disk that radiates intensely, classifying the object as an Active Galactic Nucleus or quasar.
The rate at which a black hole can consume matter is self-regulated by the amount of energy released during accretion, which pushes back on the infalling material. However, this mechanism alone may not fully explain the existence of ultramassive black holes in the early universe, suggesting that a second, more rapid growth channel must also be at play.
The second mechanism is the merger of two or more black holes, linked to the collision and coalescence of their host galaxies. When two galaxies merge, their central supermassive black holes sink toward the center of the newly formed galaxy and eventually combine, adding their masses together. This process is thought to be particularly effective in fueling the rapid growth of the most massive black holes, such as TON 618, especially in the matter-rich environment of the young universe.