Dec 06, 2023

Weird black holes may reveal secrets of the early universe

Big black holes may not always sit in the hearts of massive galaxies. Recent observations (and computer models) suggest that some are found at the centers of dwarf galaxies.

Alfred Pasieka/Science Photo Library/Getty Images Plus

By Ashley Yeager

11 hours ago

A black hole at the heart of our galaxy is a hungry monster. This super dense pit of matter weighs as much as 4 million suns. The pull of its gravity is so strong that it has swallowed nearly everything around it. And it gets heftier and heftier the more it eats. In that sense, the Milky Way’s central black hole is a lot like the mythical Kammapa of the Sotho people of southern Africa.

But it’s not alone.

Similar supermassive black holes sit at the centers of nearly all known galaxies. Each one has a mass that’s thousands, millions — sometimes even billions — of times as great as our sun’s. For decades, scientists thought that such cosmic Kammapas could only be found in big galaxies. After all, only massive galaxies were thought to have enough matter to feed these beasts.

How wrong scientists were.

About two decades ago, computer models of the earliest black holes started turning up oddities. In these models, big black holes showed up where they weren’t expected. Many scientists thought those misfits were flukes. But others weren’t so sure. They thought such oddballs should not be ignored.

Now, telescopes have turned up signs of several weirdo black holes. These include massive ones in the tiniest galaxies. Some of these black holes, surprisingly, don’t even seem to sit at their galaxies’ core. Even more intriguing, astronomers have spotted evidence of black holes wandering along the edges of their galaxies. In rare cases, some look like they’re being kicked out of their galaxies altogether.

But perhaps these black holes aren’t just weirdos. Maybe, in fact, they’re big players in the story of our universe. If so, they could help probe one of the greatest mysteries of all. How did these cosmic Kammapas develop?

Our current understanding of how black holes get so big goes something like this. In the early universe, the cores of baby galaxies had baby black holes. Over time, galaxies grew, smashed into each other and merged. As they did so, the galaxies took on gobs of new stars, gas and dust. Meanwhile, the black holes in their centers merged and fed on all the new material.

As a result, as galaxies grew, so did their central black holes.

In this scenario, the black hole at the center of each galaxy should be around one thousandth of the mass of its galaxy.

Not all of today’s galaxies are as big as our Milky Way. Some “dwarf galaxies” have only about a trillionth its mass. In order to have stayed so small, they must not have merged with many other galaxies. Each dwarf galaxy’s black hole would likely have escaped merging with many other black holes.

By that logic, the black holes in dwarf galaxies should be runts — or nonexistent.

But computer models in the late 2000s started to cast doubt on this logic.

Those models showed how massive black holes could have evolved over the history of the universe. Even the smallest galaxies, they showed, could have large black holes — and right away. Some of those galaxies never grew or merged with any others. This left the galaxies small with big black holes for billions of years.

Astronomer Amy Reines found the first hints that such galaxies exist. More than a decade ago, she was looking through telescope data on a dwarf galaxy 30 million light-years from Earth. There, she spied extreme amounts of radio light and bright X-rays. That intense radiation signaled the presence of a huge black hole.

“I hadn’t seen this before,” recalls Reines, who now works at Montana State University in Bozeman. Like other scientists, she had assumed dwarf galaxies weren’t big enough to have big black holes.

But a few months later, in 2011, Reines went to a meeting of scientists. There, she heard astrophysicist Jillian Bellovary speak. Bellovary presented some new models of galaxy formation. These suggested that even scrawny galaxies could have hefty black holes.

Reines was shocked to find her observations and Bellovary’s models seemed to match.

Not long after that, Reines launched an effort to find more dwarf galaxies with outsized black holes. She is part of a team that scanned roughly 25,000 dwarf galaxies. Of those, 151 seemed to harbor a big black hole. The team shared its findings in 2013.

If models like Bellovary’s were right, the observed dwarf galaxies with big black holes most likely formed that way in the early universe. Since then, they’ve been relatively untouched by mergers.

If true, looking at these ancient relics might tell us something about the very first generation of black holes to exist. That includes those that eventually grew into today’s supermassive monsters, such as the one at the center of the Milky Way.

For instance, the mass of a black hole in some dwarf galaxy should be similar to the mass of some of the earliest black holes. So looking at black holes in dwarf galaxies could help astronomers figure out the starting mass of the original black holes. That, in turn, could help scientists tease out how those first black holes formed.

Right now, there are two leading ideas for how the first black holes developed. Each idea creates black holes of different masses.

