If, as astronomers believe, the death of large stars leave behind black holes, there should be hundreds of millions of them scattered throughout the Milky Way galaxy.
The problem is that isolated black holes are invisible.
Following six years of meticulous observations, an international research team has for the first time found evidence of a lone black hole drifting through interstellar space.
The discovery was made by combining observations from NASA’s Hubble Space Telescope with ground telescope data – including the University of Tasmania’s former one-metre Canopus telescope on Mount Rumney, near Hobart
Telescopes can't photograph a wayward black hole because it doesn't emit any light. However, a black hole warps space, which then deflects and amplifies starlight from anything that momentarily lines up exactly behind it. This detection technique is called gravitational microlensing.
University of Tasmania Warren Chair in Astronomy, Professor Jean-Philippe Beaulieu said the isolated black hole was seven times the mass of our Sun and located about 5,000 light years away towards the centre of the Milky Way.
“Black holes have always captivated the imagination of the public and scientists,” Professor Beaulieu said.
“We know that a monster black hole of four million times the mass of the Sun is a resident of the centre of our galaxy, however isolated black holes which are expected to be abundant had eluded astronomers until now.
Some of the ground-based data that revealed this black hole have been obtained as part of an observing campaign a decade ago. At the time we were searching for extrasolar planets, but we were surprised to come back with a black hole.
Black holes roaming our galaxy are born from rare, monstrous stars that are at least 20 times more massive than our Sun. These stars explode as supernovae, and the remnant core is crushed by gravity into a black hole.
Because the self-detonation is not perfectly symmetrical, the black hole may get a kick, and go careening through our galaxy like a blasted cannonball.
Ground-based telescopes, which monitor the brightness of millions of stars in the rich star fields toward the central bulge of our Milky Way, look for a tell-tale sudden brightening of one of them when a massive object passes between us and the star. Using ground-based telescopes, we monitor the most interesting events, and finally identify those potentially revealing a planet or a black hole. If it is a black hole candidate, astronomers then use the Hubble Space Telescope to reveal its nature.
It is expected this will be the first of many such discoveries, with NASA's upcoming Nancy Grace Roman Space Telescope likely to discover several thousand microlensing events, some of which will be black holes, and the deflections will be measured with very high accuracy.
The paper, An Isolated Stellar-Mass Black Hole Detected Through Astrometric Microlensing, was published in The Astrophysical Journal.