First Evidence Of Gigantic Remains From Star Explosions

Astrophysicists have found the first ever evidence of gigantic remains being formed from repeated explosions on the surface of a dead star in the Andromeda Galaxy, 2.5 million light years from Earth. The remains or “super-remnant” measures almost 400 light years across. For comparison, it takes just 8 minutes for light from the Sun to reach us.

A white dwarf is the dead core of a star. When it is paired with a companion star in a binary system, it can potentially produce a nova explosion. If the conditions are right, the white dwarf can pull gas from its companion star and when enough material builds up on the surface of the white dwarf, it triggers a thermonuclear explosion or “nova,” shining a million times brighter than our Sun and initially moving at up to 10,000 km per second.

Astrophysicists including Dr Steven Williams from Lancaster University in the UK examined the nova M31N 2008-12a in the Andromeda Galaxy, one of our nearest neighbours.

They used Hubble Space Telescope imaging, accompanied by spectroscopy from telescopes on Earth, to help uncover the nature of a gigantic super-remnant surrounding the nova. This is the first time such a huge remnant has been associated with a nova, and their research appears in Nature.

Dr Williams worked on Liverpool Telescope observations of the nova as well as helping to interpret the results.

He said: “This result is significant, as it is the first such remnant that has been found around a nova. This nova also has the most frequent explosions of any we know — once a year. The most frequent in our own Galaxy in only once every 10 years.

“It also has potential links to Type Ia supernovae, as this is how we would expect a nova system to behave when it is nearly massive enough to explode as a supernova.”

A Type Ia supernova is caused when the entire white dwarf is blown apart when it reaches a critical upper mass, rather than an explosion on its surface as in the case of the nova in this work. Type Ia supernovae are relatively rare. We have not observed one in our own Galaxy since Kepler’s supernova of 1604, named after the famous astronomer Johannes Kepler, who observed it shortly after it exploded and for the following year.

The team simulated how such a nova can create a vast, evacuated cavity around the star, by continually sweeping up the surrounding medium within a shell at the edge of a growing super-remnant.

The models show that the super-remnant — larger than almost all known remnants of supernova explosions — is consistent with being built up by frequent nova eruptions over millions of years.

Dr Matt Darnley from Liverpool John Moores University in the UK, who led the work, said: “Studying M31N 2008-12a and its super-remnant could help us to understand how some white dwarfs grow to their critical upper mass and how they actually explode as a Type Ia Supernova once they get there. Type Ia supernovae are critical tools used to work out how the universe expands and grows.”

Astronomers Find Signatures Of A ‘Messy’ Star That Made Its Companion Go Supernova

Many stars explode as luminous supernovae when, swollen with age, they run out of fuel for nuclear fusion. But some stars can go supernova simply because they have a close and pesky companion star that, one day, perturbs its partner so much that it explodes.

These latter events can happen in binary star systems, where two stars attempt to share dominion. While the exploding star gives off lots of evidence about its identity, astronomers must engage in detective work to learn about the errant companion that triggered the explosion.

On Jan. 10 at the 2019 American Astronomical Society meeting in Seattle, an international team of astronomers announced that they have identified the type of companion star that made its partner in a binary system, a carbon-oxygen white dwarf star, explode. Through repeated observations of SN 2015cp, a supernova 545 million light years away, the team detected hydrogen-rich debris that the companion star had shed prior to the explosion.

“The presence of debris means that the companion was either a red giant star or similar star that, prior to making its companion go supernova, had shed large amounts of material,” said University of Washington astronomer Melissa Graham, who presented the discovery and is lead author on the accompanying paper accepted for publication in The Astrophysical Journal.

The supernova material smacked into this stellar litter at 10 percent the speed of light, causing it to glow with ultraviolet light that was detected by the Hubble Space Telescope and other observatories nearly two years after the initial explosion. By looking for evidence of debris impacts months or years after a supernova in a binary star system, the team believes that astronomers could determine whether the companion had been a messy red giant or a relatively neat and tidy star.

The team made this discovery as part of a wider study of a particular type of supernova known as a Type Ia supernova. These occur when a carbon-oxygen white dwarf star explodes suddenly due to activity of a binary companion. Carbon-oxygen white dwarfs are small, dense and — for stars — quite stable. They form from the collapsed cores of larger stars and, if left undisturbed, can persist for billions of years.

