Astronomers unveil growing black holes in colliding galaxies

Peering through thick walls of gas and dust that surround the messy nuclei of fusion of galaxies, astronomers are getting their best view of pairs of supermassive black holes as they march to the coalescence in mega black holes.

A team of researchers led by Michael Koss of Eureka Scientific Inc., in Kirkland, Washington, performed the largest investigation of nearby galaxies in near-infrared light, using high-resolution images taken from NASA's Hubble Space Telescope and WM Keck Observatory in Hawaii. Hubble's observations represent over 20 years of snapshots from his vast archive.

"Seeing the fusion pairs of the galaxy cores associated with these huge black holes so close together was quite surprising," Koss said. "In our study we see two nuclei of galaxies right at the time the images were taken. You can not discuss it, it's a very clean result, which is not based on interpretation."

The images also provide a close-up preview of a phenomenon that must have been more common in the primordial universe, when galaxies mergers were more frequent. When galaxies collide, their black hole monsters can release powerful energy in the form of gravitational waves, the kind of space-time ripples that have recently been discovered by revolutionary experiments.

The new study also offers a preview of what will probably happen in our cosmic courtyard, over billions of years, when our Milky Way combines with the neighboring Andromeda galaxy and their respective central black holes collapse together.

"The computer simulations of galaxy smashups show us that black holes grow faster during the final stages of mergers, close to the time when black holes interact, and that's what we found in our survey," said Laura Blecha study team from the University of Florida, in Gainesville.

"The fact that black holes grow faster and faster as the merger progresses tells us that galaxy meetings are really important for our understanding of how these objects have become so monstrously large."

A merger of galaxies is a slow process that lasts more than a billion years when two galaxies, under the inexorable attraction of gravity, dance one towards the other before finally joining. The simulations reveal that galaxies raise a lot of gas and dust while they are in slow motion.

The expelled material often forms a thick curtain around the centers of coalescent galaxies, protecting them from sight in visible light. Part of the material also falls on black holes in the nuclei of the galaxies that come together.

Black holes develop into a quick clip while they kneel with their cosmic food, and, being messy eaters, they ignite the burning gas. This rapid growth occurs in the last 10 million to 20 million years of the union. The Hubble and Keck Observatory images have captured a close-up view of this final stage when the massed black holes are only about 3,000 light years away – an almost cosmic hug.

It is not easy to find the nucleus of the galaxy so close to each other. Most of the previous observations of colliding galaxies captured coalescent black holes in the earlier phases when they were about 10 times more distant. The last phase of the fusion process is so elusive because the interacting galaxies are enclosed in dense dust and gas and require high resolution observations in the infrared light they can see through the clouds and locate the positions of the two fusion cores.

The team first searched for black holes that were visually obscured and active by analyzing X-ray data from the Burst Alert Telescope (BAT) on board the NASA Neil Gehrels Swift Telescope, a high-energy space observatory.

"The gas that falls on the black holes emits X-rays, and the brightness of the X-rays tells you how fast the black hole is growing," Koss explained. "I did not know if we would find hidden fusions, but we suspected, based on computer simulations, that they would be in very windy galaxies, so we tried to scrutinize the dust with the clearest images possible, in the hope of finding black holes coalescence. "

The researchers examined the Hubble archive, identifying those that linked the galaxies identified in the X-ray data. They then used the super-clear near-infrared vision of the Keck observatory to observe a larger sample of holes. blacks that produce X-rays not found in the Hubble archive.

"People had conducted studies to look for these interacting black holes first, but what really allowed this particular study were X-rays that can break the cocoon of dust," Koss said. "We also looked a little farther away in the universe so we could examine a large volume of space, giving us more chances to find brighter and rapidly growing black holes."

The team has targeted galaxies with an average distance of 330 million light years from Earth. Many galaxies have similar dimensions to the Milky Way and Andromeda galaxies. The team analyzed 96 galaxies from the Keck Observatory and 385 galaxies from the Hubble archive found in 38 different Hubble observing programs. The example galaxies are representative of what astronomers would have found by conducting an open-air investigation.

To test their results, the Koss team compared the galaxies of the survey with 176 other galaxies in the Hubble archive that lack actively growing black holes. The comparison confirmed that the light cores found in the researchers' census on the dusty interacting galaxies are in effect the signature of rapidly growing pairs of black holes for a collision.

When the two supermassive black holes in each of these systems finally come together over millions of years, their encounters will produce strong gravitational waves. The gravitational waves produced by the collision of two stellar mass black holes have already been detected by the gravitational laser interfering wave observatory (LIGO).

Observatories such as the projected spatial laser space antenna (LISA) with NASA / ESA space lasers will be able to detect low frequency gravitational waves from supermassive fusions of black holes, which are a million times more massive than those detected by LIGO.

Future infrared telescopes, such as NASA's James Webb Space Telescope and a new generation of giant land-based telescopes, will provide an even better probe for dusty collisions of galaxies by measuring masses, the rate of growth and the dynamics of black hole pairs.

The Webb telescope could also be able to look into the light of the mid-infrared to discover more galactic interactions so enclosed in dense gases and dust that even the near infrared light can not penetrate.

The results of the team will appear online in the November 7, 2018 issue of the journal Nature.

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