Black holes are some of the most bizarre and mysterious phenomena in the Universe – they have captivated the imaginations of scientists and the public alike, and have been featured in many science fiction movies and stories. Just like planets or stars, some are larger and some are smaller, but all generally share the same fascinating characteristics, being objects so dense that they “suck in” anything that comes too close to them, including light. Now, for the first time, astronomers have discovered something they had been searching for – two massive black holes orbiting each other in a kind of “cosmic dance.”
The two black holes were found by astronomers at The University of New Mexico and are hundreds of millions of miles from Earth. The new results have been published in a paper entitled ‘Constraining the Orbit of the Supermassive Black Hole Binary 0402+379’, in The Astrophysical Journal. The paper is by UNM Department of Physics & Astronomy graduate student Karishma Bansal (first author) along with UNM Professor Greg Taylor and colleagues at Stanford, the U.S. Naval Observatory and the Gemini Observatory. The findings are the result of more than 20 years of work.
“For a long time, we’ve been looking into space to try and find a pair of these supermassive black holes orbiting as a result of two galaxies merging,” said Taylor. “Even though we’ve theorized that this should be happening, nobody had ever seen it until now.”
Massive black holes have already been known about for a long time now, there’s even one in the center of our galaxy. But this is the first time that two of them have been observed orbiting each other. In 2016, however, two stellar-mass black holes (~30 solar mass) were found colliding, by international astronomers working on the LIGO project which detected the existence of gravitational waves. The gravity waves were caused by the collision of the two black holes. Now, with the discovery of two black holes close together but not yet colliding, scientists can better study what happens during that (pre-collisions) time and how it affects the surrounding fabric of space-time. Gravitational waves had first been predicted by Albert Einstein and now had finally been confirmed.
So how were these orbiting black holes discovered? Researchers used the Very Long Baseline Array (VLBA), a series of 10 radio telescopes across the U.S. to track radio signals generated by supermassive black holes (SMBH). BY watching the paths of these two black holes, astronomers determined that they actually orbited each other.
“When Dr. Taylor gave me this data I was at the very beginning of learning how to image and understand it,” said Bansal. “And, as I learned there was data going back to 2003, we plotted it and determined they are orbiting one another. It’s very exciting.”
The two black holes are about 750 million light-years from Earth, in a galaxy called 0402+379. They have an orbital period of about 24,000 years and their combined mass is 15 billion times that of our Sun. The long orbital time means that even though astronomers have been observing them for over ten years, they still haven’t been able to see much actual movement along those orbits.
“If you imagine a snail on the recently discovered Earth-like planet orbiting Proxima Centauri – 4.243 light years away – moving at 1 cm a second, that’s the angular motion we’re resolving here,” said Roger W. Romani, professor of physics at Stanford University and member of the research team.
“What we’ve been able to do is a true technical achievement over this 12-year period using the VLBA to achieve sufficient resolution and precision in the astrometry to actually see the orbit happening,” said Taylor. “It’s a bit of triumph in technology to have been able to do this.”
The discovery could also help astronomers understand more about binary stars and galaxies.
“The orbits of binary stars provided tremendous insights about stars,” said Bob Zavala, an astronomer at the U.S. Naval Observatory. “Now we’ll be able to use similar techniques to understand super-massive black holes and the galaxies they reside within.”
Black holes on their own can be difficult to wrap our minds around, where their density is so great that not even light can escape their gravitational pull. Supermassive ones even more so, and the sight of two of those orbiting close to each other is the stuff that science fiction fantasies are made of – but they’re real.
“Supermassive black holes have a lot of influence on the stars around them and the growth and evolution of the galaxy,” said Taylor. “So, understanding more about them and what happens when they merge with one another could be important for our understanding for the universe.”
These orbiting black holes will be observed again in three or four years to help further confirm their motions and determine their orbits more precisely.
Since they are so bizarre, even just the reality of black holes has been questioned by some, despite all of the accumulating evidence for them over the years. How can we know they exist if we can’t even see them? A study in 2015, using the Event Horizon Telescope (EHT), helped to solidify the case that they are very real, even if still poorly understood.
A research team led by Dr. Avery Broderick, an Assistant Professor at the University of Waterloo’s Department of Physics & Astronomy in Canada, used the EHT to probe deep into the center of M87, a nearby elliptical galaxy which is located approximately 53 million light-years away. The supermassive black hole in the center of that galaxy has a mass about 6 billion times that of the Sun. Broderick’s team wanted to see if the dense, compact object at the center of M87 really had the characteristics attributed to black holes.
“Accretion onto compact objects with a surface, e.g., white dwarfs, neutrons stars, results in the formation of a boundary layer in which any remaining kinetic energy contained within the accretion flow is thermalized and radiated,” said Broderick’s team in the study which was published online on arxiv. “In contrast, gas accreting onto a black hole is free to advert any excess energy across the horizon without further observational consequence. If the mass accretion rate can be independently estimated, this difference provides an observational means to distinguish between the presence of a surface, or more accurately a ‘photosphere’, and a horizon.”
The result? Their observations showed that the luminosity of the dense object at the center of M87 was indeed several orders of magnitude lower than what would be expected if there was no black hole there.
“Despite the [current] astrophysical uncertainty in the relationship between the jet power and mass accretion rate, we have shown in this paper that, within the context of current jet launching paradigms, we can rule out the existence of an observable photosphere in M87* within which the kinetic energy of the accreting gas is deposited,” they said. “The implication is that the kinetic energy of the gas is advected past an event horizon, beyond which it is no longer visible to distant observers. In other words, M87* must have an event horizon.” Or put another way, there is a black hole at the center of M87.
Also in 2015, it was reported that the supermassive black hole at the center of our own galaxy, called Sgr A*, displayed an unexpected increase in activity. It was thought that the new activity might be linked to the close passage of a massive hydrogen gas cloud called G2, but that is still a matter of debate.
Black holes have been one of the most mysterious and elusive phenomena in the Universe, but thanks to new studies such as this, astronomers are finally beginning to better understand how they work and how they relate to other cosmic phenomena as well. The Universe is indeed a strange place.