Ton 618, a hyperluminous quasar, represents one of the most distant and massive objects known in the universe. We observe this object as it was approximately 10.4 billion years ago, when the cosmos was significantly younger. At its core is an Ultra Massive Black Hole (UMBH) whose sheer size and early existence challenge established theories of black hole formation and growth. The existence of such a colossal object so early in cosmic history raises a fundamental question: how could a black hole accumulate such immense mass in a comparatively short time? The answer lies in combining theories about its initial formation with mechanisms for hyper-accelerated growth.
Defining the Ultra Massive Scale
Ton 618 is powered by an active galactic nucleus, known as a quasar, which is a phase where the central black hole is vigorously consuming surrounding matter. This process generates an enormous amount of radiation, making it shine with the luminosity of 140 trillion Suns, easily outshining its entire host galaxy. The light we detect is emitted from an accretion disk of gas spiraling inward, which is heated to millions of degrees by friction and gravity.
The black hole at the quasar’s center is estimated to be around 66 billion times the mass of our Sun, which places it firmly in the “Ultra Massive Black Hole” category. This mass dictates the size of its event horizon, or Schwarzschild radius, the point of no return. For Ton 618, this radius is calculated to be approximately 1,300 astronomical units (AU).
The event horizon is vast enough to contain our entire solar system many times over. For comparison, the distance from the Sun to Neptune is approximately 30 AU. The existence of an object this large, formed when the universe was only a few billion years old, forces scientists to look for exotic formation and growth pathways.
The Puzzle of Initial Black Hole Seeds
The formation of a black hole as massive as Ton 618 requires a large initial “seed” to kickstart the growth process in the early universe. Scientists generally consider two main competing theories for these initial seeds. The first model involves “light seeds” formed from the collapse of the universe’s first stars, known as Population III (Pop III) stars.
These metal-free Pop III stars were extremely massive, and their collapse would have produced black hole seeds in the range of 100 to 1,000 solar masses. A 100-solar-mass seed would need to sustain nearly continuous, maximum-rate feeding over billions of years to reach the 66-billion-solar-mass scale of Ton 618. This presents a major time-scale problem for early, massive quasars.
The preferred alternative model is the formation of “heavy seeds,” known as Direct Collapse Black Holes (DCBHs). This mechanism proposes that massive clouds of primordial, metal-free gas (primarily hydrogen and helium) bypassed the star-forming phase entirely. Specific environmental conditions, such as a strong ultraviolet background from nearby star formation, were necessary to prevent the gas from cooling and fragmenting into smaller stellar-mass black holes.
This monolithic collapse of a giant gas cloud under its own gravity would have resulted in an initial black hole mass ranging from 10,000 to 1,000,000 solar masses. This heavy seed provides a significantly larger starting point, greatly reducing the time needed to accrete matter and reach the Ultra Massive Black Hole scale observed in Ton 618. This larger initial mass is considered a prerequisite for explaining the presence of such a massive object so early in cosmic time.
Mechanisms for Rapid Growth
Regardless of whether the seed was light or heavy, the black hole at Ton 618’s core required a period of hyper-accelerated growth to achieve its current mass. Black hole growth is primarily governed by accretion, the process of drawing in surrounding gas and dust. This process is naturally limited by the Eddington Limit.
The Eddington Limit defines the maximum rate at which a black hole can accrete matter, a point where the outward pressure of the radiation generated by the infalling material perfectly balances the inward pull of gravity. If the black hole were to accrete continuously at this maximum rate, it would still struggle to reach 66 billion solar masses in the available time unless it began with a heavy seed.
Therefore, the formation of Ton 618 likely involved prolonged periods of “super-Eddington accretion,” consuming matter at a rate exceeding the theoretical limit. This occurs when the high density of the surrounding gas reservoir allows matter to be funneled into the black hole faster than the radiation pressure can push it away. This sustained, rapid feeding must have been virtually continuous over billions of years.
Another necessary mechanism for rapid growth involves the frequent merging of the black hole with other black holes and their host galaxies. The early universe was a much denser, more active environment, where galaxies were constantly colliding and merging. These mergers provide two benefits: they directly increase the black hole’s mass, and they funnel colossal amounts of gas and stellar material toward the center of the newly merged galaxy.
This influx of fresh material acts as the fuel source needed to sustain the hyper-accretion phase. The black hole at the center of Ton 618 must have resided in a particularly dense and active region of the early universe, constantly fed by the merging of smaller galaxies and their central black holes. The formation of this Ultra Massive Black Hole is therefore a story of a massive initial seed nourished by super-Eddington accretion and a continuous series of cosmic collisions.