Very Strong magnetic fields challenge the pull of supermassive black holes

This is a computer simulation of gas (in yellow) falling into a black hole (too small to be seen). 

Twin jets are also shown with magnetic field lines. 

Credit: Alexander Tchekhovskoy, Berkeley Lab

A new study of supermassive black holes at the centers of galaxies has found magnetic fields play an impressive role in the systems' dynamics.

In fact, in dozens of black holes surveyed, the magnetic field strength matched the force produced by the black holes' powerful gravitational pull, says a team of scientists from the U.S. Department of Energy's Lawrence Berkeley National Laboratory (LBNL) and Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany.

The findings "Dynamically important magnetic fields near accreting supermassive black holes" are published in this week's issue of Nature.

"This paper for the first time systematically measures the strength of magnetic fields near black holes," says Alexander Tchekhovskoy, the Berkeley Lab researcher who helped interpret the observational data within the context of existing computational models.

"This is important because we had no idea, and now we have evidence from not just one, not just two, but from 76 black holes."

Previously, Tchekhovskoy, who is also a postdoctoral fellow at the University of California, Berkeley, had developed computational models of black holes that included magnetic fields.

His models suggested a black hole could sustain a magnetic field that was as strong as its gravity, but there was not yet observational evidence to support this prediction.

With the two forces balancing out, a cloud of gas caught on top of the magnetic field would be spared the pull of gravity and instead levitate in place.

The magnetic field strength was confirmed by evidence from jets of gas that shoot away from supermassive black holes.

Formed by magnetic fields, these jets produce a radio emission. "We realized that the radio emission from black holes' jets can be used to measure the magnetic field strength near the black hold itself," says Mohammad Zamaninasab, the lead author of the study, who did the work while at MPIfR.

Other research teams had previously collected radio-emission data from "radio-loud" galaxies using the Very Long Baseline Array, a vast network of radio telescopes in the United States.

The researchers analyzed this pre-existing data to create radio-emission maps at different wavelengths. Shifts in jet features between different maps let them calculate the field strength near the black hole.

Based on the results, the team found not only that the measured magnetic fields can be as strong as a black hole's gravity, but that they are also comparable in strength to those produced inside MRI machines found in hospitals, roughly 10,000 times greater than the field of the Earth itself.

Tchekhovskoy says the new results mean theorists must re-evaluate their understanding of black-hole behaviour.

"The magnetic fields are strong enough to dramatically alter how gas falls into black holes and how gas produces outflows that we do observe, much stronger than what has usually been assumed," he says. "We need to go back and look at our models once again."

More information: Paper: Dynamically important magnetic fields near accreting supermassive black holes, DOI: 10.1038/nature13399

ESO VLT: The dusty heart of Circinus, an active galaxy

Nuclear region of the Circinus galaxy. 

The right image shows the inner 1000 light years of the Circinus galaxy. 

Blinding light and gaseous material are ejected by the active nucleus (located at the black box). 

They escape only along a conical region towards the northwest (upper right part of the image), leading to the white V-shaped structure in this image. 

Along other directions, the nuclear region is hidden by dense gas and dust.

This obscuring dust has now been investigated with unprecedented detail with the ESO Very Large Telescope (VLT) Interferometer. 

The false-colour model image on the left shows the dust emission and corresponds to the region marked by the black box in the right image.

The emission comes from a relatively thin, disk-like structure (white) as well as dust elongated perpendicular to it. 

The disk is also seen by water emission (red-green-blue line). 

The dust emission is more absorbed towards the southeast (bottom left) than the northwest (top right), illustrated by the change from violet to green colours. 

Credit: Konrad Tristram; Right: NASA HST, STScI.

An international research team led by Konrad Tristram from the Max-Planck-Institute for Radio Astronomy in Bonn, Germany, obtained the most detailed view so far of the warm dust in the environment of a supermassive black hole in an active galaxy.

Konrad Tristram

The observations of the Circinus galaxy show, for the first time, that the dust directly illuminated by the central engine of the active galaxy is located in two distinct components: an inner warped disk and a surrounding larger distribution of dust.

Most likely, the larger component is responsible for most of the obscuration of the inner regions close to the supermassive black hole.

Such a configuration is significantly more complex than the simple dusty doughnut, which has been favoured for the last few decades.

The results are published in the current issue of Astronomy & Astrophysics.

In active galactic nuclei, enormous amounts of energy are released due to the feeding of the supermassive black hole in the centre of the galaxy.

Such black holes have masses of a million or billion times the mass of the sun.

The matter spiralling in onto the black hole becomes so hot and luminous that it outshines its entire galaxy with billions of stars. The huge amounts of energy released also affect the surrounding galaxy.

Active galactic nuclei are therefore thought to play an important role in the formation and evolution of galaxies and hence in the formation of the universe as presently seen.

Using the MIDI instrument at the ESO Very Large Telescope (VLT) Interferometer in the Atacama Desert of Chile, the research team obtained an unprecedented clear view of the warm dust in the nucleus of the Circinus galaxy.

At a distance of only 13 million light years, the Circinus galaxy contains one of the closest and brightest active galactic nuclei.

"We obtained at least twice the amount of interferometric data than for any other galaxy", proudly reports Konrad Tristram from the Max-Planck-Institute for Radio Astronomy (MPIfR), the lead author of the paper.

"Our observations make the Circinus galaxy by far the best observed extragalactic source in optical and infrared interferometry."

By combining the light of two telescopes, the interferometric observations increase the resolution to that of a telescope of 92 meters in diameter.

In the case of the Circinus galaxy, the scientists could, for the first time, show that the emission of the nuclear dust comes from two distinct components, an inner disk-like component and an extended component significantly elongated in polar direction.

The dust disk in the Circinus galaxy has a size of about 3 light years and agrees well with a warped molecular disk revealed by water emission.

More information: "The Dusty Torus in the Circinus Galaxy: A Dense Disk and the Torus Funnel," K. R. W. Tristram, L. Burtscher, W. Jaffe, K. Meisenheimer, S. F. Hönig, M. Kishimoto, M. Schartmann, and G. Weigelt, 2014, Astronomy & Astrophysics. dx.doi.org/10.1051/0004-6361/201322698; Preprint: arxiv.org/abs/1312.4534