Category: Black Holes

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Obviously, today is the day for news on black holes. While one group of astronomers studied the violent flares of energy sent out by black holes in the near infrared and submillimeter wavelengths, another group used the Chandra X-Ray Observatory to see how black holes can pump energy in a gentler and rhythmic fashion, rather than violently. These scientists say the powerful black holes at the center of massive galaxies act as hearts to the systems, pumping energy out at regular intervals to regulate the growth of the black holes themselves, as well as star formation. “Just like our hearts periodically pump our circulatory systems to keep us alive, black holes give galaxies a vital warm component. They are a careful creation of nature, allowing a galaxy to maintain a fragile equilibrium,” said Alexis Finoguenov, of the Max-Planck Institute for Extraterrestrial Physics in Germany.

The scientists observed and simulated how the black hole at the center of elliptical galaxy M84 dependably sends bubbles of hot plasma into space, heating up interstellar space.

Here’s an animation of the regular pulses of bubbles.

This heat is believed to slow both the formation of new stars and the growth of the black hole itself, helping the galaxy remain stable. Interstellar gases only coalesce into new stars when the gas is cool enough. The heating is more efficient at the sites where it is most needed, the scientists say.

This finding helps to explain a decades-long paradox of the existence of large amounts of warm gas around certain galaxies, making them appear bright to the Chandra X-ray telescope.

“For decades astronomers were puzzled by the presence of the warm gas around these objects. The gas was expected to cool down and form a lot of stars” said Mateusz Ruszkowski, an assistant professor in the University of Michigan Department of Astronomy.

“Now, we see clear and direct evidence that the heating mechanism of black holes is persistent, producing enough heat to significantly suppress star formation. These plasma bubbles are caused by bursts of energy that happen one after another rather than occasionally, and the direct evidence for such periodic behavior is difficult to find.”

The bubbles form one inside another, for a sort of Russian doll effect that has not been seen before, Ruszkowski said. One of the bubbles of hot plasma appears to be bursting and its contents spilling out, further contributing to the heating of the interstellar gas.

“Disturbed gas in old galaxies is seen in many images that NASA’s Chandra observatory obtained, but seeing multiple events is a really impressive evidence for persistent black hole activity,” says Christine Jones, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics.

A paper on the research called “In-depth Chandra study of the AGN feedback in Virgo Elliptical Galaxy M84” has been published in Astrophysical Journal.

Source: University of Michigan

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Two different telescopes simultaneously observed violent flares from the supermassive black hole in the center of the Milky Way. The outbursts from this region, known as Sagittarius A*, reveal material being stretched like bread dough out as it orbits in the intense gravity close to the central black hole. Using ESO’s Very Large Telescope (VLT) and the Atacama Pathfinder Experiment (APEX) telescope, both in Chile, to study light from Sagittarius A* at near-infrared wavelengths and the longer submillimeter wavelengths, astronomers have for the first time concurrently caught a flare with these telescopes. “Observations like this, over a range of wavelengths, are really the only way to understand what’s going on close to the black hole,” says Andreas Eckart of the University of Cologne, who led the team.

Sagittarius A* is located at the centre of our own Milky Way Galaxy at a distance from Earth of about 26,000 light-years. It is a supermassive black hole with a mass of about four million times that of the Sun. Most, if not all, galaxies are thought to have a supermassive black hole in their center.

“Sagittarius A* is unique, because it is the nearest of these monster black holes, lying within our own galaxy,” explains team member Frederick K. Baganoff of the Massachusetts Institute of Technology (MIT) in Cambridge, USA. “Only for this one object can our current telescopes detect these relatively faint flares from material orbiting just outside the event horizon.”

The emission from Sagittarius A* is thought to come from gas thrown off by stars, which then orbits and falls into the black hole.

The VLT pointed their telescope at Sagittarius A* and saw it was active, and getting brighter by the minute. They contacted their colleagues at the APEX telescope, who were able to also catch the flares. Both telescopes are in the southern hemisphere, which provides the best vantage point for studying the Galactic Center.

Over the next six hours, the team detected violently variable infrared emission, with four major flares from Sagittarius A*. The submillimeter-wavelength results also showed flares, but, crucially, this occurred about one and a half hours after the infrared flares.