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The first idea has black holes forming as the first stars collapsed. This process would tend to create pretty lightweight black holes. The other idea: Those first black holes could have formed from giant gas clouds that collapsed in on themselves. Those black holes should start out heavier.

The dwarf galaxy that Reines observed seems to have a mass equal to a few million suns. That’s pretty heavy. It’s one point in favor of the cloud-collapse idea. But it’s just one data point. And measuring the mass of black holes is not easy.

Luckily, there are other clues scientists can use to learn how the first black holes formed. These hints come from an even odder type of black hole: massive ones that don’t sit right at the centers of their dwarf galaxies.

Remember Bellovary’s computer models of the earliest black holes? Big black holes in scrawny galaxies weren’t the only surprise her models turned up. Those models also suggested that some of those large black holes wandered near the edges of dwarf galaxies.

“I always like to think about the outliers, or the weird little rejects,” says Bellovary. She’s now based at Queensborough Community College in New York City.

Bellovary reran her models. She zoomed in on the littlest galaxies. When she did, half of the massive black holes in dwarf galaxies would be off-center. She and her colleagues shared this finding in 2019.

As if on cue, a few months later Reines had data to support those models. Her team’s observations came from radio telescopes in New Mexico known as the Very Large Array. The researchers peered at 111 dwarf galaxies. Thirteen most likely had big black holes. Of those 13, a few seemed to sit off-center from their galaxies’ cores.

Finding wanderers was a jackpot. “Once a black hole starts wandering, it does not grow in mass anymore,” says Marta Volonteri. That means stray black holes should match the starting masses of the first black holes even more closely than those at the centers of dwarf galaxies. Volonteri is an astrophysicist. She works for the Institute of Astrophysics in Paris, France. It’s at Sorbonne University.

Sadly, weighing wandering black holes is even harder than weighing ones at their galaxies’ cores. That makes it difficult to judge whether they match the star-collapse or cloud-collapse theory, based on their weight. But the overall number of wanderers can offer hints about how they formed.

If the earliest black holes formed from huge gas clouds collapsing, then wanderers should be pretty rare in dwarf galaxies. That’s because converting a gas cloud into a black hole is quite hard. So that should make it a rare event. If the first black holes came from collapsing stars, though, they should be more common. The reason is that stars can collapse into black holes pretty easily.

Signs of wanderers keep popping up. That has researchers leaning away from the cloud-collapse idea. But scientists don’t just want to know how the first black holes formed. They also want to know how some of them grew to become among the most massive black holes.

To find out, astronomers must look to a different type of cosmic weirdo.

Dwarf galaxies aren’t the only ones with wandering black holes. Some massive galaxies appear to have them too. And some of those wanderers — or rogues — seem to be flying across their host galaxies at 10 times the speed of wanderers in dwarf galaxies.

Such sprinting rogues turned up in models, such as Bellovary’s. Several years ago, the Hubble Space Telescope and other observatories also saw evidence of a huge black hole getting booted to the edge of its galaxy. Earlier this year, Hubble and the Keck Observatory might have seen the aftermath of a trio of interacting supermassive black holes. One of them seemed to have been kicked out of its galaxy.

Not all scientists have accepted those data as evidence that rogue black holes exist. But if they do, rogues could help explain how today’s biggest black holes grew up.

Imagine that astronomers find many slow-moving wanderers. Those black holes probably haven’t interacted with any others. Otherwise, they would have picked up an extra zing of speed. If astronomers find many black holes that haven’t interacted with others, that might mean interactions between big black holes are rare. It would follow, then, that mergers between big black holes are rare. And that would put a damper on scientists’ current idea that supermassive black holes grew through repeated mergers.

Now imagine the opposite. Lots of supermassive black holes are being shot out from the centers of their galaxies. That would suggest black hole interactions are pretty common. In turn, black hole mergers would likely be common, too. That would support the current merger theory of how the biggest black holes came to be.

The complete story of cosmic Kammapas remains a mystery. So far, scientists have little to go on — only a few dozen possible oddballs in dwarf galaxies and a few possible far-flung rogues. More data would help clear things up. Luckily, more astronomers have joined the search.

Future observatories may aid in the hunt, too. The Vera C. Rubin Observatory is supposed to turn on next year. That telescope in Chile could sweep the skies looking for wanderers. And the Laser Interferometer (In-tur-fur-AAHM-eh-tur) Space Antenna, or LISA, will try to detect massive black-hole smashups.

