Category: Black Holes

The debate of whether or not a supermassive black hole (SMBH) was kicked out of the centre of a galaxy continues in the Black Holes I session at the A A S. According to Stefanie Komossa and her team at the Max Plank Institute for extraterrestrial Physics (MPE) back in May 2008, spectroscopic data of a galactic core appeared to show a collision event between two SMBHs. In this case, the smaller SMBH was propelled out of its host galaxy by an intense and focused “superkick” by gravitational waves.

However, the delegates attending Session 328 have other ideas…

Tamara Bogdanovic, University of Maryland, kicked off the Black Hole I Session with an investigation into the spectroscopic data derived by Komossa et al. Bogdanovic presented her research on the possibility that rather than showing a superkick, the data could be showing the motion of binary SMBHs around the galactic core after a galactic merger. She made the rather sobering statement that there were, “more publications than data,” highlighting the fact that far from being conclusive evidence of a superkick, that more subtle mechanisms may be at work. Model data of orbiting binaries appear to fit the same spectroscopic analysis just as well as the superkick situation. As binary SMBHs would be long-lived objects, there’s a good (statistical) chance of observing them. Further work is required, however, possibly using the Very Long Baseline Array (VLBA).

Dipanker Maitra, of the University of Amsterdam, then presented his results of time-dependent modelling of Sagittarius A* (the SBH at the centre of our galaxy). It turns out that there are more high energy flare events detected from Sag A* than expected from the predicted accretion rate. Maitra models the time lag observed in radio data between the first high-energy flares and the following low energy flares.

Jen Blum, from the University of Maryland, then took on the emissions from a stellar black hole in the X-ray binary GRS 1915+105. Key to Blum’s research is to investigate the strange asymmetric iron emission line. It looks like this asymmetry can be explained by a combination of special relativity and general relativity effects near the space-time warping black hole.

David Garofalo, who works at JPL/Caltech, then followed quickly with his research of the “central engine” inside galactic nuclei, investigating how strong a SMBH’s magnetic field can be. In his models, he finds the spin of the black hole is key to magnetic field strength. Counter-intuitively, Garofalo’s work suggests that the fastest spinning black holes may have the weakest magnetic field. Also, slowly spinning SMBHs appear to have a larger gap region. He is quick to point out that his model only shows us what configurations are possible, but concludes with the suggestion that you don’t need a fast-spinning SMBH for powerful jets to be generated. “[It’s a] tug-o-war between gravity and the Lorentz forces,” he said when referring to his model, “but other [unaccounted for] physics may significantly modify the model.”

Avery Broderick, from the Canadian Institute for Theoretical Astrophysics, examines jets produced by the Milky Way’s SMBH and M87. Both are fantastic objects to study as they are relatively close. However, the angular resolution of instrumentation needs to be boosted, or new techniques are needed to understand jet mechanisms.

Massimo Dotti, from the University of Michigan, re-explored Komossa’s research, also supporting Tamara Bogdanovic’s work that a superkick may not have caused the emissions studied by Komossa. He also shows that a galactic merger and then SMBH binary can generate similar red-shifted and blue-shifted components of emission profiles. Dotti then showed details of his model and proposed some observational constrains.

Bonus speaker and NASA scientist Teddy Cheung then discussed his search for “offset galactic nuclei” that may be evidence for SMBH collisions in the centre of galaxies. According to Cheung, the calculations to find the black hole masses can be “done on the back of an envelope… the flap of the envelope!” He then showed some results of the observation campaign, pointing to a few candidates that might reveal a SMBH binary partner may have achieved escape velocity (i.e. been kicked out of the galaxy), but he emphasised that this number was small. Radio data of pre-merger and post-merger lobes were also presented, helping future studies characterize collision and merger events.

All in all, Session 328 was a superb start to the conference for me, really opening my eyes to the cutting edge supermassive black hole research going on around the world. There’s a lot more where that came from…

Article source: AAS meeting.

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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