The researchers explain that this time delay is probably caused by the rapid expansion, at speeds of about 5 million km/h, of the clouds of gas that are emitting the flares. This expansion causes changes in the character of the emission over time, and hence the time delay between the infrared and submillimetre flares.

Although speeds of 5 million km/h may seem fast, this is only 0.5% of the speed of light. To escape from the very strong gravity so close to the black hole, the gas would have to be travelling at half the speed of light – 100 times faster than detected – and so the researchers believe that the gas cannot be streaming out in a jet. Instead, they suspect that a blob of gas orbiting close to the black hole is being stretched out, like dough in a mixing bowl, and this is causing the expansion.

The team hopes that future observations will help them discover more about this mysterious region at the center of our Galaxy.

Read the team’s paper here.

Source: ESO

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We’re learning more about black holes and the early universe all the time, with the help of all the amazing ground-based telescopes astronomers now have at their disposal. Astronomers think that many – perhaps all – galaxies in the universe contain massive black holes at their centers. New observations with the Submillimeter Array now suggest that such colossal black holes were common even 12 billion years ago, when the universe was only 1.7 billion years old and galaxies were just beginning to form. The new conclusion comes from the discovery of two distant galaxies, both with black holes at their centers, which are involved in a spectacular collision.

4C60.07, the first of the galaxies to be discovered, came to astronomers’ attention because of its bright radio emission. This radio signal is one telltale sign of a quasar – a rapidly spinning black hole that is feeding on its home galaxy.

When 4C60.07 was first studied, astronomers thought that hydrogen gas surrounding the black hole was undergoing a burst of star formation, forming stars at a remarkable rate – the equivalent of 5,000 suns every year. This vigorous activity was revealed by the infrared glow from smoky debris left over when the largest stars rapidly died.

The latest research, using the keen vision of the Submillimeter Array of eight radio antennas located in Hawaii, revealed a surprise. 4C60.07 is not forming stars after all. Indeed, its stars appear to be relatively old and quiescent. Instead, prodigious star formation is taking place in a previously unseen companion galaxy, rich in gas and deeply enshrouded in dust, which also has a colossal black hole at its center.

“This new image reveals two galaxies where we only expected to find one,” said Rob Ivison (UK Astronomy Technology Centre), lead author of the study that will be published in the Monthly Notices of the Royal Astronomical Society. “Remarkably, both galaxies contain supermassive black holes at their centers, each capable of powering a billion, billion, billion light bulbs. The implications are wide-reaching: you can’t help wondering how many other colossal black holes may be lurking unseen in the distant universe.”

Due to the finite speed of light, we see the two galaxies as they existed in the distant past, less than 2 billion years after the Big Bang. The new image from the Submillimeter Array captures the moment when 4C60.07 ripped a stream of material from its neighboring galaxy, as shown in the accompanying artist’s conception. By now the galaxies have merged to create a football-shaped elliptical galaxy. Their black holes are likely to have merged and formed a single, more massive black hole.

The galaxies themselves show surprising differences. One is a dead system that has formed all of its stars already and used up its gaseous fuel. The second galaxy is still alive and well, holding plenty of dust and gas that can form new stars.

“These two galaxies are fraternal twins. Both are about the size of the Milky Way, but each one is unique,” said Steve Willner of the Harvard-Smithsonian Center for Astrophysics, a co-author of the paper.

“The superb resolution of the Submillimeter Array was key to our discovery,” he added.

Source: Smithsonian CfA

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What is the environment around a black hole really like? Astronomers are getting a better idea by observing the light coming from the accretion disk surrounding black holes. The light is not constant — it flares, sputters and sparkles – and this flickering provides new and surprising insights into the colossal amount of energy emanating from around black holes. By mapping out how well the variations in visible light match those in X-rays on very short timescales, astronomers have shown that magnetic fields must play a crucial role in the way black holes swallow matter.

“The rapid flickering of light from a black hole is most commonly observed at X-ray wavelengths,” says Poshak Gandhi, who led the international team that reports these results. “This new study is one of only a handful to date that also explore the fast variations in visible light, and, most importantly how these fluctuations relate to those in X-rays.”

