Massive fuel-hungry black holes feed off intergalactic gas

Research led by the University of Southampton has revealed how supermassive black holes (SMBHs) are feeding off gas clouds which reach them by travelling hundreds of thousands of light years from one galaxy to another.

An international team of scientists has shown there is a crucial link between the interaction of neighbouring galaxies and the enormous amount of gas needed to 'fuel' these giant, super-dense, space phenomena. Their findings are due to be published in the journal Nature Astronomy.

A black hole can be created when a star collapses, squeezing matter into a relatively tiny space. This increases the force of gravity to a point where nothing can escape, not even light -- hence the name.

Some black holes are gigantic, with masses millions of times greater than our sun, emitting enormous amounts of energy. These are known as 'supermassive black holes' and exactly how they are formed or gain enough fuel to power themselves is still a mystery.

Astrophysicist and lead researcher from the University of Southampton, Dr Sandra Raimundo, comments: "Supermassive black holes fuel their activity by, in part, the gradual accumulation of gas from the environment around them. Supermassive black holes can make the centres of galaxies shine very brightly when they capture gas and it's thought this process can be a major influence on the way that galaxies look today. How SMBHs get enough fuel to sustain their activity and growth still puzzles astronomers, but the work we have carried out provides a step towards understanding this."

The Southampton scientist, working with researchers at the universities of Copenhagen and California, used data from the 4-metre Anglo-Australian telescope in New South Wales, Australia* to study the orbits of gas and stars in a large sample of more than 3000 galaxies. They identified those with the presence of what is known as 'misaligned' gas -- in other words, gas which rotates in a different direction from the stars in the galaxy, signalling a past galaxy interaction. They then found that galaxies with misaligned gas had a higher fraction of active supermassive black holes.

The results showed a clear link between misaligned gas and supermassive black hole activity -- suggesting the gas is transferred where two galaxies meet, meanders vast distances through space and then succumbs to the huge gravitational forces of the supermassive black hole -- pulled in and swallowed up as a vital source of fuel. Astronomers have long suspected that a merger with another galaxy could provide this source of gas, but direct evidence for this has been elusive.

Dr Raimundo explains: "The work that we carried out shows the presence of gas that is misaligned from stars is associated with an increase in the fraction of active supermassive black holes. Since misaligned gas is a clear sign of a past interaction between two galaxies, our work shows that galaxy interactions provide fuel to power active supermassive black holes.

"This is the first time that a direct connection has been observed between the formation and presence of misaligned gas and the fuelling of active supermassive black holes."

Dr Marianne Vestergaard, a co-author in the study, highlights: "What is exciting about these observations is that we can now, for the very first time, identify the captured gas and trace it all the way to the centre where the black hole is devouring it."

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Black holes in eccentric orbit

When black holes collide in the universe, the clash shakes up space and time: the amount of energy released during the merger is so great that it causes space-time to oscillate, similar to waves on the surface of water. These gravitational waves spread out through the entire universe and can still be measured thousands of light years away, as was the case on 21 May 2019, when the two gravitational wave observatories LIGO (USA) and Virgo (Italy) captured such a signal. Named GW190521 after the date of its discovery, the gravitational wave event has since provoked discussion among experts because it differs markedly from previously measured signals.

The signal had initially been interpreted to mean that the collision involved two black holes moving in near-circular orbits around each other. "Such binary systems can be created by a number of astrophysical processes," explains Prof. Sebastiano Bernuzzi, a theoretical physicist from the University of Jena, Germany. Most of the black holes discovered by LIGO and Virgo, for example, are of stellar origin. "That means they are the remnants of massive stars in binary star systems," adds Bernuzzi, who led the current study. Such black holes orbit each other in quasi-circular orbits, just as the original stars did previously.

One black hole captures a second

"GW190521 behaves significantly differently, however," explains Rossella Gamba. The lead author of the publication is doing her doctorate in Jena Research Training Group 2522 and is part of Bernuzzi's team. "Its morphology and explosion-like structure are very different from previous observations." So, Rossella Gamba and her colleagues set out to find an alternative explanation for the unusual gravitational wave signal. Using a combination of state-of-the-art analytical methods and numerical simulations on supercomputers, they calculated different models for the cosmic collision. They came to the conclusion that it must have occurred on a strongly eccentric path instead of a quasi-circular one. A black hole initially moves freely in an environment that is relatively densely filled with matter and, as soon as it gets close to another black hole, it can be "captured" by the other's gravitational field. This also leads to the formation of a binary system, but here the two black holes do not orbit in a circle, but move eccentrically, in tumbling motions around each other.

