In our Galaxy, there should be an estimated 100 million to 1 billion stellar-mass black holes - those that form when heavy stars collapse. But they are much harder to spot than the giant black holes at the centers of galaxies. Those often give themselves away with bright radiation: matter falls onto them, heats up and shines. Such supermassive objects can also emit powerful jets, and if one of them is pointed at Earth, we see a bright beacon.
Stellar-mass black holes are far more modest. Most of them, models predict, should be solitary, without companion stars. And some of these objects may be relatively close to us.
A group of researchers led by Abdurakhmon Nosirov of Fudan University in Shanghai (China) decided to search for isolated black holes in data from the Gaia space telescope. The work is published in The Astrophysical Journal. The scientists examined a region within a radius of 15 parsecs of the Solar System - about 50 light-years.
According to the authors' calculations, this volume may contain several black holes. If such a hole is inside one of the local interstellar clouds - clumps of partially ionized gas - it could be fed by matter and produce noticeable radiation. But outside the clouds there is too little gas, and the flux from such a hole, according to the models, will be below the sensitivity threshold of current and near-future telescopes.
Another way to spot a black hole is to measure how its gravity affects the motion of nearby stars. However, in the vicinity of the Sun there are too few stars for this method to work.
The researchers then turned to the third Gaia data release (DR3) and selected five candidates. However, all of them lie close to the Galactic plane and, according to the authors, are most likely false astrometric solutions - that is, measurement errors caused, for example, by an unaccounted background or binary stars.
The scientists checked whether there is infrared or radio emission at the locations of these candidates. To do this, they used the CatWISE catalog (an infrared survey), as well as the RACS and VLASS radio surveys. None of the sources coincided with the Gaia candidates. This, according to the authors, makes it unlikely that we are dealing with black holes fed by interstellar matter.
The failure does not mean the search will stop. The researchers note that to rule out other explanations and confirm the nature of the candidates, observations in several ranges at once are needed - radio, optical and X-ray. In the future, they plan to search for isolated black holes using radio and X-ray catalogs. The authors pin particular hopes on new data from the Square Kilometre Array radio telescope, expected in early 2027.