Nearly every large galaxy has a supermassive black hole at its center, and the reason comes down to how galaxies themselves form. These black holes aren’t accidents or rare anomalies. They appear to be a fundamental part of galaxy construction, growing alongside their host galaxies from the earliest ages of the universe and shaping those galaxies in return.
How the First Black Hole Seeds Formed
The supermassive black holes we see today started as much smaller “seeds” in the young universe, though exactly how those seeds formed is one of the biggest open questions in astrophysics. There are two leading ideas.
The first is that massive early stars, perhaps 100 times the mass of our Sun or more, burned through their fuel quickly and collapsed into black holes. These stellar remnants would have been relatively small, maybe a few hundred solar masses, and would have needed billions of years of steady feeding to reach the millions or billions of solar masses we observe today.
The second idea, called direct collapse, skips the star stage almost entirely. In this scenario, enormous clouds of primordial gas in the early universe collapsed directly into black holes weighing around 100,000 solar masses. This process requires specific conditions: the gas cloud needs to avoid fragmenting into smaller clumps and forming ordinary stars. Intense ultraviolet radiation from nearby galaxies can prevent the gas from cooling and breaking apart, allowing the whole cloud to collapse as one object. Simulations of this process show a resulting mass of roughly 100,000 solar masses, with gas feeding into the collapse at about a quarter of a solar mass per year. That’s a much bigger starting seed, which makes it far easier to explain how black holes grew so massive so quickly.
Why the Center of a Galaxy?
Gravity naturally concentrates mass toward the center of any large structure, and galaxies are no exception. When the first dark matter structures formed in the early universe, gas sank to the bottom of their gravitational wells, pooling at the densest point. That’s where the first stars ignited and where the first black hole seeds either formed or migrated to. Once a massive object sits at the gravitational center, it stays there. Surrounding gas, dust, and smaller black holes spiral inward over time, continuously feeding whatever sits at the bottom of the gravity well.
When galaxies collide and merge, which happens frequently over cosmic time, their central black holes eventually sink toward the center of the newly combined galaxy through a process called dynamical friction. The surrounding stars and gas slow the black holes down, dragging them inward until they find each other and merge into an even larger black hole. This is why even galaxies that have been through multiple mergers still end up with a single supermassive black hole at their core.
How They Grew So Massive
Reaching millions or billions of solar masses requires two main growth channels: swallowing gas and merging with other black holes. Cosmological simulations suggest these play different roles at different stages. Starting from seeds of about 100,000 solar masses, black holes initially grow through mergers with other black holes, reaching roughly 10 million solar masses. Then a dramatic shift happens. Gas accretion takes over, and the black hole mass increases by a factor of 100 to 1,000 over a period of 600 to 700 million years, pushing it to between 1 billion and 10 billion solar masses.
There’s a speed limit on this growth, though. As a black hole pulls in gas, that gas heats up and radiates energy. At a certain point, the outward radiation pressure balances the inward pull of gravity. This threshold, called the Eddington limit, theoretically caps how fast a black hole can feed. That limit creates a real puzzle for the most ancient black holes, because some of them appear too massive to have grown at normal speeds given the age of the universe when they existed.
The James Webb Space Telescope recently found a striking example of how nature gets around this speed limit. A black hole called LID-568 was observed roughly 1.5 billion years after the Big Bang, and despite being relatively small at 7.2 million solar masses, it was accreting at more than 40 times the Eddington limit, with powerful outflows streaming away from it. This kind of super-Eddington feeding may have been common in the early universe and could explain how black holes packed on mass so rapidly.
JWST Is Rewriting the Timeline
Before the James Webb Space Telescope launched, astronomers had found supermassive black holes existing within the first billion years after the Big Bang, which was already hard to explain. JWST has pushed that frontier even further, revealing a large population of active black holes at redshifts of 4 to 11, corresponding to a universe that was only a few hundred million to about 1.5 billion years old. Their estimated masses range from about 1 million to 100 million solar masses.
Many of these early black holes are “overmassive,” meaning they’re far heavier relative to their host galaxies than the relationship we see in the nearby universe. This suggests they went through bursts of extremely rapid gas accretion, possibly at or above the Eddington limit, outpacing the growth of stars around them. The more typically sized early black holes, by contrast, appear to have grown more gradually through mergers with other black holes.
The Black Hole and Galaxy Grow Together
One of the most remarkable discoveries in this field is that a galaxy’s central black hole mass is tightly correlated with properties of the galaxy itself. The black hole mass scales with the fourth power of the velocity dispersion of stars in the galaxy’s central bulge. In practical terms, this means a galaxy whose central stars move twice as fast will host a black hole roughly 16 times more massive. This relationship holds across spiral galaxies, elliptical galaxies, and the giant ellipticals found at the centers of galaxy clusters, though with slightly different scaling in each type. Black hole mass also tracks at roughly one-thousandth of the total mass of the galaxy’s central bulge.
These correlations are too tight to be coincidence. The black hole and its galaxy regulate each other’s growth. The mechanism behind this is feedback: energy output from the black hole directly influences how much gas is available to form new stars.
How Black Holes Regulate Their Galaxies
Supermassive black holes don’t just passively sit at the center of a galaxy. When they’re actively feeding, they pump enormous amounts of energy into their surroundings, and this feedback operates in two distinct modes.
The first is the quasar mode, which dominates when the black hole is consuming gas near its maximum rate. Radiation pressure from the superheated gas drives powerful winds outward, pushing raw material away from the galaxy’s core and slowing or stopping star formation. This mode was most common in the early universe, when gas supplies were abundant and black holes were growing rapidly.
The second is the radio mode, which takes over when the black hole is feeding at a trickle, less than 1% of its maximum rate. In this quieter state, the black hole launches narrow jets of material at near light speed. These jets carve bubbles and cavities in the surrounding gas, reheating it and preventing it from cooling and collapsing into new stars. This mode is especially visible in galaxy clusters, where the central galaxy’s jets keep the surrounding hot gas from cooling. Without this reheating, the gas should cool and form stars at rates far higher than observed. In reality, only about 10% of the expected cooling actually occurs, and the radio jets explain the difference.
This feedback cycle is what enforces the tight relationship between black hole mass and galaxy properties. Once a black hole grows massive enough, its energy output becomes powerful enough to blow the remaining gas out of the galaxy’s central region, cutting off both its own fuel supply and the raw material for new stars. The black hole essentially sets its own final mass. Cosmological simulations that leave out this feedback consistently overproduce massive galaxies, generating far more giant galaxies than we actually see in the real universe.
The Milky Way’s Central Black Hole
Our own galaxy hosts a supermassive black hole called Sagittarius A*, with a mass of about 4 million times that of the Sun. By supermassive black hole standards, it’s modest. It’s also remarkably quiet. Less than 1% of the gas that falls within its gravitational influence actually reaches the point of no return. Most of the material gets ejected before it can be swallowed, which makes its X-ray emission extraordinarily faint. This low-activity state is typical of most giant black holes in nearby galaxies. Their most voracious feeding days are long past, and they now exist in a relatively dormant phase, occasionally flaring when a stray gas cloud or unlucky star wanders too close.