Black holes are, by their very nature, difficult to see. Some, cloaked in clouds of dust, are even harder to find.
But now a new study could have pinned down exactly how many black holes lie hidden from our view.
More on black holes
Black holes in galaxy cores
Most large galaxies are thought to house a supermassive black hole with the mass of a billion Suns at their centre.
The powerful gravitational pull of these black holes allows them to capture gas and dust, shrouding themselves with it.
Close to the black hole, friction superheats this gas, causing it to glow incredibly brightly, while a cooler doughnut or torus surrounds it.
When face-on to Earth, the bright central region is clear through the doughnut’s hole, but when positioned edge-on to Earth, the dusty torus blocks the view.
Fortunately for astronomers, absorbing this light heats the torus, causing it to glow in the infrared, meaning edge- and face-on black holes are equally bright in these wavelengths.
Black hole hunting
A team of researchers examined data taken by NASA’s Infrared Astronomical Satellite (IRAS) in 1983 and identified hundreds of potential black holes.
These were then observed with the NuSTAR X-ray space observatory, looking for radiation generated by the hottest gas near the black hole.
While X-rays of a lower energy are absorbed by the torus, more energetic X-rays can pierce the clouds, allowing the team to differentiate between face-on and edge-on black holes.
"It amazes me how useful IRAS and NuSTAR were for this project, especially despite IRAS being operational over 40 years ago," says study lead Peter Boorman, an astrophysicist at Caltech in Pasadena, California.
A third of black holes are hiding
The study found that around 35% of black holes are heavily obscured, a marked increase from previous measurements which found just 15% were hidden.
The ratio of hidden to unobscured black holes helps astronomers understand how black holes might have formed.
The current theory is they grow by consuming material surrounding them and, as such, you would expect a significant number to be obscured by a torus.
It’s estimated that around half of all supermassive black holes should be hidden.
While this is greater than the value measured, it’s close enough that no one’s returning to the drawing board to rewrite their theories just yet.
