The Milky Way, like all galaxies, has at its center a supermassive black hole. Normally, these black holes make themselves known through the accretion disk (a halo of uncondensed sediment) that encircles their event horizon — the area in which material slowly gets sucked into the black hole’s unfathomably dense center. The Milky Way’s accretion disk, however, has been elusive.
A recent paper published in the June issue of Nature, however, has reported the discovery of a long-awaited observation regarding the accretion disk. While the disk appears faint, its existence is now confirmed.
An accretion disk is a halo of cosmic detritus that orbits a sufficiently dense astronomical object. You can find them orbiting binary star systems (one star orbiting another) or, in this case, supermassive black holes.
In most cases, the accretion disks are easy to detect. Because they are so hot (anywhere between 10,000 and 10 million degrees Kelvin), they can easily be picked up by infrared telescopes.
The detritus that encircles these astronomical objects is normally composed of different types of gases and dust. The friction that these different particles experience as they spiral around whatever they orbit causes them to heat up. We can then measure this heat from terrestrial or airborne telescopes.
Most accretion disks slowly get fed into the black hole’s center. They orbit endlessly, feeding the innermost orbiting ring into the event horizon — the area where the orbit will suck you irreversibly toward the black hole’s center. This area of the black hole is delineated by the area that’s actually black (because not even light can escape beyond this point). This area grows as the black hole is fed by the cosmic detritus in the accretion disk.
The black hole at the center of the Milky Way is Sagittarius A. Sagittarius A is exceptional in its size. It’s estimated, for instance, to weigh a staggering four million solar masses. A solar mass is approximately 1.989 x 10^30 kg or 1.989 with 30 zeros after it. Let’s ponder this number for a moment.
Jeff Bezos is estimated to have a net worth of around 150 billion dollars (that’s nine zeros). Let’s pretend that each of his dollars is equivalent to one kg. You would need one billion Bezoses (18 zeros) to get a little over half of one solar mass. To get a better approximation, you would have to nearly double that. And Sagittarius A weighs four million of these solar masses. Yeah, that’s heavy.
The newly discovered disk
The discovery of this disk was made by the Atacama Large Millimeter/submillimeter Array in Northern Chili. They had set their sights on a 280 billion kilometer span of space (which for a telescope looking such distances isn’t that great) where the black hole is known to be. And within this span they had discovered, much to their surprise, an area with matter of different electromagnetic frequencies.
The researchers had realized that the area in question was emitting two different primary frequencies. One of them was more condensed, “squished,” and therefore blue-shifted. The other frequencies were red-shifted, elongating away from us. The group interprets this finding to mean that the disk is spiraling around the black hole.
The interpretation makes sense. The far side is moving away from us, and thus elongated in a red-shifted manner. The side that is moving toward us is blue-shifted, and thus squished. This is what we see for other accretion disks encircling blacks holes. It is what we find with our own.
Ultimately, the discovery is but another that sheds light on the nature of our galaxy. Like others, it has around it a giant mass of particles that slowly grow its already staggering size. We look forward to the future studies that will elaborate on how this accretion disk feeds that black hole that glues our galactic center together.