There should be 100 million black holes in our galaxy, the vast majority of them with a mass comparable to the Sun. But the fact that no light escapes them makes it difficult to find them. Usually, they are spotted through mergers or interactions with stars, but now astronomers have found one all alone; the first-ever truly isolated black hole.
In a paper submitted to the Astrophysical Journal and waiting for peer review, astronomers describe the discovery of the solitary black hole roughly 5,000 light-years away thanks to the phenomenon of astrometric microlensing.
Gravity warps space-time. The denser an object, the stronger its gravitational force is, and when the object in question is as dense as a black hole this warp is so incredible that it acts as a lens, magnifying and distorting any background light source behind it. This is how some objects are found, but so many light-years away, the effect is small and the light and position of the star have to be known extremely well.
The team used the Hubble Space Telescope and ground observatories to study a star located in the direction of the center of the Milky Way. The precise measurements of the object, which took six years, showed that the high-magnification microlensing event currently named MOA-2011-BLG191/OGLE-2011-BLG-0462 must have been created by a foreground object of high density.
The team estimates that the object weighs roughly 7.1 solar masses, give or take 1.3 Suns. The team also demonstrated that the object emits no light. The estimated mass is higher than what is possible for a neutron star or white dwarf and the lack of electromagnetic emission points at an obvious and exciting culprit: a solitary black hole moving through the Milky Way.
Actually, it’s not just simply moving, orbiting like the rest of the star system and nebulae around the galaxy's central core. It is speeding through our galaxy with at least an extra 45 kilometers (28 miles) per second compared to stars at the same distance. That’s a transverse velocity so its actual proper motion might be different.
But even with the limitations of the measurements, this velocity tells us something. The supernova explosion that created this black hole must have given it a powerful kick, sending it flying through the galaxy.
In the paper, the team stresses that while it will be difficult to follow up the object, there is a chance that it might appear in deep X-rays or radio observations. More excitingly, when the next generation observatories, like the Vera C Rubin Observatory and Nancy Grace Roman Space Telescope, come online over the coming decade, they should bring detections of more microlensing events and some of them might be other solitary black holes.
Understanding the population of these camera-shy objects in our galaxy will become significantly easier.