- In the largest survey of nearby galaxies, researchers observed hundreds of pairs of colliding galaxy nuclei associated with supermassive black holes.
- They found that black holes expand rapidly during the last stages of galaxy mergers.
The force of gravity draws two galaxies together: light-year by light-year, inch by inch, the galaxies comes closer as the cosmic clock ticks. This gravitational attraction was set by minor instabilities billion of years ago, which have only grown stronger with time.
When two massive galaxies merge, they bring hundreds of trillion of Suns’ worth of matter together. These materials collide, mix and ignite over hundreds of millions of years. But what happens to the supermassive black holes that lie at the galaxies’ center?
Recently, a team of astronomers Eureka Scientific Inc. conducted the largest survey of nearby galaxies’ cores in near-infrared light. To do this, they used high-resolution pictures captured by Hubble Space Telescope and the Keck Observatory telescopes on Mauna Kea in Hawaii.
They observed numerous pairs of colliding galaxy nuclei associated with supermassive black holes. The Hubble images suggest that such galaxy-merging processes could have been more frequent in the early universe.
When two galaxies merge, their supermassive black holes release intense energy in the form of space-time ripples known as gravitational waves. These waves were detected for the first time in 2015, which was reported around the world as a great accomplishment.
The recent advances in the technology have allowed astronomers to peer through the thick wall of dust and gas surrounding the cores the colliding galaxies. This helps astronomers study the close pair of black holes as they move towards each other during coalescence.
The research also gives a preview of what could possibly happen in the far future when our galaxy Milky Way merges with the nearby Andromeda galaxy and their respective supermassive black holes collide with each other.
Simulating the Galaxy Smashups
To visualize what they have discovered in the survey, researchers used computer simulations of galaxy smashups. They found that black holes grow faster and faster as collisions progress, and expand rapidly during the last stages of collisions.
The whole process lasts for over a billion years: 2 galaxies dance towards each other before eventually merging together. Simulations uncover that galaxies eject a vast amount of dust and gas which forms a thick layer around the cores of the merging galaxies. This layer blocks the view in visible light.
Some matter goes into the supermassive black holes (at the center of coalescing galaxies). These black holes monstrously engulf massive amounts of matter throughout the process, causing the infalling material to blaze brightly. During the final 10 to 20 years of the merger, black holes grow at an extremely fast rate. The W. M. Keck Observatory captured detailed views of this last phase, when merging black holes were only 3,000 light apart. Such close galaxy nuclei are very difficult to find.
In this study, researchers extracted 20 years’ worth of data from the Hubble archive. Then they observed larger samples of X-rays breaking through the cocoon of gas and dust, using near-infrared vision of the Keck Observatory.
Black holes in the process of galaxy merging | Credit: NASA
To find more rapidly growing, luminous black holes, they observed galaxies within a distance of 330 million light-years, and examined 385 galaxies identified by Hubble Space Telescope and 96 galaxies from the Keck Observatory.
Also, they compared these galaxies with 176 other galaxies that do not have actively expanding black holes. The comparison verified their results: luminous cores in the merging galaxies represent expeditiously expanding pairs of supermassive black holes headed for a collision.
Scientists have just learned how to detect gravitational waves emitted by merging-black holes. So far, they have detected these waves via Laser Interferometer Gravitational Wave Observatory.
ESA and NASA are currently working together to build observatories that can detect million times more massive black holes than those detected by existing observatory. One such project is the Laser Interferometer Space Antenna which is planned to be launched in 2034.
Moreover, next generation ground-based telescopes and future infrared space telescope like James Webb Space Telescope will give more insights of galaxy mergers by capturing their structures, expansion rate, characteristic of close pairs of black holes. They will be able to capture things in mid-infrared light that can’t be uncovered through near-infrared light.