A team of astronomers led by Michael Koss of Eureka Scientific Inc. has observed several pairs of galaxies in the final stages of merging together into single, larger galaxies. Peering through thick walls of gas and dust surrounding the merging galaxies’ cores, the team captured pairs of supermassive black holes — each of which once occupied the center of one of the two original smaller galaxies — drawing closer together before they coalescence into one giant black hole.
Images of four colliding galaxies, along with close-up views of their coalescing nuclei in the bright cores, are shown beneath the Hubble snapshots of NGC 6240. The left image of each pair, showing the merging galaxies, was taken by the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). The right image, showing the bright cores, was taken in near-infrared light by the W. M. Keck Observatory in Hawaii, using adaptive optics to sharpen the view. The nuclei in each of the Hubble and Keck Observatory infrared photos are only about 3,000 light-years apart — a near-embrace in cosmic terms. If there are pairs of black holes, they will likely merge within the next 10 million years to form a more massive black hole. These observations are part of the largest-ever survey of the cores of nearby galaxies using high-resolution images in near-infrared light taken by the Hubble and Keck observatories. Image credit: NASA / ESA / M. Koss, Eureka Scientific, Inc / W. M. Keck Observatory / Pan-STARRS.
“Seeing the pairs of merging galaxy nuclei associated with huge black holes so close together was pretty amazing,” Dr. Koss said.
“The images are pretty powerful since they are 10 times sharper than images from normal telescopes on the ground.”
Dr. Koss and co-authors performed the largest survey of the cores of nearby galaxies in near-infrared light, using high-resolution images taken by the NASA/ESA Hubble Space Telescope and the W. M. Keck Observatory in Hawaii. The Hubble observations represent over 20 years’ worth of snapshots from its vast archive.
The astronomers targeted galaxies with an average distance of 330 million light-years from Earth.
They analyzed 96 galaxies from the Keck Observatory and 385 galaxies from the Hubble archive.
The sample galaxies are representative of what astronomers would find by conducting an all-sky survey. Many of the galaxies are similar in size to the Milky Way and Andromeda galaxies.
“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,” Dr. Koss said.
“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.
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,” Dr. Koss said.
“But we were able to detect them thanks to X-ray data from the Burst Alert Telescope.”
“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.”
The team’s results 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,” Dr. Koss said.
“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.”
The findings appear in the journal Nature.
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Michael J. Koss et al. 2018. A population of luminous accreting black holes with hidden mergers. Nature 563: 214-216; doi: 10.1038/s41586-018-0652-7