Type Ia supernovae have been used for cosmological studies because their consistent luminosity makes them ideal “cosmic lighthouses,” according to Graham. They’ve been used to estimate the expansion rate of the universe and served as indirect evidence for the existence of dark energy.

Yet scientists are not certain what kinds of companion stars could trigger a Type Ia event. Plenty of evidence indicates that, for most Type Ia supernovae, the companion was likely another carbon-oxygen white dwarf, which would leave no hydrogen-rich debris in the aftermath. Yet theoretical models have shown that stars like red giants could also trigger a Type Ia supernova, which could leave hydrogen-rich debris that would be hit by the explosion. Out of the thousands of Type Ia supernovae studied to date, only a small fraction were later observed impacting hydrogen-rich material shed by a companion star. Prior observations of at least two Type Ia supernovae detected glowing debris months after the explosion. But scientists weren’t sure if those events were isolated occurrences, or signs that Type Ia supernovae could have many different kinds of companion stars.

“All of the science to date that has been done using Type Ia supernovae, including research on dark energy and the expansion of the universe, rests on the assumption that we know reasonably well what these ‘cosmic lighthouses’ are and how they work,” said Graham. “It is very important to understand how these events are triggered, and whether only a subset of Type Ia events should be used for certain cosmology studies.”

The team used Hubble Space Telescope observations to look for ultraviolet emissions from 70 Type Ia supernovae approximately one to three years following the initial explosion.

“By looking years after the initial event, we were searching for signs of shocked material that contained hydrogen, which would indicate that the companion was something other than another carbon-oxygen white dwarf,” said Graham.

In the case of SN 2015cp, a supernova first detected in 2015, the scientists found what they were searching for. In 2017, 686 days after the supernova exploded, Hubble picked up an ultraviolet glow of debris. This debris was far from the supernova source — at least 100 billion kilometers, or 62 billion miles, away. For reference, Pluto’s orbit takes it a maximum of 7.4 billion kilometers from our sun.

By comparing SN 2015cp to the other Type Ia supernovae in their survey, the researchers estimate that no more than 6 percent of Type Ia supernovae have such a litterbug companion. Repeated, detailed observations of other Type Ia events would help cement these estimates, Graham said.

The Hubble Space Telescope was essential for detecting the ultraviolet signature of the companion star’s debris for SN 2015cp. In the fall of 2017, the researchers arranged for additional observations of SN 2015cp by the W.M. Keck Observatory in Hawaii, the Karl G. Jansky Very Large Array in New Mexico, the European Southern Observatory’s Very Large Telescope and NASA’s Neil Gehrels Swift Observatory, among others. These data proved crucial in confirming the presence of hydrogen and are presented in a companion paper lead by Chelsea Harris, a research associate at Michigan State University.

“The discovery and follow-up of SN 2015cp’s emission really demonstrates how it takes many astronomers, and a wide variety of types of telescopes, working together to understand transient cosmic phenomena,” said Graham. “It is also a perfect example of the role of serendipity in astronomical studies: If Hubble had looked at SN 2015cp just a month or two later, we wouldn’t have seen anything.”

Graham is also a senior fellow with the UW’s DIRAC Institute and a science analyst with the Large Synoptic Survey Telescope, or LSST.

“In the future, as a part of its regularly scheduled observations, the LSST will automatically detect optical emissions similar to SN 2015cp — from hydrogen impacted by material from Type Ia supernovae,” said Graham said. “It’s going to make my job so much easier!”

Astronomers Observe Evolution Of A Black Hole As It Wolfs Down Stellar Material

On March 11, an instrument aboard the International Space Station detected an enormous explosion of X-ray light that grew to be six times as bright as the Crab Nebula, nearly 10,000 light years away from Earth. Scientists determined the source was a black hole caught in the midst of an outburst — an extreme phase in which a black hole can spew brilliant bursts of X-ray energy as it devours an avalanche of gas and dust from a nearby star.

Now astronomers from MIT and elsewhere have detected “echoes” within this burst of X-ray emissions, that they believe could be a clue to how black holes evolve during an outburst. In a study published today in the journal Nature, the team reports evidence that as the black hole consumes enormous amounts of stellar material, its corona — the halo of highly-energized electrons that surrounds a black hole — significantly shrinks, from an initial expanse of about 100 kilometers (about the width of Massachusetts) to a mere 10 kilometers, in just over a month.