"Such a scenario explains the observations much better than any other hypothesis presented so far. The probability is 1:4300," says Matteo Breschi, doctoral student and co-author of the study, who developed the infrastructure for the analysis. And postdoctoral researcher Dr Gregorio Carullo adds: "Even though we don't currently know exactly how common such dynamic movements by black holes are, we don't expect them to be a frequent occurrence." This makes the current results all the more exciting, he adds. Nevertheless, more research is needed to clarify beyond doubt the processes that created GW190521.

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Unprecedented glimpse of merging galaxies

Using the James Webb Space Telescope to look back in time at the early universe, astronomers discovered a surprise: a cluster of galaxies merging together around a rare red quasar within a massive black hole. The findings by Johns Hopkins University and an international team offer an unprecedented opportunity to observe how billions of years ago galaxies coalesced into the modern universe.

"We think something dramatic is about to happen in these systems," said co-author Andrey Vayner, a Johns Hopkins postdoctoral fellow who studies the evolution of galaxies. "The galaxy is at this perfect moment in its lifetime, about to transform and look entirely different in a few billion years."

The work is in press in Astrophysical Journal Letters and available today on the arXiv paper repository.

The James Webb Space Telescope, launched last December by NASA, the European Space Agency, and the Canadian Space Agency, is the largest, most powerful telescope ever sent into space. Its initial general observations were revealed in July, but this quasar imagery is one of just 13 "early look" projects selected through a highly competitive global competition to decide where the telescope is pointed during its first months of operation.

In Baltimore, the Johns Hopkins team heard their chosen target would be observed within days of President Biden's unveiling of the Webb's debut pictures on July 11, so stayed close to their computers. That following summer Saturday, Vayner and graduate student Yuzo Ishikawa were repeatedly refreshing the Webb database when suddenly the data arrived, leading to a hastily assembled multinational team confab on Sunday to try to make sense of the jaw-droppingly detailed raw images.

Although earlier observations of this area by NASA/ESA Hubble Space Telescope and the Near-Infrared Integral Field Spectrometer instrument on the Gemini-North telescope pinpointed the quasar and hinted at the possibility of a galaxy in transition, no one suspected that with Webb's crisp imaging they'd see multiple galaxies, at least three, swirling the region.

"With previous images we thought we saw hints that the galaxy was possibly interacting with other galaxies on the path to merger because their shapes get distorted in the process and we thought we maybe saw that," said co-principal investigator Nadia L. Zakamska, a Johns Hopkins astrophysicist who helped conceive the project back in 2017 with then-Johns Hopkins postdoc Dominika Wylezalek, who's now the group leader at the University of Heidelberg. "But after we got the Webb data, I was like, 'I have no idea what we're even looking at here, what is all this stuff!' We spent several weeks just staring and staring at these images."

The Webb revealed at least three galaxies moving incredibly fast, suggesting a large amount of mass is present. The team believes this could be one of the densest known areas of galaxy formation in the early universe.

Because light takes time to travel to us, when we look at objects like this one in the very distant regions of the universe, we're seeing light that was emitted about 11.5 billion years ago, or from the earliest stages of the universe's evolution. Massive galaxy swarms like this one were likely common then, Zakamska said.

"It's super exciting to be one of the first people to see this really cool object," said Ishikawa, who contributed to the interpretation of the galaxy swarm.

Even Vayner, who'd dreamed of using Webb data since he first heard about the telescope as an undergraduate more than a decade ago, and thought he knew what to expect, was shocked to see his long-studied spot in the universe revealed with such clarity.

"It really will transform our understanding of this object," said Vayner, who was instrumental in adapting the raw Webb data for scientific analysis.

The blindingly bright quasar, fueled by what Zakamska calls a "monster" black hole at the center of the galactic swirl, is a rare "extremely red" quasar, about 11.5 billion years old and one of the most powerful ever seen from such distance. It's essentially a black hole in formation, Vayner said, eating the gas around it and growing in mass. The clouds of dust and gas between Earth and the glowing gas near the black hole make the quasar appear red.

The team is already working on follow-up observations into this unexpected galaxy cluster, hoping to better understand how dense, chaotic galaxy clusters form, and how it is affected by supermassive black hole at its heart.

