Astronomers have had trouble seeing the biggest monsters in the universe: supermassive black holes, which lurk at the centers of galaxies and can grow to a monstrous size – billions of times the mass of our sun. Now we finally have a clue about why.
Supermassive black holes should emit some light when they eat stardust and other matter, but a new study suggests what they burp out is blocking the view of optical telescopes.
Giant black holes are hard enough to see to begin with, since not even light can escape their surface. They’re also surrounded by a flattened “accretion disk” – a graveyard of dead stars, planets, asteroids, comets, gas, and dust that the black hole is dragging in.
Although matter that falls into a supermassive black hole should emit visible light, and we should easily see it with telescopes, this hasn’t been the case; thus, astronomers suspected accretion disks might be what’s obscuring their view.
But according to a new study in Astrophysical Journal Letters, a draft of which is published on arXiv, supermassive black holes are actually cloaking themselves in a doughnut-shaped cloud of dead star debris that they spit out.
A team of researchers made the discovery by aiming the Atacama Large Millimeter/submillimeter Array (ALMA) at the center of a galaxy called NGC 1068, which is 47 million light-years distant from Earth.
Here’s what they saw in radiowaves, which can penetrate and reveal thick clouds of gas and dust:
The green blob at the center is the gas and dust around the galaxy’s dark heart: a black hole 10 million times more massive than the sun.
Green indicates the material picked up by ALMA is rotating around the supermassive black hole; blue is stuff moving toward us; red is stuff moving away.
What this image shows, in effect, is that NGC 1068’s enormous dark heart is blowing out a fountain of cloudy engine-like “exhaust” to conceal itself – at a breakneck speed of 2 million mph.
“These clouds are traveling so fast that they reach ‘escape velocity’ and are jettisoned in a cone-like spray from both sides of the disk,” Jack Gallimore, an astronomer at Bucknell University in Lewisburg, Pennsylvania, said in a press release issued by the National Radio Astronomy Observatory (which helps run ALMA). “With ALMA, we can for the first time see that it is the gas that is thrown out that hides the black hole, not the gas falling in.”
Electrons are ripped off the fountain of dead star as it leaves, allowing the gigantic black hole’s magnetic field to sculpt it into a tire- or doughnut-like torus shape.
That veil then absorbs the visible light that astronomers would like so very much to see, since it could reveal as-yet-unverified (or unknown) properties of black holes.
NRAO says astronomers hope to figure out the “fuel budget” of the black hole – how much stuff is going in, and how much is being spewed out.
Despite a the self-cloaking behavior of supermassive black holes, scientists are confident they can pierce the veil (using radiowaves emitted by black holes) and take a picture of one with the Event Horizon Telescope.
That ground-based mission hopes to enlist the help of a dozen radio telescope arrays to form a roughly Earth-size antenna sometime in 2017.