The findings are the first evidence that the corona shrinks as a black hole feeds, or accretes. The results also suggest that it is the corona that drives a black hole’s evolution during the most extreme phase of its outburst.

“This is the first time that we’ve seen this kind of evidence that it’s the corona shrinking during this particular phase of outburst evolution,” says Jack Steiner, a research scientist in MIT’s Kavli Institute for Astrophysics and Space Research. “The corona is still pretty mysterious, and we still have a loose understanding of what it is. But we now have evidence that the thing that’s evolving in the system is the structure of the corona itself.”

Steiner’s MIT co-authors include Ronald Remillard and first author Erin Kara.

X-ray echoes

The black hole detected on March 11 was named MAXI J1820+070, for the instrument that detected it. The Monitor of All-sky X-ray Image (MAXI) mission is a set of X-ray detectors installed in the Japanese Experiment Module of the International Space Station (ISS), that monitors the entire sky for X-ray outbursts and flares.

Soon after the instrument picked up the black hole’s outburst, Steiner and his colleagues started observing the event with NASA’s Neutron star Interior Composition Explorer, or NICER, another instrument aboard the ISS, which was designed partly by MIT, to measure the amount and timing of incoming X-ray photons.

“This boomingly bright black hole came on the scene, and it was almost completely unobscured, so we got a very pristine view of what was going on,” Steiner says.

A typical outburst can occur when a black hole sucks away enormous amounts of material from a nearby star. This material accumulates around the black hole, in a swirling vortex known as an accretion disk, which can span millions of miles across. Material in the disk that is closer to the center of the black hole spins faster, generating friction that heats up the disk.

“The gas in the center is millions of degrees in temperature,” Steiner says. “When you heat something that hot, it shines out as X-rays. This disk can undergo avalanches and pour its gas down onto the central black hole at about a Mount Everest’s worth of gas per second. And that’s when it goes into outburst, which usually lasts about a year.”

Scientists have previously observed that X-ray photons emitted by the accretion disk can ping-pong off high-energy electrons in a black hole’s corona. Steiner says some of these photons can scatter “out to infinity,” while others scatter back onto the accretion disk as higher-energy X-rays.

By using NICER, the team was able to collect extremely precise measurements of both the energy and timing of X-ray photons throughout the black hole’s outburst. Crucially, they picked up “echoes,” or lags between low-energy photons (those that may have initially been emitted by the accretion disk) and high-energy photons (the X-rays that likely had interacted with the corona’s electrons). Over the course of a month, the researchers observed that the length of these lags decreased significantly, indicating that the distance between the corona and the accretion disk was also shrinking. But was it the disk or the corona that was shifting in?

To answer this, the researchers measured a signature that astronomers know as the “iron line” — a feature that is emitted by the iron atoms in an accretion disk only when they are energized, such as by the reflection of X-ray photons off a corona’s electrons. Iron, therefore, can measure the inner boundary of an accretion disk.

When the researchers measured the iron line throughout the outburst, they found no measurable change, suggesting that the disk itself was not shifting in shape, but remaining relatively stable. Together with the evidence of a diminishing X-ray lag, they concluded that it must be the corona that was changing, and shrinking as a result of the black hole’s outburst.

“We see that the corona starts off as this bloated, 100-kilometer blob inside the inner accretion disk, then shrinks down to something like 10 kilometers, over about a month,” Steiner says. “This is the first unambiguous case of a corona shrinking while the disk is stable.”

“NICER has allowed us to measure light echoes closer to a stellar-mass black hole than ever before,” Kara adds. “Previously these light echoes off the inner accretion disk were only seen in supermassive black holes, which are millions to billions of solar masses and evolve over millions of years. Stellar black holes like J1820 have much lower masses and evolve much faster, so we can see changes play out on human time scales.”

While it’s unclear what is exactly causing the corona to contract, Steiner speculates that the cloud of high-energy electrons is being squeezed by the overwhelming pressure generated by the accretion disk’s in-falling avalanche of gas.

