Three black holes were discovered in the core of the nearby galaxy NGC 6240 by scientists at the University of Gottingen. Find out how they did it, and what it means for our understanding of the story of our universe.
The pervasive science fiction franchises have planted the notion of galaxies into all of our minds. Star Wars parodies the opening line of many story tellers, telling us that it is set “a long time ago, in a galaxy far far away.” In Star Trek, the galaxy stands in for our troubled present-day world, with its various quadrants, neutral zones and galactic barriers. It’s no coincidence that the top-grossing superhero film in 2014 was Guardians of the Galaxy.
So, we’ve all had the notion of “the galaxy” drilled into our heads from an early age. For this reason, we tend to forget that the idea of galaxies is relatively new. Before the 1920s, most astronomers thought that our Milky Way galaxy was all there was to the universe.
When Edwin Hubble studied the cepheid variable stars in the Andromeda spiral, he realized that it had to be more than 900,000 light-years away. That meant it had to be a separate galaxy from our own. When the American Astronomical Society learned about his discovery in a presentation, they gave him a standing ovation. It’s not called the Hubble Telescope for nothing.
FLUCTUATIONS LED TO BILLIONS OF GALAXIES
The fascinating thing about galaxies is how they formed. After the Big Bang, the universe was almost perfectly smooth and uniform. Even so, there were slight disturbances. As a result, the density of matter and energy in the early cosmos fluctuated a bit. Somehow, over time, these fluctuations led to the formation of the several hundred billion vast clusters of stars we call the galaxies.
I find it fitting that the space telescope bearing Edwin Hubble’s name made another fundamental discovery about galaxies. In 1994, using the Hubble Telescope’s Faint Object Spectrograph, astronomers found that there was a supermassive black hole in the core of the galaxy M-87. Today, most astronomers agree that there is a supermassive black hole in the centre of just about every sizeable galaxy.
This week, scientists at the University of Gottingen announced in the journal Astronomy and Astrophysics their discovery of something even more intriguing. A galaxy scientists call NGC 6240 contains not one, or even two, but three supermassive black holes at its core. As research team member Dr. Peter Weilbacher put it, “Up until now, such a concentration of three supermassive black holes had never been discovered in the universe.”
SAME MASS AS ABOUT 90 MILLION SUNS
The supermassive black holes at the centre of galaxies vary in mass. They can be anywhere from a few million times the mass of the sun up to several hundred million solar masses. Each of the three black holes in the core of NGC 6240 has the same mass as about 90 million suns.
NGC 6240 has an odd shape. It’s not an awe-inspiring, smooth spiral like most galaxies. Instead, it has lumpier contours. Until now, scientists had assumed that this was the result of a collision between two earlier galaxies. Once scientists knew there was a black hole at the centre of most galaxies, they assumed that there must be two of them in the core of NGC 6240.
The observations behind this discovery came from the Very Large Telescope (VLT) at the European Southern Observatory in Chile. Like the Hubble Space Telescope, the VLT includes a sophisticated spectrograph. Theirs is called 3D MUSE. Using it, researchers managed to isolate the motion and masses of each of three black holes in NGC 6240.
EXPLAINS RAPID FORMATION OF HUGE GALAXIES
The fact that there are three black holes at the centre of this galaxy helps to solve a puzzling question. It’s been hard to explain how the most massive galaxies could have formed. There didn’t seem to have been enough time between the big bang and today. Knowing that there can be merges involving multiple galaxies, it’s easier to explain the rapid evolution of the huge ones.
The researchers believe that the three black holes are in the process of merging into one. The merge will take a few million years, and the process should produce gravitation waves.
We won’t observe waves from that merger in our lifetimes. However, there must be other galaxies where this kind of collision is going on. Scientists believe that the proposed gravitational wave detector LISA can use its satellites to identify them. Then we’ll know about any other galaxies that came about through pile-ups like the ones we see on the freeway on a bad day.
UNIVERSE HAS SELF-ORGANIZING PROPERTY
The universe seems to have a mysterious, self-organizing property about it. Clouds of hydrogen and helium atoms formed out of the fluctuations in matter and energy we talked about above. Black holes concentrated from these clouds and their rapid rotation caused powerful density waves. These waves forced the remaining hydrogen and helium to condense into billions of stars.
This week’s discovery deepens our understanding of this process of self-creation, helping us to tell the story of our universe and our place in it. Even so, the team calls for further study to see how typical oddly shaped galaxies with multiples black holes are in the cosmos.
We always have more to learn if we dare to know.
University of Gottingen
NGC 6240: A triple nucleus system in the advanced or final state of merging
The Discovery of Galaxies
Cosmic Collision Makes Gravitation Waves
Are Supermassive Black Holes Collapsed Stars?
Unfolding the Milky Way