"What you see here is only a small subset of what's in the data set," Zakamska said. "There's just too much going on here so we first highlighted what really is the biggest surprise. Every blob here is a baby galaxy merging into this mommy galaxy and the colors are different velocities and the whole thing is moving in an extremely complicated way. We can now start to untangle the motions."

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The ultimate fate of a star shredded by a black hole

In 2019, astronomers observed the nearest example to date of a star that was shredded, or "spaghettified," after approaching too close to a massive black hole.

That tidal disruption of a sun-like star by a black hole 1 million times more massive than itself took place 215 million light years from Earth. Luckily, this was the first such event bright enough that astronomers from the University of California, Berkeley, could study the optical light from the stellar death, specifically the light's polarization, to learn more about what happened after the star was torn apart.

Their observations on Oct. 8, 2019, suggest that a lot of the star's material was blown away at high speed -- up to 10,000 kilometers per second -- and formed a spherical cloud of gas that blocked most of the high-energy emissions produced as the black hole gobbled up the remainder of the star.

Earlier, other observations of optical light from the blast, called AT2019qiz, revealed that much of the star's matter was launched outward in a powerful wind. But the new data on the light's polarization, which was essentially zero at visible or optical wavelengths when the event was at its brightest, tells astronomers that the cloud was likely spherically symmetric.

"This is the first time anyone has deduced the shape of the gas cloud around a tidally spaghetiffied star," said Alex Filippenko, UC Berkeley professor of astronomy and a member of the research team.

The results support one answer to why astronomers don't see high-energy radiation, such as X-rays, from many of the dozens of tidal disruption events observed to date: The X-rays, which are produced by material ripped from the star and dragged into an accretion disk around the black hole before falling inward, are obscured from view by the gas blown outward by powerful winds from the black hole.

"This observation rules out a class of solutions that have been proposed theoretically and gives us a stronger constraint on what happens to gas around a black hole," said UC Berkeley graduate student Kishore Patra, lead author of the study. "People have been seeing other evidence of wind coming out of these events, and I think this polarization study definitely makes that evidence stronger, in the sense that you wouldn't get a spherical geometry without having a sufficient amount of wind. The interesting fact here is that a significant fraction of the material in the star that is spiraling inward doesn't eventually fall into the black hole -- it's blown away from the black hole."

Polarization reveals symmetry

Many theorists have hypothesized that the stellar debris forms an eccentric, asymmetric disk after disruption, but an eccentric disk is expected to show a relatively high degree of polarization, which would mean that perhaps several percent of the total light is polarized. This was not observed for this tidal disruption event.

"One of the craziest things a supermassive black hole can do is to shred a star by its enormous tidal forces," said team member Wenbin Lu, UC Berkeley assistant professor of astronomy. "These stellar tidal disruption events are one of very few ways astronomers know the existence of supermassive black holes at the centers of galaxies and measure their properties. However, due to the extreme computational cost in numerically simulating such events, astronomers still do not understand the complicated processes after a tidal disruption."

A second set of observations on Nov. 6, 29 days after the October observation, revealed that the light was very slightly polarized, about 1%, suggesting that the cloud had thinned enough to reveal the asymmetric gas structure around the black hole. Both observations came from the 3-meter Shane telescope at Lick Observatory near San Jose, California, that is fitted with the Kast spectrograph, an instrument that can determine the polarization of light over the full optical spectrum. The light becomes polarized -- its electrical field vibrates primarily in one direction -- when it scatters off electrons in the gas cloud.

"The accretion disk itself is hot enough to emit most of its light in X-rays, but that light has to come through this cloud, and there are many scatterings, absorptions and reemissions of light before it can escape out of this cloud," Patra said. "With each of these processes, the light loses some of its photon energy, going all the way down to ultraviolet and optical energies. The final scatter then determines the polarization state of the photon. So, by measuring polarization, we can deduce the geometry of the surface where the final scatter happens."

Patra noted that this deathbed scenario may apply only to normal tidal disruptions -- not "oddballs," in which relativistic jets of material are expelled out the poles of the black hole. Only more measurements of the polarization of light from these events will answer that question.

"Polarization studies are very challenging, and very few people are well-versed enough in the technique around the world to utilize this," he said. "So, this is uncharted territory for tidal disruption events."

Patra, Filippenko, Lu and UC Berkeley researcher Thomas Brink, graduate student Sergiy Vasylyev and postdoctoral fellow Yi Yang reported their observations in a paper that has been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

A cloud 100 times larger than Earth's orbit

The UC Berkeley researchers calculated that the polarized light was emitted from the surface of a spherical cloud with a radius of about 100 astronomical units (au), 100 times farther from the star than Earth is from the sun. An optical glow from hot gas emanated from a region at about 30 au.

