by An active black hole is one of the greatest wonders of the cosmos.
A dense, invisible object that may be billions of times the mass of our Sun is surrounded by a massive, churning disk and torus of material, burning light as it spirals toward the center of the black hole. But how big are these buildings?
First, a faint detection of near-infrared light reveals the exterior of a massive accretion disk surrounding a supermassive black hole a hundred million times the mass of our Sun, in a galaxy called III Zw 002 about 1.17 billion light-years away.
This discovery, led by astronomer Denimara Dias dos Santos of the National Center for Space Research in Brazil, revealed the accretion disk to about 52 light-days from the black hole. This measurement will allow a better understanding of how large black holes feed.
The material around a black hole is difficult to reproduce. Despite their size and the sheer brightness of their surroundings, the distances between us and their galaxy mean they are still too small to resolve much detail.
Unable to image the instrument directly, light taken from the surrounding galaxy is analyzed for specific signatures indicating the presence of an accretion disk.
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One such signature is what is known as a double peak in the emission spectrum. This happens as a result of rotation. Emission is the light emitted when an excited atom loses energy; that energy appears as radiation, with wavelengths that depend on the atomic composition.
Now, imagine an accretion disk around a black hole like a record on a turntable. Part of the disc moves towards you; the other is moving. The part of the disk moving towards us pushes the light so that the wavelengths are shorter; while the moving surface stretches.
This means that the emission of a substance appears at two wavelengths, producing a double peak in the spectrum.
Double peaks around supermassive black holes have been detected before, but the first detection is from the area closest to the black hole, known as the narrow-line region. This does not give much information about the total size of the expansion disk.
Dias dos Santos and his colleagues found two double peaks – and both were not from the region of the narrow line, but far from the black hole, in the area known as the wide line region of the accretion disk.
This is the first detection of double peaks in the broad-line region, and the first detection of double peaks using a near-infrared instrument.
Astronomer Alberto Rodriguez-Ardila of Brazil’s National Astrophysics Laboratory says: “For the first time, the discovery of such double-peaked data sets constraints on the geometry of space that are impossible to solve.
“And now we have clear evidence of a food system and the inner structure of an active galaxy.”
The first double peak, from the inner part of the broad line region, was hydrogen. The model suggests that this was a distance of 16.77 light days from the black hole.
The second, discovery of oxygen, was from outer space, about 18.86 light days from the black hole. The model also shows that the broad line region reaches a radius of 52.43 light days from the black hole.
9,078 star units. To put that in perspective, Pluto is 40 star units from the Sun.
That seems big, and to us it is, but it is consistent with efforts to measure the size of growth disks using echoes of light coming from the inner edge of the torus, the size that the researchers refer to in their paper as “compact” .
The team will continue to monitor the galaxy to see if its ongoing behavior matches their predictions.
“This discovery gives us valuable insight into the composition and behavior of the wider region of this galaxy, shedding light on the interesting phenomena that occur around supermassive black holes in active galaxies,” said Rodriguez-Ardila.
The research was published in The Astrophysical Journal Letters.
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