A cosmic census triples the known number of black holes in dwarf galaxies

A colossal census of the cosmos has more than tripled the number of active black holes known to inhabit miniature galaxies and found the largest number of medium-mass black holes to date.

The survey found about 2,500 dwarf galaxies with black holes actively feeding at their centers, up from about 500 previously known, the researchers report in a paper submitted Oct. 31 to arXiv.org. The team also found about 300 new intermediate-mass black hole candidates, an increase from about 70 possible previous discoveries. (SN: 9/2/20).

That’s enough to start studying these black holes as a group, rather than in isolation, says astronomer Ragadeepika Pucha of the University of Utah in Salt Lake City. Such large-scale studies could solve many mysteries about how black holes and their galaxies evolve together.

Every large galaxy appears to center around a supermassive black hole. These giants are more than a million times larger than the sun. But astronomers aren’t sure how black holes got there.

“It’s a bit of a chicken-or-egg question between galaxies and black holes,” says co-author Stéphanie Juneau, an astronomer at NOIRLab in Tucson. “Which was formed first? Do they control each other?”

Dwarf galaxies and medium-sized black holes may be the best places to look for answers. These runs are thought to represent the earliest stages of galaxy and black hole growth, objects that have largely escaped mergers with other black holes and galaxies over cosmic time.

“To get a complete picture of galaxy formation and evolution, we need to understand how these small galaxies evolve and grow,” says Pucha.

Pucha and her colleagues turned to early data from a project at the Mayall Telescope in Arizona to create a 3-D map of the universe. In its first year alone, the Dark Energy Spectroscopic Instrument, or DESI, observed about 1.5 million galaxies (SN: 4/4/24). The team searched some of that collection for dwarf galaxies that emit wavelengths of light associated with the hot disk of gas and dust swirling around an actively feeding black hole.

About 2 percent of the nearly 115,000 dwarf galaxies in the study emit such light, the team found. Previous surveys detected active black holes in only 0.5 percent of dwarf galaxies.

The team also determined the masses of black holes in more than 4,000 galaxies, dwarf and non-dwarf. He then looked for intermediate-mass black holes, with masses between 100 and one million times that of the Sun. (SN: 2/8/17).

“These are very important because they tell us how the first black holes in the universe formed,” says Pucha. If black holes start out small and grow by merging with other black holes, then the universe should be filled with these average weights that represent the stages between growth stages.

Pucha and colleagues found about 150 confident detections in the DESI data and roughly the same number of attempts, a yield that suggests the first black holes in the universe were relatively easy. But the James Webb Space Telescope, or JWST, has also discovered surprisingly massive black holes in the very early universe (SN: 11/1/24). Pucha says this could simply mean that JWST has yet to see the earliest black holes in the universe.

This survey probably only found the tip of the iceberg. The full first year of data is expected to be released in March or April 2025 and will contain many more galaxies.

Plus, there are definitely more black holes to find. “They’ve found all the ones that are really spotlights,” says astronomer Mallory Molina of Vanderbilt University in Nashville, who is not part of the DESI project. “Even with the simplest detection tool we have, [the researchers are] still finding a large number in this poll. There is so much more to explore.”