The 2019 spectropolarimetric observations -- a technique that measures polarization across many wavelengths of light -- were of AT2019qiz, a tidal disruption located in a spiral galaxy in the constellation of Eridanus. The zero polarization of the entire spectrum in October indicates a spherically symmetric cloud of gas -- all the polarized photons balance one another. The slight polarization of the November measurements indicates a small asymmetry. Because these tidal disruptions occur so far away, in the centers of distant galaxies, they appear as only a point of light, and polarization is one of few indications of the shapes of objects.

"These disruption events are so far away that you can't really resolve them, so you can't study the geometry of the event or the structure of these explosions," Filippenko said. "But studying polarized light actually helps us to deduce some information about the distribution of the matter in that explosion or, in this case, how the gas -- and possibly the accretion disk -- around this black hole is shaped."

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Astronomers discover strangely massive black hole in Milky Way satellite galaxy

Astronomers at The University of Texas at Austin's McDonald Observatory have discovered an unusually massive black hole at the heart of one of the Milky Way's dwarf satellite galaxies, called Leo I. Almost as massive as the black hole in our own galaxy, the finding could redefine our understanding of how all galaxies -- the building blocks of the universe -- evolve. The work is published in a recent issue of The Astrophysical Journal.

The team decided to study Leo I because of its peculiarity. Unlike most dwarf galaxies orbiting the Milky Way, Leo I does not contain much dark matter. Researchers measured Leo I's dark matter profile -- that is, how the density of dark matter changes from the outer edges of the galaxy all the way into its center. They did this by measuring its gravitational pull on the stars: The faster the stars are moving, the more matter there is enclosed in their orbits. In particular, the team wanted to know whether dark matter density increases toward the galaxy's center. They also wanted to know whether their profile measurement would match previous ones made using older telescope data combined with computer models.

Led by recent UT Austin doctoral graduate María José Bustamante, the team includes UT astronomers Eva Noyola, Karl Gebhardt and Greg Zeimann, as well as colleagues from Germany's Max Planck Institute for Extraterrestrial Physics (MPE).

For their observations, they used a unique instrument called VIRUS-W on McDonald Observatory's 2.7-meter Harlan J. Smith Telescope.

When the team fed their improved data and sophisticated models into a supercomputer at UT Austin's Texas Advanced Computing Center, they got a startling result.

"The models are screaming that you need a black hole at the center; you don't really need a lot of dark matter," Gebhardt said. "You have a very small galaxy that is falling into the Milky Way, and its black hole is about as massive as the Milky Way's. The mass ratio is absolutely huge. The Milky Way is dominant; the Leo I black hole is almost comparable." The result is unprecedented.

The researchers said the result was different from the past studies of Leo I due to a combination of better data and the supercomputer simulations. The central, dense region of the galaxy was mostly unexplored in previous studies, which concentrated on the velocities of individual stars. The current study showed that for those few velocities that were taken in the past, there was a bias toward low velocities. This, in turn, decreased the inferred amount of matter enclosed within their orbits.

The new data is concentrated in the central region and is unaffected by this bias. The amount of inferred matter enclosed within the stars' orbits skyrocketed.

The finding could shake up astronomers' understanding of galaxy evolution, as "there is no explanation for this kind of black hole in dwarf spheroidal galaxies," Bustamante said.

The result is all the more important as astronomers have used galaxies such as Leo I, called "dwarf spheroidal galaxies," for 20 years to understand how dark matter is distributed within galaxies, Gebhardt added. This new type of black hole merger also gives gravitational wave observatories a new signal to search for.

"If the mass of Leo I's black hole is high, that may explain how black holes grow in massive galaxies," Gebhardt said. That's because over time, as small galaxies like Leo I fall into larger galaxies, the smaller galaxy's black hole merges with that of the larger galaxy, increasing its mass.

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Cracking a mystery of massive black holes and quasars with supercomputer simulations

At the center of galaxies, like our own Milky Way, lie massive black holes surrounded by spinning gas. Some shine brightly, with a continuous supply of fuel, while others go dormant for millions of years, only to reawaken with a serendipitous influx of gas. It remains largely a mystery how gas flows across the universe to feed these massive black holes.

UConn Assistant Professor of Physics Daniel Anglés-Alcázar, lead author on a paper published today in The Astrophysical Journal, addresses some of the questions surrounding these massive and enigmatic features of the universe by using new, high-powered simulations.

