Wandering Black Holes Found in Dwarf Galaxies Less Than a Billion Light Years from Earth

Scientists are hoping to find out how supermassive black holes with masses millions or billions of times larger than the sun came into being—and came across black holes "wandering" dwarf galaxies in the process.

The science of black hole formation is obscured by billions of years of growth driven by "mergers" (i.e. the collision of two galaxies to form one bigger galaxy). What "seed" black holes might exist in the Universe are too far away to offer much in the way of clues to black hole formation.

And so to better understand how they may have originated, a team of astronomers identified 13 dwarf galaxies within a billion light year radius of the Earth that they say "almost certainly" harbor a black hole.

Their findings have now been presented at the American Astronomical Society's meeting in Honolulu, Hawaii, (4-8 January, 2020) after being published in pre-print last year. The paper is awaiting peer review.

"This work has taught us that we must broaden our searches for massive black holes in dwarf galaxies beyond their centers to get a more complete understanding of the population and learn what mechanisms helped form the first massive black holes in the early Universe," co-author Amy Reines, an assistant professor at Montana State University, said in a statement.

Black hole
The first ever image of a black hole, captured on April 10, 2019 by the Event Horizon Telescope. Scientists have found "wandering" black holes in dwarf galaxies. National Science Foundation/Getty

With masses approximately 100 times smaller than the Milky Way, dwarf galaxies are thought to be some of the smallest in the Universe to store black holes.

The black holes themselves have been cautiously estimated to have masses that are around 400,000 times the size of the sun. To put it into perspective, the supermassive black hole at the center of our galaxy is thought to contain the mass of some 4.6 million suns.

Over a hundred dwarf galaxies were identified from the NASA-Sloan Atlas (a galaxy catalogue) and observed using high-res images from the Karl G. Jansky Very Large Array (VLA). Reine and colleagues then whittled down the list to the 13 they found had the strongest evidence for hosting massive black holes actively consuming the material around them.

Dwarf Galaxy
An illustration of a dwarf galaxy with its shape distorted, most likely by a past interaction with another galaxy, and a massive black hole in its outskirts. NRAO/AUI/NSF/Sophia Dagnello

In approximately half of the galaxies studied, the black hole was not at the center in a well-defined nuclei. Many were significantly off-from-center. This is a positioning that is highly unusual in larger galaxies like the Milky Way but more common in dwarf galaxies. In fact, according to the study's authors, as many as half of massive black holes in dwarf galaxies are considered "wandering" and are located on the outskirts of the galaxy.

This could suggest that the galaxies had merged with another at some point in time, knocking the black hole off kilter.

"We hope that studying them and their galaxies will give us insights into how similar black holes in the early Universe formed and then grew, through galactic mergers over billions of years, producing the supermassive black holes we see in larger galaxies today, with masses of many millions or billions of times that of the sun," said Reines.

The paper is still waiting to complete the peer-review process, the vetting process for scientific studies.

The current consensus among astronomers is that all galaxies equal to or bigger than the Milky Way contain a supermassive black hole at their center. These are millions, if not billions, the mass of the sun.

But there are also countless stellar black holes, just 10 to 24 times the mass of the sun. According to NASA, there may be ten million to a billion of these in the Milky Way alone.

There also appear to be intermediate-sized black holes, which sit somewhere between.

A lot about the formation of black holes—including why they may exist in such different sizes—remains a mystery.

"Currently the most likely situation is that a 'small' seed black hole formed, though by what method is unclear, that then grew from thousands or hundreds of thousands of times the mass of our sun into the billion or more times that they are today," Gregory Brown, an astronomer at the Royal Observatory Greenwich, who was not involved in the latest study, told Newsweek. "Determining how the initial seeds formed is one of the larger tasks in the field today."

Current technology is not advanced enough to allow astronomers to observe how black holes formed in the early universe. Research like this use nearby dwarf galaxies and their black holes as "proxies," said Brown. "It certainly makes a lot of sense. Being nearby they are easier to study—just about easy enough for current telescopes."

What's more, the black holes in these galaxies are thought to be less evolved than those in nearby large galaxies.

"However, this research does show an immediate problem. Of the galaxies they study, about half have non-central black holes and many are irregular, indicating mergers or interactions. Galaxies can occasionally collide in space, as our own Milky Way galaxy will with the Andromeda galaxy in a few billion years time," said Brown. "This is one of the main methods for galaxy growth, but it also can cause their black holes to merge.

"The question then is: are dwarf galaxy black holes truly representative of pristine black hole seeds? This research shows very clearly that while the idea has promise, we must be able to account for those dwarf galaxies that are not as untouched as we would like for this analysis."

"In the local universe we observe a wide range of different galaxy types, for example, grand design spirals or smooth red ellipticals. One of the biggest outstanding questions in astronomy is why galaxies end up with the properties that they do. It is thought that supermassive black holes that reside in the center of all massive galaxies play a key role in determining what galaxies ultimately end up looking like, by pumping lots of energy into the gas inside the galaxies and either expelling it or preventing it from forming more new stars," Vivienne Wild, an astrophysicist at St. Andrews University, Scotland, who was not involved in the study, told Newsweek.

"What's exciting about this result is that a very high fraction of the black holes that the team have found in these small galaxies are not actually in the centers of the galaxies."

"Such off-center black holes may have a far greater impact on their host galaxies than central black holes."

However, the sample is biased towards galaxies, which are bright at radio wavelengths, she added. "The next steps are to obtain confirmation that the radio emission is indeed due to black holes...and also to study a more complete sample of galaxies to see how common these wandering black holes really are."

This article has been updated to include quotes from Gregory Brown and Vivienne Wild, more information on the peer review process and more information on black holes.