Astronomers spot signs of supermassive black hole mergers

New research, published Wednesday, 24 October, in the journal Monthly Notices of the Royal Astronomical Society, has found evidence for a large number of double supermassive black holes, likely precursors of gigantic black hole merging events. This confirms the current understanding of cosmological evolution - that galaxies and their associated black holes merge over time, forming bigger and bigger galaxies and black holes. Astronomers from the University of Hertfordshire, together with an international team of scientists, have looked at radio maps of powerful jet sources and found signs that would usually be present when looking at black holes that are closely orbiting each other. Before black holes merge they form a binary black hole, where the two black holes orbit around each other. Gravitational wave telescopes have been able to evidence the merging of smaller black holes since 2015, by measuring the strong bursts of gravitational waves that are emitted when binary black holes merge, but current technology cannot be used to demonstrate the presence of supermassive binary black holes. Supermassive black holes emit powerful jets. When supermassive binary black holes orbit it causes the jet emanating from the nucleus of a galaxy to periodically change its direction. 

Astronomers from the University of Hertfordshire studied the direction that these jets are emitted in, and variances in these directions; they compared the direction of the jets with the one of the radio lobes (that store all the particles that ever went through the jet channels) to demonstrate that this method can be used to indicate the presence of supermassive binary black holes.

Dr. Martin Krause, lead author and senior lecturer in Astronomy at the University of Hertfordshire, said: "We have studied the jets in different conditions for a long time with computer simulations. In this first systematic comparison to high-resolution radio maps of the most powerful radio sources, we were astonished to find signatures that were compatible with jet precession in three quarters of the sources."

The fact that the most powerful jets are associated with binary black holes could have important consequences for the formation of stars in galaxies; stars form from cold gas, jets heat this gas and thus suppress the formation of stars. A jet that always heads in the same direction only heats a limited amount of gas in its vicinity.

However, jets from binary black holes change direction continuously. Therefore, they can heat much more gas, suppressing the formation of stars much more efficiently, and thus contributing towards keeping the number of stars in galaxies within the observed limits.

Сделан первый качественный снимок кольца у черной дыры

Астрономам впервые удалось получить качественный снимок газопылевой структуры, окружающей активную сверхмассивную черную дыру в центре галактики. Известно, что сверхмассивные черные дыры, находящиеся в центре галактик, "поглощают" все, что приближается к ним достаточно близко, однако возможность наблюдать такие явления в Млечном Пути предоставляется достаточно редко. Теперь же, однако, с помощью радиотелескопа ALMA астрономы смогли заснять очень активную черную дыру в центре спиральной галактики M77, находящейся в 47 млн световых лет от Земли. В центре M77 находится активное галактическое ядро; это означает, что газ и материя постоянно поглощаются центральной черной дырой и излучается интенсивный свет. Такие активные области Вселенной могут помочь ученым понять, как ведут себя галактики и сверхмассивные черные дыры, находящиеся в их центре. Новое открытие было сделано научной группой из Японии; исследователи при помощи радиотелескопа ALMA смогли заснять активное ядро M77. Об исследовании кратко сообщает портал New Atlas.

Специалисты обнаружили небольшую газопылевую структуру, окружающую черную дыру; радиус этого облака, вращающегося вокруг черной дыры, составлял 20 световых лет. Существование данных структур предполагалось на протяжении десятилетий, однако, как отмечают исследователи, впервые качественный снимок такой структуры удалось получить лишь сейчас.

Радиотелескоп ALMA позволяет делать снимки в очень высоком разрешении; при этом, как подчеркивают исследователи, важно было регистрировать микроволновое излучение от молекул цианистого водорода (HCN) и формил-ионов (HCO+). Отмечается, что данные молекулы "светятся" в микроволновом диапазоне лишь при достаточной плотности и, таким образом, позволяют сказать о плотности данного газопылевого облака.

Кроме того, исследователи заметили, что обнаруженная структура вращается несколько хаотично и движение газа не всегда регулируется гравитацией черной дыры. По предположению ученых, это может быть связано с тем, что в прошлом M77 столкнулась с другим объектом, возможно, с небольшой галактикой.