"Supermassive black holes play a key role in galaxy evolution and we are trying to understand how they grow at the centers of galaxies," says Anglés-Alcázar. "This is very important not just because black holes are very interesting objects on their own, as sources of gravitational waves and all sorts of interesting stuff, but also because we need to understand what the central black holes are doing if we want to understand how galaxies evolve."

Anglés-Alcázar, who is also an Associate Research Scientist at the Flatiron Institute Center for Computational Astrophysics, says a challenge in answering these questions has been creating models powerful enough to account for the numerous forces and factors that play into the process. Previous works have looked either at very large scales or the very smallest of scales, "but it has been a challenge to study the full range of scales connected simultaneously."

Galaxy formation, Anglés-Alcázar says, starts with a halo of dark matter that dominates the mass and gravitational potential in the area and begins pulling in gas from its surroundings. Stars form from the dense gas, but some of it must reach the center of the galaxy to feed the black hole. How does all that gas get there? For some black holes, this involves huge quantities of gas, the equivalent of ten times the mass of the sun or more swallowed in just one year, says Anglés-Alcázar.

"When supermassive black holes are growing very fast, we refer to them as quasars," he says. "They can have a mass well into one billion times the mass of the sun and can outshine everything else in the galaxy. How quasars look depends on how much gas they add per unit of time. How do we manage to get so much gas down to the center of the galaxy and close enough that the black hole can grab it and grow from there?"

The new simulations provide key insights into the nature of quasars, showing that strong gravitational forces from stars can twist and destabilize the gas across scales, and drive sufficient gas influx to power a luminous quasar at the epoch of peak galaxy activity.

In visualizing this series of events, it is easy to see the complexities of modeling them, and Anglés-Alcázar says it is necessary to account for the myriad components influencing black hole evolution.

"Our simulations incorporate many of the key physical processes, for example, the hydrodynamics of gas and how it evolves under the influence of pressure forces, gravity, and feedback from massive stars. Powerful events such as supernovae inject a lot of energy into the surrounding medium and this influences how the galaxy evolves, so we need to incorporate all of these details and physical processes to capture an accurate picture."

Building on previous work from the FIRE ("Feedback In Realistic Environments") project, Anglés-Alcázar explains the new technique outlined in the paper that greatly increases model resolution and allows for following the gas as it flows across the galaxy with more than a thousand times better resolution than previously possible,

"Other models can tell you a lot of details about what's happening very close to the black hole, but they don't contain information about what the rest of the galaxy is doing, or even less, what the environment around the galaxy is doing. It turns out, it is very important to connect all of these processes at the same time, this is where this new study comes in."

The computing power is similarly massive, Anglés-Alcázar says, with hundreds of central processing units (CPUs) running in parallel that could have easily taken the length of millions of CPU hours.

"This is the first time that we have been able to create a simulation that can capture the full range of scales in a single model and where we can watch how gas is flowing from very large scales all the way down to the very center of the massive galaxy that we are focusing on."

For future studies of large statistical populations of galaxies and massive black holes, we need to understand the full picture and the dominant physical mechanisms for as many different conditions as possible, says Anglés-Alcázar.

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First black hole ever detected is more massive than we thought

 New observations of the first black hole ever detected have led astronomers to question what they know about the Universe's most mysterious objects.

Published today in the journal Science, the research shows the system known as Cygnus X-1 contains the most massive stellar-mass black hole ever detected without the use of gravitational waves.

Cygnus X-1 is one of the closest black holes to Earth. It was discovered in 1964 when a pair of Geiger counters were carried on board a sub-orbital rocket launched from New Mexico.

The object was the focus of a famous scientific wager between physicists Stephen Hawking and Kip Thorne, with Hawking betting in 1974 that it was not a black hole. Hawking conceded the bet in 1990.

In this latest work, an international team of astronomers used the Very Long Baseline Array -- a continent-sized radio telescope made up of 10 dishes spread across the United States -- together with a clever technique to measure distances in space.

"If we can view the same object from different locations, we can calculate its distance away from us by measuring how far the object appears to move relative to the background," said lead researcher, Professor James Miller-Jones from Curtin University and the International Centre for Radio Astronomy Research (ICRAR).

"If you hold your finger out in front of your eyes and view it with one eye at a time, you'll notice your finger appears to jump from one spot to another. It's exactly the same principle."

