Approximately 16,000 light years from Earth lies a spherical glob of millions of stars dating back to the early years of the universe. This dense cluster, called 47 Tucanae, has a radius of about 200 light years and is one of the brightest clusters in our night sky. Inside 47 Tucanae, intense gravitational forces have sorted stars over time, pushing less dense stars to the outside and creating a very dense inner core that resists outside scrutiny.
"Studying globular clusters is notoriously challenging," says Bülent Kiziltan, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics. There are so many stars packed next to each other, he says, that capturing radiation from the center of one is next to impossible. So while scientists have long suspected that 47 Tucanae might contain a black hole at its center, as many other globular clusters appear to, they haven’t been able to prove it.
Now, in a study published yesterday in the journal Nature, Kiziltan and his colleagues have helped peer into the heart of 47 Tucanae to find the first of a new class of medium-sized black holes.
Despite their name, black holes aren’t actually that black, Kiziltan says. As they tear apart stars unlucky enough to wander into their pull, he says, they form a disk of bright, hot gases around them known as an accretion disk. Black holes don’t let any visible light escape, but they usually emit X-rays as they consume these gases. However, 47 Tucanae is so dense that it has no gases left at its center for the black hole to consume.
Kiziltan used his expertise in another quirky type of space object—pulsars—to try a new way of detecting these elusive kinds of black holes.
Pulsars "provide us with a platform that we can use to study very minute changes in the environment," Kiziltan says. These stars, which emit "pulses" of radiation at very regular intervals, can be used as reference points to map out cosmic formations, including globular clusters; Kiziltan likens them to "cosmic atomic clocks."
With two dozen pulsars on the edges of 47 Tucanae as guides, Kiziltan and his team were able to build simulations of how the globular cluster evolved over time, and particularly how the denser and less dense stars sorted themselves into their present-day positions.
These simulations were massive undertakings, Kiziltan says, requiring roughly six to nine months to complete even on extremely powerful computers. Which is why he wasn’t thrilled, he says, when reviewers at Nature asked for further simulations that ended up taking another year to complete.
But that effort was worth it, Kiziltan says, because it led to something unprecedented: the first discovery of a black hole inside a globular cluster. After running hundreds of simulations, he says, the only possible scenario that could lead to the development of today's 47 Tucanae featured a black hole at the global cluster's dense, gas-less center. This previously unconsidered environment for a black hole opens up new places to look for them, Kiziltan says.
"One can only imagine what is lurking in the centers of other global clusters," Kiziltan says.
What is also exciting, Kiziltan notes, is the size of the black hole his simulations predicted. So far, scientists have mostly found small black holes (those roughly the size of the stars that collapsed to form them) and supermassive black holes (those thousands of times larger than our Sun). Intermediate-sized black holes have mostly eluded scientists—though not for lack of trying.
The black hole predicted at the center of 47 Tucanae falls within this rare middle ground, Kiziltan says. Further study of this potential black hole could provide new insights on how and why these largely unknown type of black holes form.
Perhaps even more important than the discoveries themselves is how Kiziltan and his team arrived at them. Kiziltan and his collaborators drew on a mathematical theory developed in the 1950s by two American cryptographers to help chart the probable distributions of stars in 47 Tucanae. "They developed this mathematical method to piece together incomplete information to see the bigger picture," Kiziltan says.
Kiziltan is working to refine their new approach and use this new method to look at other populations of stars for previously unseen black holes. Powerful new scientific computers and other instruments that will go online in the coming years will help with this quest, he says.
"We've done many things for the first time in this work," Kiziltan says. At the same time, “there are still so many things that need to be done.”