Intermediate-mass black holes are hypothetical objects, with masses in the range of 100 to one million times that of our Sun. If these objects really do haunt the Cosmos, they would have masses significantly greater than black holes of stellar mass–but much less than enormous supermassive black holes that weigh-in at millions to billions of times more than our Sun. Most large galaxies, including our own Milky Way, host supermassive gravitational beasts within their hearts of darkness–lurking there in sinister, greedy, voracious secret, waiting for their next meal–an unfortunate gas cloud, perhaps, or a doomed star that has wandered too close to where it lies in wait. In January 2016, a team of astronomers led by Dr. Tomoharu Oka, a professor at Kelo University in Japan, announced that they have detected signs of an invisible black hole, with a mass of about 100,000 times that of our Sun, lurking at the center of our Milky Way. The astronomers assume that this potential intermediate mass object provides a valuable key to our scientific understanding of how the supermassive beasts haunting the centers of galaxies are born.
To make their discovery, the astronomers used the Nobeyama 45-m Radio Telescope to detect an enigmatic gas cloud, dubbed CO-0.40-0.22. This mysterious cloud of gas is only 200 light-years away from the center of our Galaxy, and what makes it weird is its amazingly wide velocity dispersion–the cloud harbors gas with a very wide range of speeds. The team found this unusual feature with a duo of radio telescopes: the Nobeyama 45-m Telescope in Japan and the ASTE Telescope in Chile, both operated by the National Astronomical Observatory in Japan. The team of astronomers performed supercomputer simulations and concluded that a model with an intermediate mass hole of about 100,000 solar-masses would be the best explanation for the observed velocity distribution.
Black holes of intermediate mass are too massive to be formed as the result of the collapse of a single star–which is what happens when a stellar mass hole forms. The environments of intermediate mass holes lack the extreme conditions–that is, the high density and velocities observed at the centers of galaxies–which seemingly would result in the formation of gigantic supermassive beasts. Currently, there are three popular formation scenarios explaining the possible origins of black holes of intermediate mass. The first is the merging of stellar mass black holes and other compact objects as the result of accretion. The second theory postulates the runaway collision of massive stars, inhabiting dense stellar clusters, and the collapse of the object formed by this wreck into an intermediate mass black hole. The third scenario suggests that these intermediate mass objects are primordial black holes that were born in the Big Bang birth of the Universe almost 14 billion years ago.
The best evidence for the existence of intermediate mass holes is derived from a few low-luminosity active galactic nuclei (AGN). Because of their activity, these galaxies are almost certain to host accreting black holes, and in some cases the black hole masses can be estimated using a technique termed reverberation mapping. For example, the spiral galaxy named NGC 4395 appears to harbor a black hole in its center that sports a mass of approximately 3.6 X 10 to the fifth power solar-masses.
There are also some ultra-luminous X-ray sources (ULXs) in nearby galaxies that are potential intermediate mass holes. These objects are about a hundred to a thousand solar masses. The ULXs are seen inhabiting star-birthing regions–such as the starburst galaxy M82–and are thought to be associated with youthful stellar clusters that have also been seen in these regions. The problem is that only a dynamical mass measurement from a study of the optical spectrum of the companion star can uncover the presence of an intermediate mass black hole, thus demonstrating it to be the compact accretor of the ULX.
There are a few globular clusters that may host candidate intermediate objects. These observations are based on measurements of the velocities of stars near their centers.
In November 2004, a team of astronomers reported their discovery of an intermediate-mass black hole in our Galaxy. The object, named GCIRS 13E, orbits only three light years from Sagittarius A*(Sgr A*, for short, pronounced Saj-A-Star), which is our Galaxy’s resident supermassive black hole. Sgr A* is relatively light– as far as supermassive beasts go–weighing-in at millions, as opposed to billions, of times more than our Sun. GCIRS 13E is approximately 1,300 solar-masses, and is tucked within a cluster of seven stars. These stars are possibly the remains of a massive stellar cluster that has been stripped down as a result of its voyage near the Galactic Center. This observation may add support to the theory that supermassive black holes grow by accreting nearby smaller black holes and stars. However, in 2005, a team of German astrophysicists proposed that the presence of an intermediate mass black hole near the Galactic center is unlikely, based on a dynamical study of the stellar cluster in which GCIRS 13E dwells. Such an object near the Galactic center could also be spotted by the way it disrupts stars in orbit around Sgr A*.
