Astronomers have identified the smallest supermassive black hole ever detected in a galaxy’s center.
At the heart of every big galaxy sits a supermassive black hole. These leviathans are essentially pits in the fabric of spacetime. They contain millions to billions of times the mass of the Sun, yet squash all their mass deep inside a boundary whose size is on par with scales from the solar system. (The event horizon of the Milky Way’s central black hole is only 20% as wide as Mercury’s orbit, in case you’re curious.)
These beasts don’t dwell just in big galaxies, however. Some dwarf galaxies also have supermassive black holes at their centers. Last year, for example, we reported on the discovery of 151 active black holes in these runt galaxies. At the time, Amy Reines (then at NRAO) and her team could only determine masses for about two dozen of the black holes, which weighed in at 80,000 to 6.3 million solar masses.
As part of the ongoing investigation of the other 100-plus objects, Vivienne Baldassare (University of Michigan), Reines, and their colleagues have now determined the mass of the black hole sitting in the dwarf disk galaxy SDSS J1523+1145. (The team calls the stellar city RGG 118, for the surname initials of the three authors of the previous study.) And in the words of coauthor Elena Gallo (also at Michigan), this supermassive black hole is downright “teeny.”
Small Supermassive Black Hole
The team used the 6.5-meter Clay Telescope in Chile and the Chandra X-ray Observatory to study the black hole’s energy output and clock how fast gas is moving near it. Astronomers calculate a black hole’s mass using the gas’s velocity: the gas emits radiation at certain wavelengths, and its whiplash speed smears out these spectral lines. The lines’ width tells astronomers how fast the gas is moving and, thus, how massive the black hole is.
The team estimates that RGG 118’s central black hole has a mass between 27,000 and 62,000 Suns — so, about 50,000. That makes it the smallest beast ever found in a galaxy’s nucleus.
Depending on how you define it, this object could be considered an intermediate-mass black hole (IMBH). IMBHs are the middle children between stellar-mass black holes and their supermassive brothers and should have hundreds to hundreds of thousands of solar masses. Astronomers have been hunting for these in-betweeners for some time, but they only have a few good candidates.
But “‘intermediate-mass black hole’ is a bit of a loaded term and means something different to everyone,” cautions Kayhan Gültekin (University of Michigan), who wasn’t involved with the study. “IMBH” can (and often does) instead signify a black hole that’s too big to have formed from a collapsing star but too small to have formed and evolved the way supermassive black holes do.
From this angle, the new object isn’t an IMBH. RGG 118’s black hole lives at its galaxy’s center, just like bigger ones do. Given its mass, its energy output is also the same. It even influences its galaxy the same way, somehow governing the speeds of stars that, while in the galaxy’s bulge-like core, should be well out of its gravitational reach. This golden rule of galaxy–black hole symbiosis, called M-sigma, has made astronomers suspect a close, coevolutionary bond exists between the two.
In fact, it’s intriguing just how well RGG 118’s black hole mimics its larger siblings. Take, for instance, the smeared-out emission lines the team used to calculate the object’s mass. Astronomers call this emission the broad-line region, and it comes from clouds near the black hole. They don’t yet know what “near” precisely means: the gas could live in clouds orbiting the black hole, or it could be a wind streaming up and out from the tutu-like accretion disk girding the object.
Either way, this broad-line region doesn’t appear around stellar-mass black holes. Maybe that’s because the accretion disks around them are too hot — the spacetime ditches they dig out for themselves are smaller but deeper than those around their bigger brethren, which allows gas to swirl in toward the event horizon at higher temperatures. The fact that RGG 118’s beast also boasts this emission yet has only a hundredth or less the mass of “normal” central black holes gets astronomers closer to finding out just how small black holes can be before the smeared emission disappears. That, in turn, could tell astronomers just what’s going on in this mysterious region.
From Whence Cometh the Dragons?
Also up for debate is just how the beasts in galaxies’ cores arise — do they grow from dead-star seeds, or do big clouds of tens to hundreds of thousands of Suns’ worth of gas collapse directly to create them?
If big black holes come from stellar seeds, they’ll initially be “too light” compared with the star speeds in their host galaxies. But if they form from collapsing clouds, they’ll begin as “too heavy.” RGG 118’s black hole is “just right,” which doesn’t tell us much in this regard. But others like it might not be. Finding more, and determining how many such objects lurk in dwarf galaxies overall, will help us understand how the first supermassive black holes form.
Below, you can also watch a ~2-minute video summarizing the result.
Reference: V. F. Baldassare et al. “A ~50,000 Solar Mass Black Hole in the Nucleus of RGG 118.” Astrophysical Journal Letters. August 10, 2015.
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