"Over six days we observed a full orbit of the black hole and used observations taken of the same system with the same telescope array in 2011," Professor Miller-Jones said. "This method and our new measurements show the system is further away than previously thought, with a black hole that's significantly more massive."

Co-author Professor Ilya Mandel from Monash University and the ARC Centre of Excellence in Gravitational Wave Discovery (OzGrav) said the black hole is so massive it's actually challenging how astronomers thought they formed.

"Stars lose mass to their surrounding environment through stellar winds that blow away from their surface. But to make a black hole this heavy, we need to dial down the amount of mass that bright stars lose during their lifetimes" he said.

"The black hole in the Cygnus X-1 system began life as a star approximately 60 times the mass of the Sun and collapsed tens of thousands of years ago," he said. "Incredibly, it's orbiting its companion star -- a supergiant -- every five and a half days at just one-fifth of the distance between the Earth and the Sun.

"These new observations tell us the black hole is more than 20 times the mass of our Sun -- a 50 per cent increase on previous estimates."

Xueshan Zhao is a co-author on the paper and a PhD candidate studying at the National Astronomical Observatories -- part of the Chinese Academy of Sciences (NAOC) in Beijing.

"Using the updated measurements for the black hole's mass and its distance away from Earth, I was able to confirm that Cygnus X-1 is spinning incredibly quickly -- very close to the speed of light and faster than any other black hole found to date," she said.

"I'm at the beginning of my research career, so being a part of an international team and helping to refine the properties of the first black hole ever discovered has been a great opportunity."

Next year, the world's biggest radio telescope -- the Square Kilometre Array (SKA) -- will begin construction in Australia and South Africa.

"Studying black holes is like shining a light on the Universe's best kept secret -- it's a challenging but exciting area of research," Professor Miller-Jones said.

"As the next generation of telescopes comes online, their improved sensitivity reveals the Universe in increasingly more detail, leveraging decades of effort invested by scientists and research teams around the world to better understand the cosmos and the exotic and extreme objects that exist.

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When galaxies collide

 It was previously thought that collisions between galaxies would necessarily add to the activity of the massive black holes at their centers. However, researchers have performed the most accurate simulations of a range of collision scenarios and have found that some collisions can reduce the activity of their central black holes. The reason is that certain head-on collisions may in fact clear the galactic nuclei of the matter which would otherwise fuel the black holes contained within.

When you think about gargantuan phenomena such as the collision of galaxies, it might be tempting to imagine it as some sort of cosmic cataclysm, with stars crashing and exploding, and destruction on an epic scale. But actually it is closer to a pair of clouds combining, usually a larger one absorbing a smaller one. It's unlikely any stars within them would collide themselves. But that said, when galaxies collide, the consequences can be enormous.

Galaxies collide in different ways. Sometimes a small galaxy will collide with the outer part of a larger one and either pass through or merge, in either case exchanging a lot of stars along the way. But galaxies can also collide head-on, where the smaller of the two will be torn apart by overpowering tidal forces of the larger one. It's in this scenario that something very interesting can happen within the galactic nucleus.

"At the heart of most galaxies lies a massive black hole, or MBH," said Research Associate Yohei Miki from the University of Tokyo. "For as long as astronomers have explored galactic collisions, it has been assumed that a collision would always provide fuel for an MBH in the form of matter within the nucleus. And that this fuel would feed the MBH, significantly increasing its activity, which we would see as ultraviolet and X-ray light amongst other things. However, we now have good reason to believe that this sequence of events is not inevitable and that in fact the exact opposite might sometimes be true."

It seems logical that a galactic collision would only increase the activity of an MBH, but Miki and his team were curious to test this notion. They constructed highly detailed models of galactic collision scenarios and ran them on supercomputers. The team was pleased to see that in some circumstances, an incoming small galaxy might actually strip away the matter surrounding the MBH of the larger one. This would reduce instead of increase its activity.

"We computed the dynamic evolution of the gaseous matter which surrounds the MBH in a torus, or donut, shape," said Miki. "If the incoming galaxy accelerated this torus above a certain threshold determined by properties of the MBH, then the matter would be ejected and the MBH would be starved. These events can last in the region of a million years, though we are still unsure about how long the suppression of MBH activity may last."

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A 'bang' in LIGO and Virgo detectors signals most massive gravitational-wave source yet

 For all its vast emptiness, the universe is humming with activity in the form of gravitational waves. Produced by extreme astrophysical phenomena, these reverberations ripple forth and shake the fabric of space-time, like the clang of a cosmic bell.