The findings offer new insights into an important phase of a black hole’s outburst, known as a transition from a hard to a soft state. Scientists have known that at some point early on in an outburst, a black hole shifts from a “hard” phase that is dominated by the corona’s energy, to a “soft” phase that is ruled more by the accretion disk’s emissions.

“This transition marks a fundamental change in a black hole’s mode of accretion,” Steiner says. “But we don’t know exactly what’s going on. How does a black hole transition from being dominated by a corona to its disk? Does the disk move in and take over, or does the corona change and dissipate in some way? This is something people have been trying to unravel for decades And now this is a definitive piece of work in regards to what’s happening in this transition phase, and that what’s changing is the corona.”

This research is supported, in part, by NASA through the NICER mission and the Astrophysics Explorers Program.

X-Ray Pulse Detected Near Event Horizon As Black Hole Devours Star

On Nov. 22, 2014, astronomers spotted a rare event in the night sky: A supermassive black hole at the center of a galaxy, nearly 300 million light years from Earth, ripping apart a passing star. The event, known as a tidal disruption flare, for the black hole’s massive tidal pull that tears a star apart, created a burst of X-ray activity near the center of the galaxy. Since then, a host of observatories have trained their sights on the event, in hopes of learning more about how black holes feed.

Now researchers at MIT and elsewhere have pored through data from multiple telescopes’ observations of the event, and discovered a curiously intense, stable, and periodic pulse, or signal, of X-rays, across all datasets. The signal appears to emanate from an area very close to the black hole’s event horizon — the point beyond which material is swallowed inescapably by the black hole. The signal appears to periodically brighten and fade every 131 seconds, and persists over at least 450 days.

The researchers believe that whatever is emitting the periodic signal must be orbiting the black hole, just outside the event horizon, near the Innermost Stable Circular Orbit, or ISCO — the smallest orbit in which a particle can safely travel around a black hole.

Given the signal’s stable proximity to the black hole, and the black hole’s mass, which researchers previously estimated to be about 1 million times that of the sun, the team has calculated that the black hole is spinning at about 50 percent the speed of light.

The findings, reported today in the journal Science, are the first demonstration of a tidal disruption flare being used to estimate a black hole’s spin.

The study’s first author, Dheeraj Pasham, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research, says that most supermassive black holes are dormant and don’t usually emit much in the way of X-ray radiation. Only occasionally will they release a burst of activity, such as when stars get close enough for black holes to devour them. Now he says that, given the team’s results, such tidal disruption flares can be used to estimate the spin of supermassive black holes — a characteristic that has been, up until now, incredibly tricky to pin down.

“Events where black holes shred stars that come too close to them could help us map out the spins of several supermassive black holes that are dormant and otherwise hidden at the centers of galaxies,” Pasham says. “This could ultimately help us understand how galaxies evolved over cosmic time.”

Pasham’s co-authors include Ronald Remillard, Jeroen Homan, Deepto Chakrabarty, Frederick Baganoff, and James Steiner of MIT; Alessia Franchini at the University of Nevada; Chris Fragile of the College of Charleston; Nicholas Stone of Columbia University; Eric Coughlin of the University of California at Berkeley; and Nishanth Pasham, of Sunnyvale, California.

A real signal

Theoretical models of tidal disruption flares show that when a black hole shreds a star apart, some of that star’s material may stay outside the event horizon, circling, at least temporarily, in a stable orbit such as the ISCO, and giving off periodic flashes of X-rays before ultimately being fed by the black hole. The periodicity of the X-ray flashes thus encodes key information about the size of the ISCO, which itself is dictated by how fast the black hole is spinning.

Pasham and his colleagues thought that if they could see such regular flashes very close to a black hole that had undergone a recent tidal disruption event, these signals could give them an idea of how fast the black hole was spinning.

They focused their search on ASASSN-14li, the tidal disruption event that astronomers identified in November 2014, using the ground-based All-Sky Automated Survey for SuperNovae (ASASSN).

“This system is exciting because we think it’s a poster child for tidal disruption flares,” Pasham says. “This particular event seems to match many of the theoretical predictions.”

The team looked through archived datasets from three observatories that collected X-ray measurements of the event since its discovery: the European Space Agency’s XMM-Newton space observatory, and NASA’s space-based Chandra and Swift observatories. Pasham previously developed a computer code to detect periodic patterns in astrophysical data, though not for tidal disruption events specifically. He decided to apply his code to the three datasets for ASASSN-14li, to see if any common periodic patterns would rise to the surface.