The observations tracked the flickering of the black holes simultaneously using two different instruments, one on the ground and one in space. The X-ray data were taken using NASA’s Rossi X-ray Timing Explorer satellite. The visible light was collected with the high speed camera ULTRACAM, a visiting instrument at ESO’s Very Large Telescope (VLT), recording up to 20 images a second. ULTRACAM was developed by team members Vik Dhillon and Tom Marsh. “These are among the fastest observations of a black hole ever obtained with a large optical telescope,” says Dhillon.

To their surprise, astronomers discovered that the brightness fluctuations in the visible light were even more rapid than those seen in X-rays. In addition, the visible-light and X-ray variations were found not to be simultaneous, but to follow a repeated and remarkable pattern: just before an X-ray flare the visible light dims, and then surges to a bright flash for a tiny fraction of a second before rapidly decreasing again.

Watch a movie of the fluctuations.

None of this radiation emerges directly from the black hole, but from the intense energy flows of electrically charged matter in its vicinity. The environment of a black hole is constantly being reshaped by a competing forces such as gravity, magnetism and explosive pressure. As a result, light emitted by the hot flows of matter varies in brightness in a muddled and haphazard way. “But the pattern found in this new study possesses a stable structure that stands out amidst an otherwise chaotic variability, and so, it can yield vital clues about the dominant underlying physical processes in action,” says team member Andy Fabian.

The visible-light emission from the neighborhoods of black holes was widely thought to be a secondary effect, with a primary X-ray outburst illuminating the surrounding gas that subsequently shone in the visible range. But if this were so, any visible-light variations would lag behind the X-ray variability, and would be much slower to peak and fade away. “The rapid visible-light flickering now discovered immediately rules out this scenario for both systems studied,” asserts Gandhi. “Instead the variations in the X-ray and visible light output must have some common origin, and one very close to the black hole itself.”

Strong magnetic fields represent the best candidate for the dominant physical process. Acting as a reservoir, they can soak up the energy released close to the black hole, storing it until it can be discharged either as hot (multi-million degree) X-ray emitting plasma, or as streams of charged particles travelling at close to the speed of light. The division of energy into these two components can result in the characteristic pattern of X-ray and visible-light variability.

Papers on this research: Here and Here

Source: ESO

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Sorry, couldn’t resist that title. Astronomers studying black holes are able to “see” them due to the fact that the gas getting sucked in gets extremely hot and emits X-rays. These X-ray pulses are commonly seen among smaller black holes, but until now, had not been detected from super-massive black holes. But astronomers using the XMM Newton X-ray satellite have discovered a strong X-ray pulse emitting from a giant black hole in a galaxy 500 million light years from Earth, created by gas being sucked in by gravity. “Scientists have been looking for such behaviour for the past 20 years and our discovery helps us begin to understand more about the activity around such black holes as they grow,” said Dr. Marek Gierlinski from Durham University. Gierlinski and his colleagues say this finding is the “missing link” between small and super-massive black holes.

The astronomers were looking at the center of the galaxy REJ1034+396 galaxy and found that X-rays are being emitted as a regular signal from the super-massive black hole. They say the frequency of the pulse is related to the size of the black hole. “Such signals are a well known feature of smaller black holes in our Galaxy when gas is pulled from a companion star,” said Gierlinski. “The really interesting thing is that we have now established a link between these light-weight black holes and those millions of times as heavy as our Sun.”

The scientists hope future research will tell them why some super-massive black holes show this behavior while others do not. Most galaxies, including the Milky Way, are believed to contain super-massive black holes at their centers.

The researchers, who publish their findings in the journal Nature on September 18, say their discovery will increase the understanding of how gas behaves before falling on to a black hole as it feeds and develops.

Source: Durham University

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Black holes are thought to exist throughout the universe, with the largest and most massive found at the centers of the largest galaxies. These supermassive black holes have been shown to have masses upwards of one billion times that of our own Sun. But an astronomer studying black holes says there’s an upper limit to how big a black hole can get. Priyamvada Natarajan, an associate professor of astronomy and physics at Yale University has shown that even the biggest of these gravitational monsters can’t keep growing forever. Instead, they appear to curb their own growth – once they accumulate about 10 billion times the mass of the Sun.

