Hidden Black Hole May Be The “Missing Link”
By Judith E Braffman-Miller
Black holes are powerful, greedy gravitational beasts that are known to come in only two sizes: stellar mass and supermassive. “Small” stellar mass black holes weigh-in at only a few Suns, while sinister supermassive monsters–that usually lurk hungrily in the dark hearts of probably every large galaxy in the Universe–can weigh-in at millions to billions of times solar-mass. Astronomers have long suspected that black holes of intermediate mass (IMBH), weighing-in at 100 to 10,000 Suns, also haunt the Universe in certain secretive places, but so far there has been no conclusive evidence of their true existence in nature. However, in February 2017, a team of astronomers announced that they have obtained new evidence that an IMBH, weighing about 2,200 times that of our Sun, is playing hide and seek in the center of the globular cluster 47 Tucanae. Intermediate mass holes would provide the “missing link” between small black holes of only stellar mass and their enormous, and much heavier supermassive kin.
“We want to find intermediate-mass black holes because they are the missing link between stellar-mass and supermassive black holes. They may be the primordial seeds that grew into the monsters we see in the centers of galaxies today,” noted study lead author Dr. Bulent Kiziltan in a February 8, 2017 Harvard-Smithsonian Center for Astrophysics (CfA) Press Release. Dr. Kiziltan is of the CfA in Cambridge, Massachusetts.
47 Tucanae is a 12-billion-year-old star cluster that is situated about 13,000 light-years from our planet in the southern constellation of Tucana the Toucan. The cluster hosts literally hundreds of thousands of fiery, brilliant stars, that have been pushed by gravity into a ball of only approximately 120 light-years in diameter. The globular also contains about 24 pulsars that proved to be valuable targets for the astronomers to use in their observations. Pulsars are very young neutron stars, which are the ghostly, dense remains of what was once a massive star that perished in a supernova blast, after having depleted its necessary supply of nuclear-fusing fuel. Born spinning wildly, pulsars cast beams of light into the space between stars with a regularity that is frequently compared to the beam of a lighthouse on Earth.
Globular clusters are spherical collections of stars that are found in orbit around the core of their host galaxy as satellites. Gravity is responsible for giving these clusters their lovely spherical shape, as well as their relatively high stellar densities toward their centers. Globular clusters are fairly common, and there are approximately 150 to 158 of these systems known to be in orbit around our Milky Way Galaxy–with, perhaps, 10 to 20 more still waiting to be discovered.
Every large galaxy of sufficient mass, inhabiting our Milky Way’s Local Cluster of galaxies, is orbited by its own retinue of globular clusters. Furthermore, almost every large galaxy surveyed has been found to host a system of these spherical, starlit, celestial objects.
Even though globular clusters host some of the most ancient stars to be born in a galaxy, their origins and the role that they play in galactic evolution are not well understood. However, it does seem clear that these clusters are different from dwarf elliptical galaxies, and that they formed as part of the star-birthing process of their particular galactic host, rather than as separate galaxies in their own right. Most of our Milky Way’s globulars dwell within a halo encircling the Galactic core, and the great majority of these systems are situated in the celestial sky centered on our Galaxy’s core. Because the formation of globular clusters remains poorly understood, it is uncertain whether the stellar inhabitants of these clusters are born in a single generation, or are born across many generations over the passage of several hundred million years. Since, in many clusters, most stars are at roughly the same stage of stellar evolution, it seems to indicate that they were all born at about the same time.
Black Hole Hide And Seek
Astronomers have known for decades that it is likely every large galaxy in the observable Universe holds a supermassive monster in its secretive dark heart. The beast waits there, hidden in the center of its galactic host, waiting for its next banquet to travel into its powerful gravitational trap. Anything unlucky enough to wander too close to this hungry supermassive black hole is doomed. These unfortunate captured objects cannot free themselves from the fierce, strong gravitational embrace of their greedy predator and captor. Not even light can liberate itself once it has passed the point of no return called the event horizon.
It is thought that supermassive black holes already lurked in the Cosmos when it was very young. Clouds of gas and snared stars tumble down, whirling and swirling into the black hole’s embrace, never to return from the prison of the churning maelstrom encircling this feasting gravitational monster. As the material swirls down to its inevitable doom, it forms a wild and violent storm of brilliantly glaring material surrounding the black hole–the enormous accretion disk. This material becomes hotter and hotter, and hurls out a veritable storm of radiation, especially as it approaches the black hole’s event horizon–located at the innermost region of the blazing, brilliant disk.