What he observed was a surprisingly strong, stable, and periodic burst of X-ray radiation that appeared to come from very close to the edge of the black hole. The signal pulsed every 131 seconds, over 450 days, and was extremely intense — about 40 percent above the black hole’s average X-ray brightness.

“At first I didn’t believe it because the signal was so strong,” Pasham says. “But we saw it in all three telescopes. So in the end, the signal was real.”

Based on the properties of the signal, and the mass and size of the black hole, the team estimated that the black hole is spinning at least at 50 percent the speed of light.

“That’s not super fast — there are other black holes with spins estimated to be near 99 percent the speed of light,” Pasham says. “But this is the first time we’re able to use tidal disruption flares to constrain the spins of supermassive black holes.”

Illuminating the invisible

Once Pasham discovered the periodic signal, it was up to the theorists on the team to find an explanation for what may have generated it. The team came up with various scenarios, but the one that seems the most likely to generate such a strong, regular X-ray flare involves not just a black hole shredding a passing star, but also a smaller type of star, known as a white dwarf, orbiting close to the black hole.

Such a white dwarf may have been circling the supermassive black hole, at ISCO — the innermost stable circular orbit — for some time. Alone, it would not have been enough to emit any sort of detectable radiation. For all intents and purposes, the white dwarf would have been invisible to telescopes as it circled the relatively inactive, spinning black hole.

Sometime around Nov. 22, 2014, a second star passed close enough to the system that the black hole tore it apart in a tidal disruption flare that emitted an enormous amount of X-ray radiation, in the form of hot, shredded stellar material. As the black hole pulled this material inward, some of the stellar debris fell into the black hole, while some remained just outside, in the innermost stable orbit — the very same orbit in which the white dwarf circled. As the white dwarf came in contact with this hot stellar material, it likely dragged it along as a luminous overcoat of sorts, illuminating the white dwarf in an intense amount of X-rays each time it circled the black hole, every 131 seconds.

The scientists admit that such a scenario would be incredibly rare and would only last for several hundred years at most — a blink of an eye in cosmic scales. The chances of detecting such a scenario would be exceedingly slim.

“The problem with this scenario is that, if you have a black hole with a mass that’s 1 million times that of the sun, and a white dwarf is circling it, then at some point over just a few hundred years, the white dwarf will plunge into the black hole,” Pasham says. “We would’ve been extremely lucky to find such a system. But at least in terms of the properties of the system, this scenario seems to work.”

The results’ overarching significance is that they show it is possible to constrain the spin of a black hole, from tidal disruption events, according to Pasham. Going forward, he hopes to identify similar stable patterns in other star-shredding events, from black holes that reside further back in space and time.

“In the next decade, we hope to detect more of these events,” Pasham says. “Estimating spins of several black holes from the beginning of time to now would be valuable in terms of estimating whether there is a relationship between the spin and the age of black holes.”

This research was supported, in part, by NASA.

Study Reveals One Of Universe’s Secret Ingredients For Life

A new study led by ANU has investigated the nature of a cosmic phenomenon that slows down star formation, which helps to ensure the universe is a place where life can emerge.

Lead researcher Dr. Roland Crocker from the ANU Research School of Astronomy and Astrophysics said the research team studied a particular way stars provide a counter-pressure to gravity that slows down the star-formation process.

“If star formation happened rapidly, all stars would be bound together in massive clusters, where the intense radiation and supernova explosions would likely sterilise all the planetary systems, preventing the emergence of life,” he said.

“The conditions in these massive star clusters would possibly even prevent planets from forming in the first place.”

The study found that ultraviolet and optical light from young and massive stars spreads out into the gas from which the stars have recently formed and hits cosmic dust, which then scatters infrared light that acts effectively as a kind of pressure that pushes against gravity.

“The phenomenon we studied occurs in galaxies and star clusters where there’s a lot of dusty gas that is forming heaps of stars relatively quickly,” Dr. Crocker said.

“In galaxies forming stars more slowly—such as the Milky Way—other processes are slowing things down. The Milky Way forms two new stars every year, on average.”

Other galaxies in our vicinity and elsewhere in the universe continuously form new stars at a relatively slow and steady rate.