New simulation sheds light on spiraling supermassive black holes

A new model is bringing scientists a step closer to understanding the kinds of light signals produced when two supermassive black holes, which are millions to billions of times the mass of the Sun, spiral toward a collision. For the first time, a new computer simulation that fully incorporates the physical effects of Einstein's general theory of relativity shows that gas in such systems will glow predominantly in ultraviolet and X-ray light. Just about every galaxy the size of our own Milky Way or larger contains a monster black hole at its center. Observations show galaxy mergers occur frequently in the universe, but so far no one has seen a merger of these giant black holes. "We know galaxies with central supermassive black holes combine all the time in the universe, yet we only see a small fraction of galaxies with two of them near their centers," said Scott Noble, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "The pairs we do see aren't emitting strong gravitational-wave signals because they're too far away from each other. Our goal is to identify - with light alone - even closer pairs from which gravitational-wave signals may be detected in the future." Scientists have detected merging stellar-mass black holes - which range from around three to several dozen solar masses - using the National Science Foundation's Laser Interferometer Gravitational-Wave Observatory (LIGO).

Gravitational waves are space-time ripples traveling at the speed of light. They are created when massive orbiting objects like black holes and neutron stars spiral together and merge.
Supermassive mergers will be much more difficult to find than their stellar-mass cousins. One reason ground-based observatories can't detect gravitational waves from these events is because Earth itself is too noisy, shaking from seismic vibrations and gravitational changes from atmospheric disturbances. The detectors must be in space, like the Laser Interferometer Space Antenna (LISA) led by ESA (the European Space Agency) and planned for launch in the 2030s.

Observatories monitoring sets of rapidly spinning, superdense stars called pulsars may detect gravitational waves from monster mergers. Like lighthouses, pulsars emit regularly timed beams of light that flash in and out of view as they rotate. Gravitational waves could cause slight changes in the timing of those flashes, but so far studies haven't yielded any detections.

But supermassive binaries nearing collision may have one thing stellar-mass binaries lack - a gas-rich environment. Scientists suspect the supernova explosion that creates a stellar black hole also blows away most of the surrounding gas. The black hole consumes what little remains so quickly there isn't much left to glow when the merger happens.

Supermassive binaries, on the other hand, result from galaxy mergers. Each supersized black hole brings along an entourage of gas and dust clouds, stars and planets. Scientists think a galaxy collision propels much of this material toward the central black holes, which consume it on a time scale similar to that needed for the binary to merge. As the black holes near, magnetic and gravitational forces heat the remaining gas, producing light astronomers should be able to see.

"It's very important to proceed on two tracks," said co-author Manuela Campanelli, director of the Center for Computational Relativity and Gravitation at the Rochester Institute of Technology in New York, who initiated this project nine years ago.

"Modeling these events requires sophisticated computational tools that include all the physical effects produced by two supermassive black holes orbiting each other at a fraction of the speed of light. Knowing what light signals to expect from these events will help modern observations identify them. Modeling and observations will then feed into each other, helping us better understand what is happening at the hearts of most galaxies."

The new simulation shows three orbits of a pair of supermassive black holes only 40 orbits from merging. The models reveal the light emitted at this stage of the process may be dominated by UV light with some high-energy X-rays, similar to what's seen in any galaxy with a well-fed supermassive black hole.

Three regions of light-emitting gas glow as the black holes merge, all connected by streams of hot gas: a large ring encircling the entire system, called the circumbinary disk, and two smaller ones around each black hole, called mini disks. All these objects emit predominantly UV light. When gas flows into a mini disk at a high rate, the disk's UV light interacts with each black hole's corona, a region of high-energy subatomic particles above and below the disk. This interaction produces X-rays. When the accretion rate is lower, UV light dims relative to the X-rays.

Based on the simulation, the researchers expect X-rays emitted by a near-merger will be brighter and more variable than X-rays seen from single supermassive black holes. The pace of the changes links to both the orbital speed of gas located at the inner edge of the circumbinary disk as well as that of the merging black holes.

"The way both black holes deflect light gives rise to complex lensing effects, as seen in the movie when one black hole passes in front of the other," said Stephane d'Ascoli, a doctoral student at Ecole Normale Superieure in Paris and lead author of the paper. "Some exotic features came as a surprise, such as the eyebrow-shaped shadows one black hole occasionally creates near the horizon of the other."

The simulation ran on the National Center for Supercomputing Applications' Blue Waters supercomputer at the University of Illinois at Urbana-Champaign. Modeling three orbits of the system took 46 days on 9,600 computing cores. Campanelli said the collaboration was recently awarded additional time on Blue Waters to continue developing their models.

The original simulation estimated gas temperatures. The team plans to refine their code to model how changing parameters of the system, like temperature, distance, total mass and accretion rate, will affect the emitted light. They're interested in seeing what happens to gas traveling between the two black holes as well as modeling longer time spans.

"We need to find signals in the light from supermassive black hole binaries distinctive enough that astronomers can find these rare systems among the throng of bright single supermassive black holes," said co-author Julian Krolik, an astrophysicist at Johns Hopkins University in Baltimore.

