A New Method Has Revealed the First 'Silent' Black Hole Lurking Beyond Our Galaxy
The stealthiest monsters are often the most interesting.
And most stellar-mass black holes are quiet monsters, floating invisibly through the big abyssal depths of space, showing no sign except the bending of light via photons that stray too close. This has forced astronomers to seek alternative means of detecting them, like stars that appear to be locked in a strong binary orbit with what appears to be nothing at all.
And, for the first time, astronomers have successfully identified a black hole beyond our galaxy using this unconventional technique, according to a recent study published in the journal Monthly Notices of the Royal Astronomical Society.
This could become a crucial step in revealing the evolution of black holes within and without our Milky Way.
How to spot a stealthy stellar-mass black hole
The suspicious movements of an orbiting star have revealed a comparably small black hole within the Large Magellanic Cloud, which is a dwarf galaxy in orbit around ours, roughly 160,000 light-years away. Called NGC 1850, the black hole was found in a star cluster called NGC 1850 (celestial cartography is a logical practice), which contains thousands of stars. This recent detection hints that the method might be crucial in the search for black holes within highly-populated star clusters, both within and beyond our enormous Milky Way. "Similar to Sherlock Holmes tracking down a criminal gang from their missteps, we are looking at every single star in this cluster with a magnifying glass in one hand trying to find some evidence for the presence of black holes but without seeing them directly," said Sara Saracino, an astrophysicist at the U.K.'s Liverpool John Moores University, in a report from Science Alert.
"The result shown here represents just one of the wanted criminals, but when you have found one, you are well on your way to discovering many others, in different clusters," added Sarecino. The majority of black holes cataloged so far beyond our Milky Way were easy to spot, because they're hurling unconscionable volumes of deadly radiation, which means they are actively sucking in indescribable scales of material, which are the real source of the radiation (since the black holes themselves give practically nothing away). Astronomers have identified more black holes via gravitational waves since the first ones were detected in 2015. This is when subtle ripples in the very fabric of space-time are cast out in our direction in the aftermath of a violent collision of two black holes. But despite all our progress, these mapped black holes don't even comprise the tip of the cosmic iceberg.
Baby black holes lie ahead
There might be 100 million stellar-mass black holes in our galaxy alone. Obviously, we have a lot more counting to do. And this also means we have a lot to learn about these seemingly malevolent maws in the ancient depths of the darkest corners of the galaxy. But we don't have to look them in the proverbial face to understand their properties, since the things they take with them, like gravitationally trapped stars, will give away their secrets by the way they move.
From hundreds of thousands of light-years away, these stars look like they're stationary. But the light of the stars themselves will change, its wavelength stretching and compressing as the solar furnace moves closer and farther away from us. And then we know they're in the grasp of a black hole. Continuing to study black holes within young star clusters might reveal more about how colossal stars and neutron stars are forged into the black holes we know and fear. And, since many of these star clusters are very young — NGC 1850 is only 100 million years old — there is a possibility of discovering young black holes, which would provide a unique window into their complex and haunting evolution.