Intermediate-Mass Black Hole Found at Center of Star Cluster 47 Tucanae

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47 Tucanae — a huge, ancient globular star cluster about 15,300 light-years away from us — harbors a central black hole about 2,200 times more massive than our Sun, according to a new report in the journal Nature.

This artist’s conception shows an intermediate-mass black hole in the center of the globular star cluster 47 Tucanae. Image credit: B. Kiziltan / T. Karacan.

This artist’s conception shows an intermediate-mass black hole in the center of the globular star cluster 47 Tucanae. Image credit: B. Kiziltan / T. Karacan.

Astronomers know that small black holes — black holes ranging from 10 times to 100 times the Sun’s mass — are the remnants of dying stars, and that supermassive black holes, more than 1,000,000 times the mass of the Sun, inhabit the centers of most galaxies.

But scattered across the Universe are a few apparent black holes of a more mysterious type. Ranging from 100 to 10,000 solar masses, these intermediate-mass black holes are so hard to measure that even their existence is sometimes disputed.

“We want to find intermediate-mass black holes because they are the missing link between stellar-mass and supermassive black holes,” said Dr. Bulent Kiziltan of the Harvard-Smithsonian Center for Astrophysics, 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 in the southern constellation of Tucana, 47 Tucanae orbits our Milky Way.

At about 120 light-years across it is so large that, despite its distance, it looks about as big as the full Moon.

Hosting millions of stars, 47 Tucanae is one of the brightest and most massive globular clusters known and is visible to the naked eye. It also contains about two dozen pulsars that were important targets of this investigation.

47 Tucanae, otherwise known as NGC 104, has been examined for a central black hole before without success.

The heart of the giant globular star cluster 47 Tucanae in the Hubble image at left reveals the glow of 200,000 stars. The green box outlines the cluster’s crowded core, where Hubble spied a parade of young white dwarfs starting their slow-paced 40-million-year journey to the less populated suburbs. The stellar relics are too faint to be seen clearly in visible light, as shown in the Hubble image at top right. But in ultraviolet light the stars glow brightly because they are extremely hot, as shown in the image at bottom right. The green circles in the image outline the brightest of the young white dwarfs spied by Hubble. Image credit: NASA / ESA / H. Richer  J. Heyl, University of British Columbia / J. Mack, STScI / G. Piotto, University of Padova.

The heart of the giant globular star cluster 47 Tucanae in the Hubble image at left reveals the glow of 200,000 stars. The green box outlines the cluster’s crowded core, where Hubble spied a parade of young white dwarfs starting their slow-paced 40-million-year journey to the less populated suburbs. The stellar relics are too faint to be seen clearly in visible light, as shown in the Hubble image at top right. But in ultraviolet light the stars glow brightly because they are extremely hot, as shown in the image at bottom right. The green circles in the image outline the brightest of the young white dwarfs spied by Hubble. Image credit: NASA / ESA / H. Richer J. Heyl, University of British Columbia / J. Mack, STScI / G. Piotto, University of Padova.

“We show there is evidence for a central black hole in 47 Tucanae with a mass of 2,200 solar masses when the dynamical state of the globular cluster is probed with pulsars,” the astronomers said.

“The existence of an intermediate-mass black hole in the centre of one of the densest clusters with no detectable electromagnetic counterpart suggests that the black hole is not accreting at a sufficient rate to make it electromagnetically bright and therefore, contrary to expectations, is gas-starved.”

In most cases, a black hole is found by looking for X-rays coming from a hot disk of material swirling around it. This method only works if the black hole is actively feeding on nearby gas.

The center of 47 Tucanae is gas-free, effectively starving any black hole that might lurk there.

The supermassive black hole at Milky Way’s center also betrays its presence by its influence on nearby stars. Years of infrared observations have shown a handful of stars at our galactic center whipping around an invisible object with a strong gravitational tug.

But the crowded center of 47 Tucanae makes it impossible to watch the motions of individual stars.

The new research relies on two lines of evidence. The first is overall motions of stars throughout the cluster.

A globular cluster’s environment is so dense that heavier stars tend to sink to the center of the cluster.

An intermediate-mass black hole at the cluster’s center acts like a cosmic ‘spoon’ and stirs the pot, causing those stars to slingshot to higher speeds and greater distances. This imparts a subtle signal that astronomers can measure.

By employing computer simulations of stellar motions and distances, and comparing them with visible-light observations, Dr. Kiziltan and co-authors find evidence for just this sort of gravitational stirring.

The second line of evidence comes from pulsars in 47 Tucanae.

These objects get flung about by the gravity of the central intermediate-mass black hole, causing them to be found at greater distances from the cluster’s center than would be expected if no black hole existed.

Combined, this evidence suggests the presence of an intermediate-mass black hole of about 2,200 solar masses within the star cluster.

Since this black hole has eluded detection for so long, similar black holes may be hiding in other globular clusters. Locating them will require similar data on the positions and motions of both the stars and any pulsars within the clusters.

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Bülent Kiziltan et al. 2017. An intermediate-mass black hole in the centre of the globular cluster 47 Tucanae. Nature 542, 203-205; doi: 10.1038/nature21361

This article is based on a press-release from the Harvard-Smithsonian Center for Astrophysics.