Dr. Crocker said the study’s mathematical findings indicated the phenomenon set an upper limit on how quickly stars can form in a galaxy or giant gas cloud.

“This and other forms of feedback help to keep the universe alive and vibrant,” he said.

“We are investigating other ways stars might feed back into their environment to slow down the overall rate of star formation.”

Professor Mark Krumholz and Dr Dougal Mackey from the ANU Research School of Astronomy and Astrophysics, Professor Todd Thompson from Ohio State University in the United States and Associate Professor Holger Baumgardt at the University of Queensland contributed to the study, which was published in the Monthly Notices of the Royal Astronomical Society.

Ultra-Hot Gas Around Remnants Of Sun-Like Stars

Solving a decades-old mystery, an international team of astronomers have discovered an extremely hot magnetosphere around a white dwarf, a remnant of a star like our Sun. The work was led by Dr Nicole Reindl, Research Fellow of the Royal Commission 1851, based at the University of Leicester, and is published today (7 November) in the journal Monthly Notices of the Royal Astronomical Society.

White dwarfs are the final stage in the lives of stars like our Sun. At the end of their lives, these stars eject their outer atmospheres, leaving behind a hot, compact and dense core that cools over billions of years. The temperature on their surfaces is typically around 100,000 degrees Celsius (in comparison the surface of the Sun is 5500 degrees).

Some white dwarfs though challenge scientists, as they show evidence for highly ionised metals. In astronomy ‘metals’ describe every element heavier than helium, and high ionisation here means that all but one of the outer electrons usually in their atoms have been stripped away. That process needs a temperature of 1 million degrees Celsius, so far higher than the surface of even the hottest white dwarf stars.

Reindl’s team used the 3.5-metre Calar Alto telescope in Spain to discover and observe a white dwarf in the direction of the constellation of Triangulum, catalogued as GALEXJ014636.8+323615, located 1200 light years from the Sun. Analysing the light from the white dwarf with a technique known as spectroscopy, where the light is dispersed into its constituent colours, revealed the signatures of highly ionised metals. Intriguingly these varied over a period of six hours — the same time it takes for the white dwarf to rotate.

Reindl and her team conclude that the magnetic field around the star — the magnetosphere — traps material flowing from its surface. Shocks within the magnetosphere heat the material dramatically, stripping almost all the electrons from the metal atoms.

“It’s like a doughnut made up of ultra-hot material that surrounds the already very hot star” explains Reindl.

“The axis of the magnetic field of the white dwarf is tilted from its rotational axis. This means that the amount of shock-heated material we see varies as the star rotates.

‘After decades of finding more and more of these obscure stars without having a clue where these highly ionised metals come from,” she continues, “our shock-heated magnetosphere model finally explains their origin.”

Magnetospheres are found around other types of stars, but this is the first report of one around a white dwarf. The discovery might have far-reaching consequences. “We simply didn’t take this into account,” admits Reindl. “Ignoring their magnetospheres could mean measurements of other basic properties of white dwarfs are wrong, like their temperatures and masses.”

It may be that a quarter of white dwarfs go through a stage of trapping and super-heating material. Reindl and her team now plan to model them in detail and to extend their research by studying more of these fascinating objects.

Astronomers Find Pairs Of Black Holes At The Centers Of Merging Galaxies

For the first time, a team of astronomers 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’ messy cores, the research 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.

Led by University of Maryland alumnus Michael Koss (M.S. ’07, Ph.D. ’11, astronomy), a research scientist at Eureka Scientific, Inc., with contributions from UMD astronomers, the team surveyed hundreds of nearby galaxies using imagery from the W.M. Keck Observatory in Hawaii and NASA’s Hubble Space Telescope. The Hubble observations represent more than 20 years’ worth of images from the telescope’s lengthy archive. The team described their findings in a research paper published on November 8, 2018, in the journal Nature.

“Seeing the pairs of merging galaxy nuclei associated with these huge black holes so close together was pretty amazing,” Koss said. “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 high-resolution images also provide a close-up preview of a phenomenon that astronomers suspect was more common in the early universe, when galaxy mergers were more frequent. When the black holes finally do collide, they will unleash powerful energy in the form of gravitational waves — ripples in space-time recently detected for the first time by the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors.