Now researchers have detected a signal from what may be the most massive black hole merger yet observed in gravitational waves. The product of the merger is the first clear detection of an "intermediate-mass" black hole, with a mass between 100 and 1,000 times that of the sun.

They detected the signal, which they have labeled GW190521, on May 21, 2019, with the National Science Foundation's Laser Interferometer Gravitational-wave Observatory (LIGO), a pair of identical, 4-kilometer-long interferometers in the United States; and Virgo, a 3-kilometer-long detector in Italy.

The signal, resembling about four short wiggles, is extremely brief in duration, lasting less than one-tenth of a second. From what the researchers can tell, GW190521 was generated by a source that is roughly 5 gigaparsecs away, when the universe was about half its age, making it one of the most distant gravitational-wave sources detected so far.

As for what produced this signal, based on a powerful suite of state-of-the-art computational and modeling tools, scientists think that GW190521 was most likely generated by a binary black hole merger with unusual properties.

Almost every confirmed gravitational-wave signal to date has been from a binary merger, either between two black holes or two neutron stars. This newest merger appears to be the most massive yet, involving two inspiraling black holes with masses about 85 and 66 times the mass of the sun.

The LIGO-Virgo team has also measured each black hole's spin and discovered that as the black holes were circling ever closer together, they could have been spinning about their own axes, at angles that were out of alignment with the axis of their orbit. The black holes' misaligned spins likely caused their orbits to wobble, or "precess," as the two Goliaths spiraled toward each other.

The new signal likely represents the instant that the two black holes merged. The merger created an even more massive black hole, of about 142 solar masses, and released an enormous amount of energy, equivalent to around 8 solar masses, spread across the universe in the form of gravitational waves.

"This doesn't look much like a chirp, which is what we typically detect," says Virgo member Nelson Christensen, a researcher at the French National Centre for Scientific Research (CNRS), comparing the signal to LIGO's first detection of gravitational waves in 2015. "This is more like something that goes 'bang,' and it's the most massive signal LIGO and Virgo have seen."

The international team of scientists, who make up the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration, have reported their findings in two papers published today. One, appearing in Physical Review Letters, details the discovery, and the other, in The Astrophysical Journal Letters, discusses the signal's physical properties and astrophysical implications.

"LIGO once again surprises us not just with the detection of black holes in sizes that are difficult to explain, but doing it using techniques that were not designed specifically for stellar mergers," says Pedro Marronetti, program director for gravitational physics at the National Science Foundation. "This is of tremendous importance since it showcases the instrument's ability to detect signals from completely unforeseen astrophysical events. LIGO shows that it can also observe the unexpected."

In the mass gap

The uniquely large masses of the two inspiraling black holes, as well as the final black hole, raise a slew of questions regarding their formation.

All of the black holes observed to date fit within either of two categories: stellar-mass black holes, which measure from a few solar masses up to tens of solar masses and are thought to form when massive stars die; or supermassive black holes, such as the one at the center of the Milky Way galaxy, that are from hundreds of thousands, to billions of times that of our sun.

However, the final 142-solar-mass black hole produced by the GW190521 merger lies within an intermediate mass range between stellar-mass and supermassive black holes -- the first of its kind ever detected.

The two progenitor black holes that produced the final black hole also seem to be unique in their size. They're so massive that scientists suspect one or both of them may not have formed from a collapsing star, as most stellar-mass black holes do.

According to the physics of stellar evolution, outward pressure from the photons and gas in a star's core support it against the force of gravity pushing inward, so that the star is stable, like the sun. After the core of a massive star fuses nuclei as heavy as iron, it can no longer produce enough pressure to support the outer layers. When this outward pressure is less than gravity, the star collapses under its own weight, in an explosion called a core-collapse supernova, that can leave behind a black hole.

This process can explain how stars as massive as 130 solar masses can produce black holes that are up to 65 solar masses. But for heavier stars, a phenomenon known as "pair instability" is thought to kick in. When the core's photons become extremely energetic, they can morph into an electron and antielectron pair. These pairs generate less pressure than photons, causing the star to become unstable against gravitational collapse, and the resulting explosion is strong enough to leave nothing behind. Even more massive stars, above 200 solar masses, would eventually collapse directly into a black hole of at least 120 solar masses. A collapsing star, then, should not be able to produce a black hole between approximately 65 and 120 solar masses -- a range that is known as the "pair instability mass gap."

But now, the heavier of the two black holes that produced the GW190521 signal, at 85 solar masses, is the first so far detected within the pair instability mass gap.

