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Astronomers peer through thick walls of gas and dust that surround the chaotic cores of merging galaxies, glancing at close pairs of supermassive black holes as they march toward giant black holes in the direction of merging.
A research team led by Michael Koss of Eureka Scientific Inc. of Kirkland, Washington, carried the largest investigation of the nuclei of nearby near-infrared galaxies with high-resolution images of NASA's Hubble Space Telescope and WM Keck Observatory in Hawaii. The Hubble observations represent over 20 years of snapshots from his extensive archive.
"It was amazing to see the pairs of converging galaxy cores that are so close together with these huge black holes," Koss said. "In our study, we see two galaxy nuclei when the images were taken, and you can not argue about that, it's a very" clean "result that does not rely on interpretation."
The images also provide a close-up view of a phenomenon that was more frequent in the early Universe when galaxy fusions occurred more frequently. When galaxies collide, their black monster holes can unleash vast amounts of energy in the form of gravitational waves that form a wave in space-time that has recently been discovered in pioneering experiments.
The new study also provides a preview of what is likely to happen in our own cosmic backyard in billions of years as our Milky Way connects to the neighboring Andromeda galaxy and collapse their respective central black holes.
"Computer simulations of galaxy – Smashups show us that black holes in the final phase of mergers, near the interaction between the black holes, are growing fastest, as we found in our survey," said Laura Blecha, Florida study group member in Gainesville. "The fact that black holes are growing faster and faster in the wake of mergers shows that galaxy encounters are really important in understanding how these objects become so vast."
A galaxy fusion is a slow process that takes more than a billion years. Two galaxies dance under the merciless gravity of each other before finally joining together. Simulations show that galaxies stir up a lot of gas and dust while in a slow-motion track.
The ejected material often forms a thick curtain around the centers of the coalescing galaxies and shields them in visible light. Part of the material also falls on the black holes in the nuclei of the converging galaxies. The black holes grow quickly as they feed on their cosmic food, and as they are messy eaters, they make the incoming gas burn brightly. This rapid growth has taken place in the last 10 to 20 million years of the Union. The Hubble and Keck Observatories took close-up shots of this final phase when the black holes are only about 3,000 light-years apart – a nearly hugely cosmic one.
It's not easy to find galaxy cores so close together. Most previous observations of colliding galaxies have trapped the merging black holes in earlier stages when they were about ten times farther away. The late stage of the fusion process is so elusive that the interacting galaxies are trapped in dense dust and gas, requiring high-resolution infrared observations that can see through the clouds and determine the positions of the two converging nuclei.
The team initially looked for optically hidden, active black holes by performing the X-ray data of the Burst Alert Telescope (BAT) aboard NASA's Neil Gehrel's Swift Telescope, a high-energy space observatory. "Gas falling on the black holes radiates X-rays and the brightness of the X-rays indicates how fast the black hole is growing," Koss explained. "I did not know if we would find hidden fusions, but because of computer simulations we suspected that they would be in heavily-covered galaxies, so we tried to look through the dust with sharp images, hoping one would Association to find black holes. "
The researchers scoured the Hubble archive and identified the converging galaxies they discovered in the X-ray data. They then used the Keck Observatory's ultra-sharp near infrared view to observe a larger number of X-ray producing black holes not found in the Hubble archive.
"People had already done studies to search for these close-knit black holes, but what really made this study possible was the X-rays that can break the cocoon of dust," Koss said. "We also looked a little further in the Universe to capture a larger volume of space, giving us a greater chance of finding brighter, fast-growing black holes."
The team aimed at galaxies with an average distance of 330 million light-years from Earth. Many of the galaxies are similar in size to the galaxies Milky Way and Andromeda. The team analyzed 96 galaxies from the Keck Observatory and 385 galaxies from the Hubble Archive found in 38 different Hubble Observation Programs. The example galaxies are representative of what astronomers would find in an all-sky survey.
To test their results, Koss' team compared the survey galaxies to 176 other galaxies from the Hubble archive that lack active black holes. The comparison confirmed that the glowing nuclei found by the researchers in the collection of dusty, interacting galaxies are indeed a signature of rapidly growing black hole pairs heading for a collision.
When the two supermassive black holes in each of these systems finally come together in millions of years, their encounters will produce strong gravitational waves. Gravitational waves generated by the collision of two stellar mass black holes have already been detected by the Laser Interferometer Gravitational Wave Observatory (LIGO). Observatories such as NASA / ESA's proposed Spaceborne Laser Interferometer Space Antenna (LISA) will be able to detect low-frequency gravitational waves from supermassive black hole fusions that are a million times more massive than those discovered by LIGO.
Future infrared telescopes, such as James Webb's James Space Space Telescope and a new generation of giant ground telescopes, offer an even better opportunity to study dusty galaxy collisions by measuring mass, growth rate, and dynamics of narrow black hole pairs. The Webb telescope may also be able to see in mid-infrared light to expose more galaxy interactions trapped in thick gas and dust so that even near infrared light can not penetrate it.
The results of the team will be published online in the issue of the journal Nature on November 7, 2018.
The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland is conducting Hubble science operations. STScI is licensed to NASA by the Association of Universities for Astronomy Research of Washington, D.C. operated.