"If we can do that, we might be able to discover merging supermassive black holes before they're seen by a space-based gravitational-wave observatory."

Black holes ruled out as universe's missing dark matter

For one brief shining moment after the 2015 detection of gravitational waves from colliding black holes, astronomers held out hope that the universe's mysterious dark matter might consist of a plenitude of black holes sprinkled throughout the universe. University of California, Berkeley, physicists have dashed those hopes. Based on a statistical analysis of 740 of the brightest supernovas discovered as of 2014, and the fact that none of them appear to be magnified or brightened by hidden black hole "gravitational lenses," the researchers concluded that primordial black holes can make up no more than about 40 percent of the dark matter in the universe. Primordial black holes could only have been created within the first milliseconds of the Big Bang as regions of the universe with a concentrated mass tens or hundreds of times that of the sun collapsed into objects a hundred kilometers across. The results suggest that none of the universe's dark matter consists of heavy black holes, or any similar object, including massive compact halo objects, so-called MACHOs. Dark matter is one of astronomy's most embarrassing conundrums: despite comprising 84.5 percent of the matter in the universe, no one can find it. Proposed dark matter candidates span nearly 90 orders of magnitude in mass, from ultralight particles like axions to MACHOs.

Several theorists have proposed scenarios in which there are multiple types of dark matter. But if dark matter consists of several unrelated components, each would require a different explanation for its origin, which makes the models very complex.

"I can imagine it being two types of black holes, very heavy and very light ones, or black holes and new particles. But in that case one of the components is orders of magnitude heavier than the other, and they need to be produced in comparable abundance. We would be going from something astrophysical to something that is truly microscopic, perhaps even the lightest thing in the universe, and that would be very difficult to explain," said lead author Miguel Zumalacarregui, a Marie Curie Global Fellow at the Berkeley Center for Cosmological Physics.

An as-yet unpublished reanalysis by the same team using an updated list of 1,048 supernovas cuts the limit in half, to a maximum of about 23 percent, further slamming the door on the dark matter-black hole proposal.

"We are back to the standard discussions. What is dark matter? Indeed, we are running out of good options," said Uros Seljak, a UC Berkeley professor of physics and astronomy and BCCP co-director. "This is a challenge for future generations."

The analysis is detailed in a paper published this week in the journal Physical Review Letters.

Dark matter lensing
Their conclusions are based on the fact that an unseen population of primordial black holes, or any massive compact object, would gravitationally bend and magnify light from distant objects on its way to Earth.

Therefore, gravitational lensing should affect the light from distant Type Ia supernovas. These are the exploding stars that scientists have used as standard brightness sources to measure cosmic distances and document the expansion of the universe.

Zumalacarregui conducted a complex statistical analysis of data on the brightness and distance supernovas catalogued in two compilations - 580 in the Union and 740 in the joint light-curve analysis (JLA) catalogs - and concluded that eight should be brighter by a few tenths of a percent than predicted based on observations of how these supernovas brighten and fade over time. No such brightening has been detected.

Other researchers have performed similar but simpler analyses that yielded inconclusive results. But Zumalacarregui incorporated the precise probability of seeing all magnifications, from small to huge, as well as uncertainties in brightness and distance of each supernova. Even for low-mass black holes - those 1 percent the mass of the sun - there should be some highly magnified distant supernovas, he said, but there are none.

"You cannot see this effect on one supernova, but when you put them all together and do a full Bayesian analysis you start putting very strong constraints on the dark matter, because each supernova counts and you have so many of them," Zumalacarregui said.

The more supernovas included in the analysis, and the farther away they are, the tighter the constraints. Data on 1,048 bright supernovas from the Pantheon catalog provided an even lower upper limit - 23 percent - than the newly published analysis.

Seljak published a paper proposing this type of analysis in the late 1990s, but when interest shifted from looking for big objects, MACHOs, to looking for fundamental particles, in particular weakly interacting massive particles, or WIMPs, follow-up plans fell by the wayside. By then, many experiments had excluded most masses and types of MACHOs, leaving little hope of discovering such objects.

At the time, too, only a small number of distant Type Ia supernovas had been discovered and their distances measured.

Only after the LIGO observations brought up the issue again did Seljak and Zumalacarregui embark on the complicated analysis to determine the limits on dark matter.

"What was intriguing is that the masses of the black holes in the LIGO event were right where black holes had not yet been excluded as dark matter," Seljak said. "That was an interesting coincidence that got everyone excited. But it was a coincidence."