All the black holes discovered currently fall into two categories: there's the stellar-mass black holes, weighing between about five and a few dozen times the mass of our Sun, and way up the other end of the scale are the so-called supermassive black holes with the mass of millions or billions of stars. These behemoths are thought to grow either by merging with other objects or by growing from their smaller stellar-mass brethren as they absorb matter, leading scientists to believe that "middleweight" black holes must be lurking out there somewhere. Now, a team of astronomers has detected the first evidence of one of these missing links.

Dubbed intermediate-mass black holes (IMBHs), these objects are theorized to weigh between 100 and 10,000 solar masses. Now, researchers from the Harvard-Smithsonian Center for Astrophysics (CfA) and the University of Queensland (UQ) believe they've found one weighing about 2,200 Suns, hiding in the center of a star cluster known as 47 Tucanae.

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"We want to find intermediate-mass black holes because they are the missing link between stellar-mass and supermassive black holes," says Bulent Kiziltan, lead author of the study. "They may be the primordial seeds that grew into the monsters we see in the centers of galaxies today."

Located about 13,000 light-years away from Earth, 47 Tucanae has been scanned for a black hole at its center in the past, but conventional observation methods have made things tricky. One of the most common techniques for finding black holes is to watch for the X-ray bursts emitted by super-heated material in the surrounding accretion disk as the black hole devours stars or gases. But with no gases in the area, any potential black hole at the center of 47 Tucanae remains virtually invisible.

The other method is to watch for the gravitational effects a black hole has on the stars that orbit it. The problem here is that 47 Tucanae is very dense, housing millions of stars within a relatively-small space of about 120 light-years across, which makes it difficult to separate the motions of individual stars.

So rather than try to watch stars individually, the team studied their movements as a whole. Due to the cluster's density, its heavier stars will move towards the center, where the black hole would bump up their speed. Comparing computer simulations of this process to visible-light observations of the real thing, the researchers found that 47 Tucanae was behaving just as the model predicted.

"An intermediate-mass black hole at the cluster's center acts like a cosmic 'spoon' that stirs the pot, causing the stars near it to slingshot to higher speeds and greater distances, imparting a subtle signal that astronomers can measure," says Holger Baumgardt, a UQ researcher who did computer simulations used in the study.

Further evidence for an IMBH at the center of 47 Tucanae comes from pulsars in the cluster. The presence of an IMBH would flick these objects to the further fringes of the cluster, but if there was no black hole, they'd be found closer to the center. Sure enough, the easily-detectable pulsars were spotted right where an IMBH would be expected to cast them.

Since it's incredibly unlikely that 47 Tucanae is home to the only intermediate black hole in the universe, the researchers are hoping to use their techniques to examine other globular clusters to find more.

The research was published in the journal Nature.

Sources: Harvard-Smithsonian CfA, University of Queensland