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Astronomers discover the smallest known supermassive black hole

Astronomers discover the smallest known supermassive black hole
An artist's illustration shows the small supermassive black hole at the center of dwarf galaxy RGG 118.
(Chandra X-ray Observatory)

Here's a cosmic contradiction: A tiny supermassive black hole. Astronomers using the Magellan II telescope in Chile and NASA's Chandra X-ray Observatory have discovered the smallest supermassive black hole yet found at the center of a dwarf galaxy.

The findings, published by the Astrophysical Journal Letters, could help scientists better understand the evolution of some of the earliest galaxies – and the black holes at their hearts.

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Black holes are such dense, massive objects that even light cannot escape. They come in two main types: the "stellar-mass" black holes, which are the remains of dead stars that were several times the mass of the sun; and the supermassive black holes, which are thought to sit at the center of every large galaxy and that can weigh millions or even billions of solar masses.

But this newly found supermassive black hole, at the center of a dwarf galaxy called RGG 118, holds a mere 50,000 suns' worth of mass – on the order of 100,000 times smaller than some of its oversized peers. (The supermassive black hole in the middle of our own Milky Way, for the sake of comparison, holds between 4 million and 5 million solar masses, and it's not even a serious heavyweight.)

Unusual as this pint-sized supermassive black hole (and its galaxy) might sound, they're probably quite common, the scientists said.

"Most galaxies are fairly small – and whereas we are fairly convinced that there is a black hole sitting at the center of every big galaxy, we're not sure whether that's the case in the small ones," said study coauthor Elena Gallo, an astrophysicist at the University of Michigan in Ann Arbor. "It's just harder to find them, so that's why we don't really know."

"And if there are black holes there," she added, "we don't know whether they behave, with respect to their host galaxy, the same way as the big ones do."

RGG 118 is itself a tiny galaxy, which means that it probably never merged with other galaxies to make the monstrous structures we see populating the cosmos today.

"They're probably relatively pristine objects," Gallo said of such small galaxies.

This dwarf galaxy, then, could be an analogue to the early galactic building blocks from the universe's history.

By studying the movement of hydrogen within RGG 118 using the Magellan II (or Clay) telescope and the high-energy X-rays coming from the center of the galaxy using the orbiting Chandra telescope, the researchers were able to calculate the mass of the black hole.

The new findings give some insight into the relationship between dwarf galaxies and their black holes. But Gallo stressed that they would need to study more than just one dwarf galaxy's black hole before they could draw broader conclusions.

"These dwarf galaxies, with their teeny black holes, give us a relatively nearby laboratory to study the process by which black holes formed very early on in the universe," she said.

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