пятница, 9 ноября 2018 г.

Astronomers unveil growing black holes in colliding galaxies

Peering through thick walls of gas and dust surrounding the messy cores of merging galaxies, astronomers are getting their best view yet of close pairs of supermassive black holes as they march toward coalescence into mega black holes. A team of researchers led by Michael Koss of Eureka Scientific Inc., in Kirkland, Washington, performed the largest survey of the cores of nearby galaxies in near-infrared light, using high-resolution images taken by NASA's Hubble Space Telescope and the W. M. Keck Observatory in Hawaii. The Hubble observations represent over 20 years' worth of snapshots from its vast archive. "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 images also provide a close-up preview of a phenomenon that must have been more common in the early universe, when galaxy mergers were more frequent. When galaxies collide, their monster black holes can unleash powerful energy in the form of gravitational waves, the kind of ripples in space-time that were just recently detected by ground-breaking experiments. The new study also offers a preview of what will likely happen in our own cosmic backyard, in several billion years, when our Milky Way combines with the neighboring Andromeda galaxy and their respective central black holes smash together.

"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 study team member Laura Blecha of the University of Florida, in Gainesville.

"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."

A galaxy merger is a slow process lasting more than a billion years as two galaxies, under the inexorable pull of gravity, dance toward each other before finally joining together. Simulations reveal that galaxies kick up plenty of gas and dust as they undergo this slow-motion train wreck.

The ejected material often forms a thick curtain around the centers of the coalescing galaxies, shielding them from view in visible light. Some of the material also falls onto the black holes at the cores of the merging galaxies.

The black holes grow at a fast clip as they engorge themselves with their cosmic food, and, being messy eaters, they cause the infalling gas to blaze brightly. This speedy growth occurs during the last 10 million to 20 million years of the union. The Hubble and Keck Observatory images captured close-up views of this final stage, when the bulked-up black holes are only about 3,000 light-years apart - a near-embrace in cosmic terms.

It's not easy to find galaxy nuclei so close together. Most prior observations of colliding 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 and require high-resolution observations in infrared light that can see through the clouds and pinpoint the locations of the two merging nuclei.

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 Telescope, a high-energy space observatory.

"Gas falling onto the black holes emits X-rays, and the brightness of the X-rays tells you how quickly the black hole is growing," Koss explained. "I didn't know if we would find hidden mergers, but we suspected, based on computer simulations, that they would be in heavily shrouded galaxies.Therefore we tried to peer through the dust with the sharpest images possible, in hopes of finding coalescing black holes."

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

"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," Koss said. "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."

The team targeted galaxies with an average distance of 330 million light-years from Earth. Many of the galaxies are similar in size to the Milky Way and Andromeda galaxies. The team analyzed 96 galaxies from the Keck Observatory and 385 galaxies from the Hubble archive found in 38 different Hubble observation programs. The sample galaxies are representative of what astronomers would find by conducting an all-sky survey.

To verify their results, Koss's team compared the survey galaxies with 176 other galaxies from the Hubble archive that lack actively growing black holes. The comparison confirmed that the luminous cores found in the researchers' census of dusty interacting galaxies are indeed a signature of rapidly growing black-hole pairs headed for a collision.

When the two supermassive black holes in each of these systems finally come together in millions of years, their encounters will produce strong gravitational waves. Gravitational waves produced by the collision of two stellar-mass black holes have already been detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO).

Observatories such as the planned NASA/ESA space-based Laser Interferometer Space Antenna (LISA) will be able to detect the lower-frequency gravitational waves from supermassive black-hole mergers, which are a million times more massive than those detected by LIGO.

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

The Webb telescope may also be able to look in mid-infrared light to uncover more galaxy interactions so encased in thick gas and dust that even near-infrared light cannot penetrate them.

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

среда, 7 ноября 2018 г.

В космосе заметили гигантский фонтан

«Фонтан» находится в центре галактики. Астрономы впервые обнаружили сверхмассивную черную дыру, которая формирует замкнутый цикл из выбрасываемого и падающего обратно холодного молекулярного газа. «Фонтан» находится в центре галактики, которая входит в состав галактического скопления Abell 2597, удаленного на миллиард световых лет от Земли. Молекулярный газ притягивается гравитацией черной дыры, образуя аккреционный диск. Часть этого материала направляется к полюсам и выбрасывается в виде струй (джетов) плазмы, скорость которых сравнима со скоростью света. Эти джеты простираются на 30 тысяч световых лет. Однако затем этот материал падает обратно в нитевидную туманность, которая питает аккреционный диск и достигает ста тысяч световых лет в диаметре, занимая весь центр галактики. Горячие струи охлаждаются, газ конденсируется и образует облака из молекул монооксида углерода (угарного газа). Эти облака устремляются к сверхмассивной черной дыре, и цикл начинается заново.

понедельник, 29 октября 2018 г.

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.

воскресенье, 14 октября 2018 г.

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.

четверг, 4 октября 2018 г.

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."