"The fact that we're seeing a black hole in this mass gap will make a lot of astrophysicists scratch their heads and try to figure out how these black holes were made," says Christensen, who is the director of the Artemis Laboratory at the Nice Observatory in France.

One possibility, which the researchers consider in their second paper, is of a hierarchical merger, in which the two progenitor black holes themselves may have formed from the merging of two smaller black holes, before migrating together and eventually merging.

"This event opens more questions than it provides answers," says LIGO member Alan Weinstein, professor of physics at Caltech. "From the perspective of discovery and physics, it's a very exciting thing."

"Something unexpected"

There are many remaining questions regarding GW190521.

As LIGO and Virgo detectors listen for gravitational waves passing through Earth, automated searches comb through the incoming data for interesting signals. These searches can use two different methods: algorithms that pick out specific wave patterns in the data that may have been produced by compact binary systems; and more general "burst" searches, which essentially look for anything out of the ordinary.

LIGO member Salvatore Vitale, assistant professor of physics at MIT, likens compact binary searches to "passing a comb through data, that will catch things in a certain spacing," in contrast to burst searches that are more of a "catch-all" approach.

In the case of GW190521, it was a burst search that picked up the signal slightly more clearly, opening the very small chance that the gravitational waves arose from something other than a binary merger.

"The bar for asserting we've discovered something new is very high," Weinstein says. "So we typically apply Occam's razor: The simpler solution is the better one, which in this case is a binary black hole."

But what if something entirely new produced these gravitational waves? It's a tantalizing prospect, and in their paper the scientists briefly consider other sources in the universe that might have produced the signal they detected. For instance, perhaps the gravitational waves were emitted by a collapsing star in our galaxy. The signal could also be from a cosmic string produced just after the universe inflated in its earliest moments -- although neither of these exotic possibilities matches the data as well as a binary merger.

Read more at Science Daily

X-rays and gravitational waves will combine to illuminate massive black hole collisions

A new study by a group of researchers at the University of Birmingham has found that collisions of supermassive black holes may be simultaneously observable in both gravitational waves and X-rays at the beginning of the next decade.

The European Space Agency (ESA) has recently announced that its two major space observatories of the 2030s will have their launches timed for simultaneous use. These missions, Athena, the next generation X-ray space telescope and LISA, the first space-based gravitational wave observatory, will be coordinated to begin observing within a year of each other and are likely to have at least four years of overlapping science operations.

According to the new study, published this week in Nature Astronomy, ESA's decision will give astronomers an unprecedented opportunity to produce multi-messenger maps of some of the most violent cosmic events in the Universe, which have not been observed so far and which lie at the heart of long-standing mysteries surrounding the evolution of the Universe.

They include the collision of supermassive black holes in the core of galaxies in the distant universe and the "swallowing up" of stellar compact objects such as neutron stars and black holes by massive black holes harboured in the centres of most galaxies.

The gravitational waves measured by LISA will pinpoint the ripples of space time that the mergers cause while the X-rays observed with Athena reveal the hot and highly energetic physical processes in that environment. Combining these two messengers to observe the same phenomenon in these systems would bring a huge leap in our understanding of how massive black holes and galaxies co-evolve, how massive black holes grow their mass and accrete, and the role of gas around these black holes.

These are some of the big unanswered questions in astrophysics that have puzzled scientists for decades.

Dr Sean McGee, Lecturer in Astrophysics at the University of Birmingham and a member of both the Athena and LISA consortiums, led the study. He said, "The prospect of simultaneous observations of these events is uncharted territory, and could lead to huge advances. This promises to be a revolution in our understanding of supermassive black holes and how they growth within galaxies."

Professor Alberto Vecchio, Director of the Institute for Gravitational Wave Astronomy, University of Birmingham, and a co-author on the study, said: "I have worked on LISA for twenty years and the prospect of combining forces with the most powerful X-ray eyes ever designed to look right at the centre of galaxies promises to make this long haul even more rewarding. It is difficult to predict exactly what we're going to discover: we should just buckle up, because it is going to be quite a ride."

During the life of the missions, there may be as many as 10 mergers of black holes with masses of 100,000 to 10,000,000 times the mass of the sun that have signals strong enough to be observed by both observatories. Although due to our current lack of understanding of the physics occurring during these mergers and how frequently they occur, the observatories could observe many more or many fewer of these events. Indeed, these are questions which will be answered by the observations.

Read more at Science Daily