In the 18th century, John Michell and Pierre-Simon Laplace contemplated the possibility that there actually could exist in the Cosmos bizarre entities like black holes. Albert Einstein, in his Theory of General Relativity (1915) later predicted the existence of objects possessing such powerful gravitational fields that anything unlucky enough to wander too close to the monster would be consumed. However, the idea that such strange objects could really exist in nature seemed so preposterous at the time that Einstein rejected the concept–even though his own calculations indicated otherwise.
In 1916, Karl Schwarzschild devised the first modern solution to General Relativity that could describe a black hole. However, its interpretation as an area of space from which nothing, nothing, nothing at all could escape from the gravitational monster was not truly comprehended for almost half a century. Up until that time, black holes were thought to be only mathematical oddities. It was not until the 1960s that theoretical work demonstrated that black holes are a generic prediction of General Relativity.
Supermassive black holes are infamous for their insatiable hunger, enormous mass, and rather sloppy table manners. But, what about their smaller kin–the still-hypothetical black holes of intermediate mass, that fall between black holes of the supermassive kind and black holes of the stellar kind?
Intermediate mass black holes are still only hypothetical objects. If the do exist in nature, they are too massive to have been born as the result of the collapse of a single, solitary, massive star–which is what happens when a stellar mass black hole forms. The environment of IMBHs lacks the same extreme conditions of high density and velocity–seen at the heart of galaxies–which would cause one of the enormous supermassive beasts to form. At present, there are a few suggestions that attempt to explain the mysterious origins of IMBHs. The first theory proposes that IMBHs result from the collision and merging of black holes of stellar mass, and other compact objects, as the result of accretion. A second suggestion proposes that the runaway collision of massive stars, dwelling within dense stellar clusters–and the collapse of the bizarre object created by this mess–gives rise to an IMBH. The third model suggests that these intermediate mass objects are really primordial black holes that were born in the Big Bang birth of the Universe almost 14 billion years ago.
Hidden Black Hole May Be The “Missing Link”
47 Tucanae had been examined by other astronomers before Dr. Kiziltan’s team’s study. Alas, these earlier examinations of the stellar cluster, conducted by other groups of scientists, came up empty-handed. Usually, a black hole is discovered by astronomers when they hunt for X-rays that are being emitted from the searing-hot disk of material that is whirling around it. However, this method can only work if the black hole is in the act of feasting on a buffet of unfortunate gas that catastrophically wandered too close to its gravitational grip. The center of 47 Tucanae has no gas, and this effectively starves any hungry black hole that may hide there in sinister, voracious secret.
The supermassive black hole that haunts the heart of our own Milky Way Galaxy–dubbed Sagittarius A, or Sgr A, for short (pronounced saj-a-star) gives its hiding place away as a result of its gravitational influence on nearby stars. Decades of infrared observations revealed a small number of stars at our Galaxy’s center that were zipping around an invisible object with a very powerful gravitational pull. However, the heavily populated core of 47 Tucanae makes it impossible for astronomers to study the motions of individual stars.
The new research depends on two lines of evidence. The first is the overall motions of stars that dwell throughout the globular cluster. A globular’s environment is so extremely dense that more massive stars have a tendency to sink down to the center of the cluster. An IMBH lurking within the cluster’s hidden heart behaves something like a cosmic “spoon” that stirs the contents of a pot of soup. This basically means that it causes those stars to shoot out to greater speeds and greater distances. This sends forth a subtle signal that astronomers are able to measure.
The second line of evidence is derived from pulsars–the dense, compact ghosts of dead stars whose radio signals are readily detectable. Pulsars also get unceremoniously hurled about by the gravity of the central IMBH. This causes the pulsars to be found at greater distances from the cluster’s center than would normally be expected if there were no hidden black hole lurking there.
When all of the clues are put together, the evidence indicates the presence of an IMBH of approximately 2,200 solar-masses haunting the hidden center of 47 Tucanae.
Because this black hole has escaped detection for so long, similar IMBHs may be hiding in other globular clusters. Seeking them will require similar data on the positions and movements of both the stars and any pulsars that are beaming within the clusters.
The paper describing this research appears in the February 9, 2017 issue of the science journal Nature.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.