Artist’s impression of a supermassive black hole at the centre of a galaxy
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Illuminating the mysterious mechanisms at play at the edge of the event horizon
The Atacama Large
Millimeter/submillimeter Array (ALMA) has revealed an extremely powerful
magnetic field, beyond anything previously detected in the core of a
galaxy, very close to the event horizon of a supermassive black hole.
This new observation helps astronomers to understand the structure and
formation of these massive inhabitants of the centres of galaxies, and
the twin high-speed jets of plasma they frequently eject from their
poles. The results appear in the 17 April 2015 issue of the journal
Science.
Supermassive black holes,
often with masses billions of times that of the Sun, are located at the
heart of almost all galaxies in the Universe. These black holes can
accrete huge amounts of matter in the form of a surrounding disc.
While most of this matter is fed into the black hole, some can escape
moments before capture and be flung out into space at close to the speed
of light as part of a jet of plasma. How this happens is not well understood, although it is thought that strong magnetic fields, acting very close to the event horizon, play a crucial part in this process, helping the matter to escape from the gaping jaws of darkness.
Up to now only weak magnetic fields far from black holes — several light-years away — had been probed [1]. In this study, however, astronomers from Chalmers University of Technology and Onsala Space Observatory in Sweden have now used ALMA
to detect signals directly related to a strong magnetic field very
close to the event horizon of the supermassive black hole in a distant
galaxy named PKS 1830-211. This magnetic field is located precisely at
the place where matter is suddenly boosted away from the black hole in
the form of a jet.
The team measured the strength of the magnetic field by studying the way in which light was polarised, as it moved away from the black hole.
“Polarisation is an important property of light and is much used in daily life, for example in sun glasses or 3D glasses at the cinema,” says Ivan Marti-Vidal, lead author of this work. “When produced naturally, polarisation can be used to measure magnetic fields, since light changes its polarisation when it travels through a magnetised medium. In this case, the light that we detected with ALMA had been travelling through material very close to the black hole, a place full of highly magnetised plasma.”
“Polarisation is an important property of light and is much used in daily life, for example in sun glasses or 3D glasses at the cinema,” says Ivan Marti-Vidal, lead author of this work. “When produced naturally, polarisation can be used to measure magnetic fields, since light changes its polarisation when it travels through a magnetised medium. In this case, the light that we detected with ALMA had been travelling through material very close to the black hole, a place full of highly magnetised plasma.”
The astronomers applied a new analysis technique that they
had developed to the ALMA data and found that the direction of
polarisation of the radiation coming from the centre of PKS 1830-211 had
rotated [2].
These are the shortest wavelengths ever used in this kind of study,
which allow the regions very close to the central black hole to be
probed [3].
"We have found clear signals of polarisation rotation
that are hundreds of times higher than the highest ever found in the
Universe," says Sebastien Muller, co-author of the paper. "Our
discovery is a giant leap in terms of observing frequency, thanks to the
use of ALMA, and in terms of distance to the black hole where the
magnetic field has been probed — of the order of only a few light-days
from the event horizon. These results, and future studies, will help us
understand what is really going on in the immediate vicinity of
supermassive black holes.”
Notes
[1] Much weaker magnetic fields have been detected in the vicinity of
the relatively inactive supermassive black hole at the centre of the
Milky Way. Recent observations have also revealed weak magnetic fields
in the active galaxy NGC 1275, which were detected at millimetre
wavelengths.
[2] Magnetic fields introduce Faraday
rotation, which makes the polarisation rotate in different ways at
different wavelengths. The way in which this rotation depends on the
wavelength tells us about the magnetic field in the region.
[3] The ALMA observations were at an
effective wavelength of about 0.3 millimetres, earlier investigations
were at much longer radio wavelengths. Only light of millimetre
wavelengths can escape from the region very close to the black hole,
longer wavelength radiation is absorbed.
More Information
This research was presented in a paper entitled “A strong
magnetic field in the jet base of a supermassive black hole” to appear
in Science on 17 April 2015
The team is composed of I. Martí-Vidal (Onsala Space
Observatory and Department of Earth and Space Sciences, Chalmers
University of Technology, Sweden), S. Muller (Onsala Space Observatory
and Department of Earth and Space Sciences, Chalmers University of
Technology, Sweden), W. Vlemmings (Department of Earth and Space
Sciences and Onsala Space Observatory, Chalmers University of
Technology, Sweden), C. Horellou (Department of Earth and Space
Sciences, Chalmers University of Technology, Sweden) and S. Aalto
(Department of Earth and Space Sciences, Chalmers University of
Technology, Sweden).
The Atacama Large Millimeter/submillimeter Array (ALMA), an
international astronomy facility, is a partnership of ESO, the US
National Science Foundation (NSF) and the National Institutes of Natural
Sciences (NINS) of Japan in cooperation with the Republic of Chile.
ALMA is funded by ESO on behalf of its Member States, by NSF in
cooperation with the National Research Council of Canada (NRC) and the
National Science Council of Taiwan (NSC) and by NINS in cooperation with
the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space
Science Institute (KASI).
ALMA construction and operations are led by ESO on behalf
of its Member States; by the National Radio Astronomy Observatory
(NRAO), managed by Associated Universities, Inc. (AUI), on behalf of
North America; and by the National Astronomical Observatory of Japan
(NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides
the unified leadership and management of the construction,
commissioning and operation of ALMA.
ESO is the foremost intergovernmental astronomy
organisation in Europe and the world’s most productive ground-based
astronomical observatory by far. It is supported by 16 countries:
Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland,
Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden,
Switzerland and the United Kingdom, along with the host state of Chile.
ESO carries out an ambitious programme focused on the design,
construction and operation of powerful ground-based observing facilities
enabling astronomers to make important scientific discoveries. ESO also
plays a leading role in promoting and organising cooperation in
astronomical research. ESO operates three unique world-class observing
sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO
operates the Very Large Telescope, the world’s most advanced
visible-light astronomical observatory and two survey telescopes. VISTA
works in the infrared and is the world’s largest survey telescope and
the VLT Survey Telescope is the largest telescope designed to
exclusively survey the skies in visible light. ESO is a major partner in
ALMA, the largest astronomical project in existence. And on Cerro
Armazones, close to Paranal, ESO is building the 39-metre European
Extremely Large Telescope, the E-ELT, which will become “the world’s
biggest eye on the sky”.
Links
Contacts
Onsala Space Observatory
Onsala, Sweden
Tel: +46 31 772 55 57
Email: ivan.marti-vidal@chalmers.se
Richard Hook
ESO, Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
Source: ESO
