Hunting Lone Black Holes Within 50 Light-Years of Earth, Hidden in Gas Clouds

Astronomers think the Milky Way holds between about 100 million and 1 billion black holes left behind by collapsed massive stars, and a few of them may sit in the Sun's immediate neighborhood. A new study in The Astrophysical Journal describes an attempt to find such lone black holes nearby, Universe Today reported. The search turned up five candidates, none of them confirmed.

The key idea is that the best place to look is not empty space but the thin gas clouds the Solar System is passing through.

Why the Milky Way's quiet black holes are so hard to spot

Supermassive black holes at the centers of galaxies are comparatively easy to find. When one actively swallows material, the gas heats up and glows, and the brightest examples, called quasars, can be seen from vast distances. The Milky Way's own supermassive black hole, Sgr A*, is quiet and gives off almost no radiation. Astronomers detected it through its gravity, which speeds up the stars closest to it and shapes their orbits.

Black holes formed from single collapsed stars are far more numerous but more subdued, according to Universe Today. Models predict that many of them are isolated, with no companion star to feed them or give them away.

What the new study looked at

The paper is titled “Searching for Isolated Black Hole Candidates within 15 pc of the Solar System in Gaia DR3.” Its lead author is Abdurakhmon Nosirov of Fudan University in Shanghai, China. The authors say there may be a few black holes within 15 parsecs of the Solar System, a distance of roughly 50 light-years.

The team argues that location matters. If a black hole sits inside one of the Local Interstellar Clouds, which fill 5% to 20% of this local volume, it could pull in gas from the surrounding material and produce emission that current or near-future observatories can detect. The Solar System is moving through one of these clouds, a mass of partially ionized gas, near its edge.

Outside the clouds, the picture changes. The authors say the rate at which a black hole could gather gas would be too low, and the resulting signal well below what current and near-future instruments can pick up.

Another method, tracking how a black hole tugs on nearby stars, is also unlikely to work here. The authors write that the local density of stars is too low for that approach to succeed.

Five candidates, and why they are not confirmed

The researchers turned to the third data release of Gaia, the space observatory that maps the positions of stars. Gaia flagged six sources, but one was excluded because it is a brown dwarf, leaving five. All of them lie close to the Galactic plane, which the authors say makes them likely to be spurious results of Gaia's position measurements, for example from unmodeled background sources or hidden binary companions.

Because Gaia's data can suggest candidates but not confirm them, the team compared the sources with predicted infrared and radio emission. The surveys involved include CatWISE, an infrared catalogue from NASA's Wide-field Infrared Survey, and two radio surveys, RACS and VLASS. RACS has repeatedly observed the radio sky with all 36 dishes of the ASKAP telescope in Western Australia, and VLASS has cataloged 10 million individual radio sources. The authors note large uncertainties in the expected emission from such black holes, which makes predictions difficult.

Why it matters

The study does not claim a discovery, and nothing in the report suggests a danger to Earth. Its value is in narrowing where to look. If black holes really number in the hundreds of millions or more, finding even one that is isolated and nearby would test the models that predict them, and would show whether black holes can be detected through the gas they pull in.

For now the work is a search strategy and a short list of candidates. Whether radio and infrared data can turn any of the five into a confirmed black hole is the question the surveys are meant to answer.

Around Prague, based on reporting by Universe Today