Don’t let the name fool you. Black holes might all have hearts of pure darkness, but many cloak themselves in rings of fire that blaze like little else in the cosmos.
That doesn’t mean all are detectable. An analysis of galaxies surveyed by a large international consortium of researchers suggests the glow surrounding a third of all supermassive black holes might be hidden by a thick pall of dust and gas.
Taken in context of previous studies that estimate the prevalence of such dusty cloaks, that figure could be as high as 50 percent, leaving a sizable portion of black hole heavyweights challenging to directly detect using current methods.
Black holes are famous for being insatiable concentrations of gravity from which not even light can escape. And if they floated through space all on their lonesome, it might be the end of the story.
It’s now thought most large galaxies in the Universe contain at least one supermassive black hole, the majority of which are probably so bloated with the bodies of consumed stars they’ve sunk to a spot where matter swirls about them like floodwaters around an open drain.
So great is their appetite, the mess of dust and gas circling their boundary accelerates to mindblowing speeds, radiating at high energies that signify their presence far across the Universe.
Seen from above or below, supermassive black holes stand out to our telescopes as donuts shining in X-rays. From the edge, however, dust and gas on the very fringes of the glowing torus hides the light, restricting our view of the galaxy’s blazing nucleus.
That means we’ve been left to guess at just how many galaxies have active supermassive black holes inside them – counting what we see in surveys of the sky is certain to leave out those facing in a direction that leaves them hard to see. By some reckoning, as few as 15 percent of supermassive black holes are hidden behind dust clouds.
In the early 1980s, NASA sent its Infrared Astronomical Satellite (IRAS) into orbit, where for 10 months it mapped the warm glow of the entire sky. It happened to also image dust heated by X-rays, providing astronomers with their first true sense of what they might be missing.
While many were undoubtedly signs of supermassive black holes, others were dust clouds cooked by other forms of cosmic activity such as intense periods of star formation.
Now researchers have put another NASA satellite to work on the problem. Called the Nuclear Spectroscopic Telescope Array, or NuSTAR, it’s an X-ray observatory capable of detecting X-rays so energetic, they can seep right through the surrounding clouds.
Guided by IRAS, the researchers focussed NuStar on a list of target galaxies within roughly 50 million light-years of the Milky Way, sorting them based on the spectra of light each emitted.
“It amazes me how useful IRAS and NuSTAR were for this project, especially despite IRAS being operational over 40 years ago,” says Peter Boorman, a Caltech astrophysicist who led the study.
“I think it shows the legacy value of telescope archives and the benefit of using multiple instruments and wavelengths of light together.”
Based solely on their analysis, between a quarter and nearly a half of black holes churning away in the hearts of galaxies are positioned in a way that rings of material obscure a good fraction of their maelstrom’s light.
Knowing the proportions of supermassive black holes that have suitably thick larders of matter to feed upon makes a significant difference in models seeking to explain their extraordinary growth rates.
“If we didn’t have black holes, galaxies would be much larger,” says study co-author Poshak Gandhi, an astrophysicist at the University of Southampton in the UK.
“So if we didn’t have a supermassive black hole in our Milky Way galaxy, there might be many more stars in the sky. That’s just one example of how black holes can influence a galaxy’s evolution.”
This research was published in The Astrophysical Journal.
