In the grand scheme of the universe, black holes are the ultimate dead-ends - nothing can escape these spacetime vacuums, not even light.
Astronomers currently know of two types of black holes. The first kind are called stellar-mass black holes, which are middle-sized spacetime regions that form after very massive stars set off gigantic explosions when they run out of nuclear fuel.
The second kind are called supermassive black holes, aptly named because of their size. These black holes are often detected at the center of galaxies, including the Milky Way. Our galaxy contains the Sgr. A*, the biggest and largest supermassive black hole with a solar mass of 400 million.
Another example of a supermassive black hole is one found at the center of the dwarf galaxy NGC 5195. NASA's Chandra X-ray telescope spotted the supermassive black hole as it was burping galactic gas.
Supermassive black holes have masses that range from several millions to billions of times the mass of our own sun. Unlike stellar-mass black holes, no one knows how supermassive black holes form.
Now, a team of experts in Japan may have found what could be considered as the second most massive black hole - next only to the Sgr. A* - in the Milky Way. The discovery may possibly help shed light on the evolution of black holes.
An Enigmatic Gas Cloud With An Interesting Characteristic
What's curious about the discovery is that the massive black hole was not directly detected by scientists, and that what they first caught present instead had an unusual characteristic.
Led by Professor Tomoharu Oka of the Keio University, the group of astronomers had located a mysterious gas cloud named CO-0.40-0.22, a space object that is about 200 light-years away from Sgr. A*.
Researchers detected the enigmatic gas cloud through the use of the Nobeyama 45-m telescope in Japan and the Atacama Submillimeter Telescope Experiment (ASTE) in Chile. Japan's National Astronomical Society operated both telescopes.
What scientists found unusual is that the gas cloud CO-0.40-0.22 had an unusually wide velocity dispersion, meaning that the cloud holds gas that has a wide variety of speeds.
Basically, velocity dispersion is the spread and velocities of stars or gas within a galaxy, a statistical average of the combined motions of many stars.
Again, the team used the Nobeyama 45-m telescope to obtain 21 emission lines from 18 molecules. The results showed that the gas cloud has an elliptical shape and consists of two components.
The first component was compact and had low density, with a very wide velocity dispersion of 100 kilometers per second or 62 miles per second. The second component was dense and extended 10 light-years, with only a narrow velocity dispersion.
The gas cloud has a wide velocity dispersion because there are no holes inside it, scientists said.
Even more so, the infrared and X-ray observations did not detect any compact objects.
This indicates that the velocity dispersion is not triggered by a local energy input such as supernova explosions or the explosion of a star, something that causes the formation of stellar-mass black holes.
Simulating Gas Clouds To Better Understand The Discovery
A simulated model using a gravity source with 100 thousand times the mass of the sun with a radius of 0.3 light-years offered the best fit to the CO-0.40-0.22 phenomenon. With that, Oka and his team performed a simulation of gas clouds tossed by a strong gravity source.
In the simulation, the gas clouds were attracted by the gravity source, and the speeds increased as they approached it, reaching maximum at the closest point to the object. The clouds continued past the gravitational source and the speeds decreased.
"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," said Oka.
The First Detection Of An Intermediate-Mass Black Hole
If the enigmatic gas cloud does contain a massive black hole, Oka said it will be the first time that an intermediate-mass black hole has been detected.
How will this affect what we know about black holes? There is a theory that supermassive black holes form from mergers of many intermediate-mass black holes. Scientists find the theory problematic as there is no firm observational evidence for the existence of intermediate-mass black holes.
Oka and his team's findings, which are featured in the Astrophysical Journal Letters, might be the key to solve this cosmic puzzle. If cloud CO-0.40-0.22 indeed encloses an intermediate-mass black hole, it might support the intermediate-mass black hole theory regarding the evolution of supermassive black holes.
A New Way To Detect Black Holes
The team's research also opened up a new way to search for black holes. Through radio telescopes, scientists found that that there are many other compact clouds with wide velocity dispersions similar to CO-0.40-0.22. Oka and his colleagues propose that some of these compact clouds might hold black holes.
The Milky Way alone is home to about 100 million black holes, but X-ray observations have only detected dozens so far. Most of the black holes may possibly be "dark" and may be very difficult to see directly at any wavelength.
In the end, Oka said that investigations of gas motion through the use of radio telescopes may provide a complementary way to search for "dark" black holes. This even has the potential to dramatically increase the number of black hole candidates in the Milky Way, he added.
