There should be black holes in our neighborhood. Can we find them?

Scientific estimates show that there should be between about 100 million and 1 billion stellar-mass black holes in the Milky Way. But they aren’t easy to locate. They’re not like supermassive black holes (SMBH) at the centers of galaxies. Those are usually easy to spot for different reasons.

When an SMBH is actively accreting material, the material gathers in a rotating accretion ring, where it heats up and emits energetic radiation. These are called active galactic nuclei (AGN). The brightest of them are called quasars, and they can be seen from vast distances.

AGN and quasars emit a pair of powerful astrophysical jets in opposite directions along their axes of rotation, and in some cases one of these jets is pointed straight at Earth. We call these blazars, and they announce their presence with a lighthouse-like beam of energy that can’t be missed.

Other SMBHs are found by their powerful gravity and the effect it has on their surroundings. Sgr A*, the Milky Way’s SMBH, is a quiescent black hole that feeds on only a trickle of matter. It emits almost no radiation. It was detected by the way its gravity accelerates the stars that are closest to it and how that shapes their orbits.

Stellar-mass black holes are much more plentiful, but they’re also more subdued than SMBHs and more difficult to detect. Still, they must be out there, and some should be in our neighborhood. Modeling predicts them and also predicts that they should be isolated.

Research in The Astrophysical Journal presented an effort to find them in Gaia data.

It’s titled “Searching for Isolated Black Hole Candidates within 15 pc of the Solar System in Gaia DR3,” and the lead author is Abdurakhmon Nosirov. Nosirov is from the Center for Astronomy and Astrophysics, Department of Physics, at Fudan University in Shanghai, China.

“Theoretical models predict that the galaxy hosts 10⁸–10⁹ black holes formed from the complete gravitational collapse of heavy stars and that most of these black holes are isolated, without any companion,” the authors write. If the upper estimate is 1 billion, and we can’t find them, what’s stopping us?

“Within 15 pc of the solar system (∼50 lt-yr), there may be a few black holes,” the authors write. “If located inside one of the Local Interstellar Clouds—which occupy 5%–20% of this local volume—an isolated black hole could produce detectable electromagnetic emission via accretion from the interstellar medium, given the capabilities of current or near-future observatories.”

The Local Interstellar Cloud (LIC) is a cloud of partially ionized gas. The solar system is moving through the LIC, near its edge. Other interstellar clouds, like the G-Cloud, are nearby, and collectively they’re referred to as Local Interstellar Clouds (LICs).

But predicting the electromagnetic emissions of these black holes is challenging because of “large uncertainties in the expected accretion spectra,” according to the authors. The situation is different for black holes within the LICs and those outside them.

Outside the LICs, there’s very little for black holes to feed on. “Outside these clouds, the accretion rate would be too low; according to our models, the resulting electromagnetic flux is well below the detection thresholds of current and near-future observational facilities,” the researchers explain. But inside the LICs, the interstellar medium should be dense enough for black hole accretion to be detectable.

Electromagnetic flux is only one way to detect a black hole. The other way is through its gravitational effect on nearby stars, which could be measured with astrometry. Sadly, that won’t work in our neighborhood. “While astrometric detection via gravitational perturbation of nearby stars is conceivable, the local stellar density is too low for this method to be realistically successful,” the authors write.

The researchers turned to Gaia and its Data Release 3 to find potential isolated black holes that are accreting from the ISM, and they found five. (Gaia found six, but one is excluded as a brown dwarf.) Again, astrometry is ruled out as a solution. “All candidates lie close to the galactic plane, making them likely spurious astrometric solutions, for instance caused by unmodeled background sources (crowding) and/or unmodeled binarity,” the researchers explain.

So it came down to their modeled infrared and radio emissions.

Gaia gathered enough spectroscopic data to detect black hole candidates, but not enough to confirm them. But there are multiple catalogs of emission data covering the 15 parsecs surrounding the solar system. CatWISE is one of them, a catalog of infrared sources from NASA’s Wide-field Infrared Survey Explorer.

RACS and VLASS are radio catalogs of the sky. RACS is the Rapid ASKAP Continuum Survey, and VLASS is the Very Large Array Sky Survey. RACS has observed the radio sky repeatedly with the entire 36-dish ASKAP radio telescope, and VLASS cataloged 10 million individual radio sources.

“Although the detection thresholds of the radio catalogs are close to the fluxes predicted by our models, both models agree that, if these sources were black holes accreting from the interstellar medium, they should be detectable in CatWISE,” the researchers write.

Sadly for black hole aficionados, no sources were found that matched the Gaia candidates.

“Our search for infrared and radio emission from these sources further suggests that they are unlikely to be black holes accreting from the interstellar medium,” the authors write.

This effort wasn’t fruitful, but that doesn’t mean the search will end. Other research shows that any isolated black hole that was accreting from the ISM would be very faint and would exhibit a fairly bland spectrum without many features. But that doesn’t mean they’re undetectable. It just shows that they’re likely undetectable with observations from single instruments.

“Multiwavelength follow-up campaigns, covering radio, optical and X-ray bands, are necessary to rule out alternative interpretations and confirm the nature of any candidate,” the authors write.

“Future efforts will focus on searching for isolated black holes accreting from the warm, partially ionized interstellar medium in radio and X-ray catalogs,” the authors write in their conclusion.

In early 2027, the Square Kilometre Array will be releasing another tranche of data. Will it be the data that allows black hole hunters to be successful?

That data “appears especially promising for this endeavor,” the researchers conclude.

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

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Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

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