These ultra-massive black holes, found at the centers of giant elliptical galaxies in huge galaxy clusters, are the biggest in the known universe. Even the large black hole at the center of our own Milky Way galaxy is thousands of times less massive than these behemoths. But these gigantic black holes, which accumulate mass by sucking in matter from neighboring gas, dust and stars, seem unable to grow beyond this limit regardless of where – and when – they appear in the universe. “It’s not just happening today,” said Natarajan. “They shut off at every epoch in the universe.”

Natarajan’s study is the first time an upper mass limit has been derived for black holes. Natarajan used existing optical and X-ray data of these ultra-massive black holes to show that, in order for those various observations to be consistent, the black holes must essentially shut off at some point in their evolution.

One possible explanation, says Natarajan, is that the black holes eventually reach the point when they radiate so much energy as they consume their surroundings that they end up interfering with the very gas supply that feeds them, which may interrupt nearby star formation. The new findings have implications for the future study of galaxy formation, since many of the largest galaxies in the universe appear to co-evolve along with the black holes at their centers.

“Evidence has been mounting for the key role that black holes play in the process of galaxy formation,” said Natarajan. “But it now appears that they are likely the prima donnas of this space opera.”

Source: PhysOrg

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An international team of astronomers has obtained the closest views ever of what is believed to be a super-massive black hole at the center of the Milky Way galaxy. The astronomers linked together radio dishes in Hawaii, Arizona and California to create a virtual telescope more than 2,800 miles across that is capable of seeing details more than 1,000 times finer than the Hubble Space Telescope. The target of the observations was the source known as Sagittarius A* (“A-star”), long thought to mark the position of a black hole whose mass is 4 million times that of the sun.

Using a technique called Very Long Baseline Interferometry (VLBI), the astronomers studied the radio waves coming from Sagittarius A*. In VLBI, signals from multiple astronomy telescopes are combined to create the equivalent of a single giant telescope, as large as the separation between the facilities. As a result, VLBI yields exquisitely sharp resolution.

They detected structure at a tiny angular scale of 37 micro-arcseconds – the equivalent of a baseball seen on the surface of the moon, 240,000 miles distant. These observations are among the highest resolution ever done in astronomy.

“This technique gives us an unmatched view of the region near the Milky Way’s central black hole,” said Sheperd Doeleman of MIT, first author of the study that will be published in the Sept. 4 issue of the journal Nature.

Though Sagittarius A* was discovered three decades ago, the new observations for the first time have an angular resolution, or ability to observe small details, that is matched to the size of the black hole “event horizon” — the region inside of which nothing, including light, can ever escape.

With three telescopes, the astronomers could only vaguely determine the shape of the emitting region. Future investigations will help answer the question of what, precisely, they are seeing: a glowing corona around the black hole, an orbiting “hot spot,” or a jet of material. Nevertheless, their result represents the first time that observations have gotten down to the scale of the black hole itself, which has a “Schwarzschild radius” of 10 million miles.

The concept of black holes, objects so dense that their gravitational pull prevents anything including light itself from ever escaping their grasp, has long been hypothesized, but their existence has not yet been proved conclusively. Astronomers study black holes by detecting the light emitted by matter that heats up as it is pulled closer to the event horizon. By measuring the size of this glowing region at the Milky Way center, the new observations have revealed the highest density yet for the concentration of matter at the center of our galaxy, which “is important new evidence supporting the existence of black holes,” said Doeleman.

“This result, which is remarkable in and of itself, also confirms that the 1.3-mm VLBI technique has enormous potential, both for probing the galactic center and for studying other phenomena at similar small scales,” said co-author Jonathan Weintroub.

The team plans to expand their work by developing novel instrumentation to make more sensitive 1.3-mm observations possible. They also hope to develop additional observing stations, which would provide additional baselines (pairings of two telescope facilities at different locations) to enhance the detail in the picture. Future plans also include observations at shorter, 0.85-mm wavelengths; however, such work will be even more challenging for many reasons, including stretching the capabilities of the instrumentation, and the requirement for a coincidence of excellent weather conditions at all sites.

Source: Harvard Smithsonian press release