Maunakea, Hawaii - Two galaxies, drawn together by the force of gravity, are merging into a tangled mass of dense gas and dust. Structure is giving way to chaos, but hiding behind this messy cloud of material are two supermassive black holes, nestled at the center of each of the galaxies, that are now excitingly close, giving astronomers the best view yet of the pair marching toward coalescence into one mega black hole.
"Seeing the pairs of merging galaxy nuclei associated with "huge" black holes so close together was pretty amazing," said study leader, Michael Koss of Eureka Scientific Inc. in Kirkland, Washington. "The images are pretty powerful since they are ten times sharper than images from normal telescopes on the ground. It's similar to going from legally blind (20/200 vision) to perfect 20/20 vision when you put on your eyeglasses. In our study, we see two galaxy nuclei right when the images were taken. You can't argue with it; it's a very clean result which doesn't rely on interpretation."
The team's results appear online in the November 7, 2018 issue of the journal Nature.
Koss and his team of researchers made the discovery after completing the largest systematic survey of nearby galaxies using high-resolution images taken with W. M. Keck Observatory's adaptive optics (AO) system and near-infrared camera (NIRC2), along with over 20 years of archival Hubble Space Telescope images. With this survey, astronomers can pinpoint the type of galaxy most likely to harbor close pairs of supermassive black holes.
"This is the first large systematic survey of 500 galaxies that really isolated these hidden late stage black hole mergers that are heavily obscured and highly luminous," said Koss. "It's the first time this population has really been discovered. We found a surprising number of supermassive black holes growing larger and faster in the final stages of galaxy mergers."
Theory states that there is a supermassive black hole at the center of every large galaxy. When galaxies merge, so do their supermassive black holes. This process takes billions of years, but ends in a blink of an eye. A supermassive black hole merger has never been directly observed via electromagnetic radiation.
It's not easy finding galaxy nuclei so close together either. The late stage of the merger process is so elusive because the interacting galaxies kick up a lot of gas and dust, especially in the final, most violent stages of the merger. A thick curtain of material forms and shields the galaxy nuclei from view in visible light. Astronomers did not have the capability to observe this type of event until now.
"Heavily obscured galaxy nuclei don't have a bright point source in the center like a lot of luminous unobscured supermassive black holes do," said Koss. "But we were able to detect them thanks to X-ray data from the Burst Alert Telescope (BAT). We then used the superior laser capability of Keck Observatory's AO system to perform high-resolution, near-infrared imaging to distinctly see a double nucleus through the gas and dust and uncover the hidden mergers."
Koss and his team's findings support the theory that galaxy mergers explain how some supermassive black holes become so monstrously large.
"There are competing ideas; one idea is that you have a bunch of gas in the galaxy that slowly feeds the supermassive black hole. The other is idea is that you need galaxy mergers to trigger large growth. Our data argues for the second case, that these galaxy mergers are really critical in fueling the growth of supermassive black holes," said Koss.
This survey may also help astronomers observe a black hole merger. When supermassive black holes collide, not only do they create a mega black hole, they can also unleash powerful energy in the form of gravitational waves. These ripples in space-time, which were predicted by Einstein, were recently detected in 2016 by groundbreaking experiments.
Like a siren before a tsunami, gravitational waves reach Earth slightly earlier than light. In order to image such an event, astronomers need to know where to look and what object to look for. Gravitational wave detectors tell astronomers what area, and Koss' research tells them whether that object is likely to host a supermassive black hole merger.
Koss and his team focused on galaxies with an average distance of 330 million light-years from Earth. Many of the galaxies are similar in size to the Milky Way. The images presage what will likely happen in our own cosmic backyard, in about a billion years, when our Milky Way merges with the neighboring Andromeda galaxy and their respective central black holes will smash together.
The next step for Koss and his team is to follow up on these hidden black hole mergers with Keck Observatory's instrument, the OH-Suppressing Infra-Red Imaging Spectrograph (OSIRIS). OSIRIS will allow them to measure both the rotation rate and mass of the black holes, which would make it possible to estimate how long it will take before the supermassive black holes collide and how strong the gravitational wave signal might be. OSIRIS data will also look for signs to see if the two black holes are both growing simultaneously.
ABOUT ADAPTIVE OPTICS
W. M. Keck Observatory is a distinguished leader in the field of adaptive optics (AO), a breakthrough technology that removes the distortions caused by the turbulence in the Earth's atmosphere. Keck Observatory pioneered the astronomical use of both natural guide star (NGS) and laser guide star adaptive optics (LGS AO) on large telescopes and current systems now deliver images three to four times sharper than the Hubble Space Telescope. Keck AO has imaged the four massive planets orbiting the star HR8799, measured the mass of the giant black hole at the center of our Milky Way Galaxy, discovered new supernovae in distant galaxies, and identified the specific stars that were their progenitors. Support for this technology was generously provided by the Bob and Renee Parsons Foundation, Change Happens Foundation, Gordon and Betty Moore Foundation, Mt. Cuba Astronomical Foundation, NASA, NSF, and W. M. Keck Foundation.
The Near-Infrared Camera, second generation (NIRC2) works in combination with the Keck II adaptive optics system to obtain very sharp images at near-infrared wavelengths, achieving spatial resolutions comparable to or better than those achieved by the Hubble Space Telescope at optical wavelengths. NIRC2 is probably best known for helping to provide definitive proof of a central massive black hole at the center of our galaxy. Astronomers also use NIRC2 to map surface features of solar system bodies, detect planets orbiting other stars, and study detailed morphology of distant galaxies.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two, 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. The data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.