The images also presage what will likely happen in a few billion years, when our Milky Way galaxy merges with the neighboring Andromeda galaxy. Both galaxies host supermassive black holes at their center, which will eventually smash together and merge into one larger black hole.

The team was inspired by a Hubble image of two interacting galaxies collectively called NGC 6240, which later served as a prototype for the study. The team first searched for visually obscured, active black holes by sifting through 10 years’ worth of X-ray data from the Burst Alert Telescope (BAT) aboard NASA’s Neil Gehrels Swift Observatory.

“The advantage to using Swift’s BAT is that it observes high-energy, ‘hard’ X-rays,” said study co-author Richard Mushotzky, a professor of astronomy at UMD and a fellow of the Joint Space-Science Institute (JSI). “These X-rays penetrate through the thick clouds of dust and gas that surround active galaxies, allowing the BAT to see things that are literally invisible in other wavelengths.”

The researchers then combed through the Hubble archive, zeroing in on the merging galaxies they spotted in the X-ray data. They then used the Keck telescope’s super-sharp, near-infrared vision to observe a larger sample of the X-ray-producing black holes not found in the Hubble archive.

The team targeted galaxies located an average of 330 million light-years from Earth — relatively close by in cosmic terms. Many of the galaxies are similar in size to the Milky Way and Andromeda galaxies. In total, the team analyzed 96 galaxies observed with the Keck telescope and 385 galaxies from the Hubble archive.

Their results suggest that more than 17 percent of these galaxies host a pair of black holes at their center, which are locked in the late stages of spiraling ever closer together before merging into a single, ultra-massive black hole. The researchers were surprised to find such a high fraction of late-stage mergers, because most simulations suggest that black hole pairs spend very little time in this phase.

To check their results, the researchers compared the survey galaxies with a control group of 176 other galaxies from the Hubble archive that lack actively growing black holes. In this group, only about one percent of the surveyed galaxies were suspected to host pairs of black holes in the later stages of merging together.

This last step helped the researchers confirm that the luminous galactic cores found in their census of dusty interacting galaxies are indeed a signature of rapidly-growing black hole pairs headed for a collision. According to the researchers, this finding is consistent with theoretical predictions, but until now, had not been verified by direct observations.

“People had conducted studies to look for these close interacting black holes before, but what really enabled this particular study were the X-rays that can break through the cocoon of dust,” explained Koss. “We also looked a bit farther in the universe so that we could survey a larger volume of space, giving us a greater chance of finding more luminous, rapidly-growing black holes.”

It is not easy to find galactic nuclei so close together. Most prior observations of merging galaxies have caught the coalescing black holes at earlier stages, when they were about 10 times farther away. The late stage of the merger process is so elusive because the interacting galaxies are encased in dense dust and gas, requiring very high-resolution observations that can see through the clouds and pinpoint the two merging nuclei.

“Computer simulations of galaxy smashups show us that black holes grow fastest during the final stages of mergers, near the time when the black holes interact, and that’s what we have found in our survey,” said Laura Blecha, an assistant professor of physics at the University of Florida and a co-author of the study. Blecha was a JSI Prize Postdoctoral Fellow in the UMD Department of Astronomy prior to joining UF’s faculty in 2017. “The fact that black holes grow faster and faster as mergers progress tells us galaxy encounters are really important for our understanding of how these objects got to be so monstrously big.”

Future infrared telescopes such as NASA’s highly anticipated James Webb Space Telescope (JWST), slated for launch in 2021, will provide an even better view of mergers in dusty, heavily obscured galaxies. For nearby black hole pairs, JWST should also be capable of measuring the masses, growth rates and other physical parameters for each black hole.

“There might be other objects that we missed. Even with Hubble, many nearby galaxies at low redshift cannot be resolved — the two nuclei just merge into one,” said study co-author Sylvain Veilleux, a professor of astronomy at UMD and a JSI Fellow. “With JWST’s higher angular resolution and sensitivity to the infrared, which can pass through the dusty cores of these galaxies, searches for these nearby objects should be easy to do. Also with JWST, we will be able to push toward larger distances, to see objects at higher redshift. With these observations, we can begin to explore the fraction of objects that are merging in the youngest, most distant regions of the universe — which should be fairly frequent.”