Time and new technology may someday tell all. For now, oddball black holes spark our imaginations. They prompt us to ask big questions and to look for evidence that may help us better understand our cosmos’ history.

With each discovery, you can’t help but wonder: What else is hidden out there? Perhaps there are other oddities not yet discovered that could help explain the early universe, Bellovary says. But only if we’re willing to chase the misfits and their stories.

antenna: (plural: antennae or antennas) (in physics) Devices for picking up (receiving) electromagnetic energy.

array: A broad and organized group of objects. Sometimes they are instruments placed in a systematic fashion to collect information in a coordinated way.

astronomer: A scientist who works in the field of research that deals with celestial objects, space and the physical universe.

astrophysicist: A scientist who works in an area of astronomy that deals with understanding the physical nature of stars and other objects in space.

black hole: A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.

colleague: Someone who works with another; a co-worker or team member.

computer model: A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.

core: Something — usually round-shaped — in the center of an object.

cosmic: An adjective that refers to the cosmos — the universe and everything within it.

galaxy: A group of stars — and usually invisible, mysterious dark matter — all held together by gravity. Giant galaxies, such as the Milky Way, often have more than 100 billion stars. The dimmest galaxies may have just a few thousand. Some galaxies also have gas and dust from which they make new stars.

gravity: The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

laser: A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.

light-year: The distance light travels in one year, about 9.46 trillion kilometers (almost 6 trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.

mass: A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

matter: Something that occupies space and has mass. Anything on Earth with matter will have a property described as "weight."

Milky Way: The galaxy in which Earth’s solar system resides.

model: A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes. Or an individual that is meant to display how something would work in or look on others.

observatory: (in astronomy) The building or structure (such as a satellite) that houses one or more telescopes. Or it can be a system of structures that make up a telescope complex.

outliers: Events or cases that fall outside some normal range. That makes them unusual and may make them seem unlikely or suspicious.

radiation: (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

radio: Referring to radio waves, or the device that receives these transmissions. Radio waves are a part of the electromagnetic spectrum that people often use for long-distance communication. Longer than the waves of visible light, radio waves are used to transmit radio and television signals. They also are used in radar. Many astronomical objects also radiate some of their energy as radio waves.

relic: Something that is a leftover from an earlier time.

rogue: An animal that wanders alone, outside its herd or the community into which it was born. Or anything, even a planet or galaxy, that unexpectedly travels alone and far from where it would be expected.

star: The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become hot enough, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

sun: The star at the center of Earth’s solar system. It is about 27,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.

technology: The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.

telescope: Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.

theory: (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.

universe: The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).

X-ray: A type of radiation analogous to gamma rays, but having somewhat lower energy.

Meeting: R. Hickox et al. Uncovering a hidden mini-monster: A heavily obscured AGN in a dwarf star-forming galaxy. Bulletin of the AAS, Vol. 54. Published April 2, 2022.

Journal: M. Mollina et al. A sample of massive black holes in dwarf galaxies detected via [Fe x] coronal line emission: Active galactic nuclei and/or tidal disruption events. ApJ. Vol. 922, December 2021, p. 155. doi: 0.3847/1538-4357/ac1ffa.

Journal: J. Bellovary et al. Multimessenger signatures of massive black holes in dwarf galaxies. MNRAS, Vol. 482, January 2019, p. 2913. doi: 10.1093/mnras/sty2842.

Journal: A. Ricarte and N. Priyamvada. The observational signatures of supermassive black hole seeds. MNRAS, Vol. 481, December 2018, p.3278. doi: 10.1093/mnras/sty2448.

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Meeting: J. Bellovary et al. The formation and evolution of massive black holes in cosmological simulations. American Astronomical Society, AAS Meeting #217, Bulletin of the American Astronomical Society, Vol. 43, January 2011.

Journal:​ M. Volonteri, G. Lodato and P. Natarajan. The evolution of massive black hole seeds. MNRAS, Vol. 383, Jan. 21, 2008, p. 1079. doi: 10.1111/j.1365-2966.2007.12589.x.

Journal: M. Volonteri, F. Haardt and P. Madau. The Assembly and Merging History of Supermassive Black Holes in Hierarchical Models of Galaxy Formation. ApJ. Vol. 582, January 2003, p. 559. doi: 10.1086/344675.

Ashley Yeager is an editor who helps writers share stories about all sorts of cutting-edge science. Ashley enjoys hiking with her dogs, swimming and reading. She is fascinated by the stars and the stuff between them — so much so she wrote a book about dark matter, that mysterious substance that pervades the universe.

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