In January 2006, a team of astronomers led by Dr. Philip Kaaret of the University of Iowa in Iowa City, announced their discovery of a quasi-periodic oscillation possibly caused by an intermediate-mass-hole candidate that they detected using NASA’s Rossi X-ray Timing Explorer. This candidate intermediate object, dubbed M82 X-1, is circled by a red giant star that is in the process of shedding its atmosphere into the jaws of the waiting, hungry black hole. However, neither the existence of the oscillation nor its interpretation as the orbital period of the system are fully accepted by the rest of the astronomical community. Red giant stars are the bloated, dying remains of a Sun-like star that has managed to have depleted its necessary supply of hydrogen fuel–and has become an enormous, swollen, red, and angry looking star. When our Sun goes red giant, it will puff itself up to become a cannibalistic stellar monstrosity–devouring two of its planetary-children, Mercury and Venus–before it goes on to possibly roast our own Earth in its fiery, outer gaseous layers.
In 2009, a team of astronomers led by Dr. Sean Farrell discovered HLX-1, a candidate intermediate-mass black hole with a smaller stellar cluster surrounding it, located in the galaxy ESO 243-49. This observation indicated that the unfortunate ESO 243-49 had experienced a galactic smash-up with HLX-1’s galaxy and adopted–as its own–most of the badly battered smaller galaxy’s material.
On July 9, 2012, a team of astronomers at the CSIRO radio telescope in Australia announced that they had detected an intermediate-mass black hole.
Finally, in January 2016, the team of astronomers at Keio University in Japan announced the important discovery of CO-0.40-0.22, with its mysteriously wide velocity dispersion. In order to study the detailed structure of this object, the team observed it with the Nobeyama 45-m Telescope a second time to obtain 21 emission lines from 18 molecules. The team’s findings reveal that the enigmatic cloud sports an elliptical shape and is composed of a duo of components: a dense component reaching out 10 light-years with a narrow velocity dispersion, and a compact, but low density, component sporting an extremely wide velocity dispersion of 100 kilometers per second.
What makes this velocity dispersion so wide? That is the question. There are no holes within the cloud. Furthermore, X-ray and infrared observations did not detect any compact objects. These attributes suggest that the velocity dispersion is not likely to be caused by a local energy input, such as supernova blasts.
The “Missing Link”!
The team of astronomers from Keio University performed a simple supercomputer simulation of gas clouds that have been hurled out by a powerful source of gravity. In the simulation, the gas clouds are, at first, attracted by the source of gravity–which makes them speed up as they approach it, attaining maximum speed at the nearest point to the object. However, after the clouds have floated past the object, their speed decreases.
The team discovered that a model using a gravity source with 100,000 times solar-mass, within an area with a radius of 0.3 light-years, best explained the observed data. “Considering the fact that no compact objects are seen in X-ray or infrared observations, as far as we know, the best candidate for the compact massive object is a black hole,” Dr. Oka, the lead author, explained in a January 15, 2016 National Observatory of Japan Press Release. The paper describing this research appeared in the Astrophysical Journal Letters.
If that is shown to be the case, this discovery represents the very first confirmed detection of an intermediate mass black hole. Astronomers already know that black holes come in two sizes: stellar mass and supermassive. Intermediate-size black holes would be the “missing link” between the two.
Although a number of supermassive black holes have been discovered, no one knows how these enormous objects formed in the first place. One theory is that they were born as the result of mergers of a large number of intermediate-mass holes. However, this raises a problem because so far there has been no robust observational evidence for the existence of these intermediate mass objects. If the mysterious cloud CO-0.40-0.22, located a mere 200 light-years from Sgr A*, hosts an intermediate mass black hole, it could strengthen the intermediate mass black hole merger scenario that explains the formation of the supermassive variety.
This study provides a new way for astronomers to hunt for black holes with radio telescopes. Recent observations have shown that there are many wide-velocity compact clouds floating around that are similar to CO-0.40-0.22. The astronomers suggest that some of those clouds might host black holes.
Another study proposed that there are 100 million black holes in our Galaxy, but X-ray observations have only spotted dozens so far. This may be because most of the objects are “dark” and are extremely hard to detect directly at any wavelength. “Investigations of gas motion with radio telescopes may provide a complementary way to search for dark black holes. The on-going wide area survey observations of the Milky Way with the Nobeyama 45-m Telescope and high-resolution observations of nearby galaxies using the Atacama Large Millimeter/submillimeter Array (ALMA) have the potential to increase the number of black hole candidates dramatically,” Dr. Oka explained in the January 15, 2016 National Observatory of Japan Press Release.
The research results were published as Oka et al. Signature of an Intermediate-Mass Black Hole in the Central Molecular Zone of Our Galaxy in the January 1, 2016 issue of the Astrophysical Journal Letters.