Using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope, astronomers have for the first time detected an extremely powerful magnetic field near the event horizon of a supermassive black hole in the centre of an active galaxy. This discovery supports the idea that strong magnetic fields help collimate the high-speed plasma jets emanating from many active galaxies.
Active galaxies are characterised by a very bright central nucleus, with high and rapidly varying luminosities on timescales of hours or days, emission across a wide range of wavelengths (and are usually most luminous in a non-optical part of the electromagnetic spectrum, e.g. UV or radio), non-thermal spectra, and often have radio jets emanating from the central region of the galaxy.
Different types of activity galaxies include quasars, Seyferts, BL Lac/blazars and radio galaxies. The central region, or nucleus, of all active galaxies are thought to be similar and are explained by the “Unified Model of AGN” (where AGN stands for Active Galactic Nuclei). The variation in AGN properties is thought to be related to the line of sight we have into the central region of the AGN. In the Unified Model, AGN have a central supermassive black hole that is fed by an accretion disk that is a few light days across and surrounded by a thick dusty torus.
Radio synchrotron emission is produced in many AGN, collimated into jets that propagate outwards from the nucleus perpendicular to the plane of the accretion disc. These radio jets are prominent in many radio galaxies and can be as large as several Mpc in size. Radiation from the jet moves close to the speed of light and can be beamed, and can vary on periods from hours to days.
Exactly what powers these jets is still somewhat of a mystery. It is believed that strong magnetic fields must exist close to the accretion disk, and the magnetic field lines must collimate and power the jets. Hence the recent results of Ivan Martí-Vidal and colleagues published in the journal Science are of great interest.
Using ALMA, Marti-Vidal et al. detected polarised light related to the strong magnetic field at the base of the jet emanating from the distant AGN, PKS 1830−211, which is at a redshift of z = 2.5. The high-resolution ALMA observations were at a wavelength of about 0.3 mm. The high-resolution was crucial to probe the region very close to the black hole, and only millimetre wavelength light can escape from the dusty region very close to the black hole, since longer wavelength radiation is absorbed.
Polarised light means that the electric field vectors of an electromagnetic wave have a preferred direction. As the polarised light propagates through a magnetised medium, the direction of polarisation can change – this phenomena is known Faraday rotation. The amount of Faraday rotation, determined by a quantity known as the rotation measure, is proportional to the magnetic field strength.
The high rotation measure derived in PKS 1830-211 suggest magnetic fields of at least tens of Gauss, and possibly considerably higher, on scales of the order of light-days (about 0.01 parsec) from the supermassive black hole.
“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.” Sebastien Muller, co-author of the Science paper.
Zooming in on the distant active galaxy PKS 1830-211 (Credit:ALMA (ESO/NAOJ/NRAO)/I. Martí-Vidal/Nick Risinger (skysurvey.org) & NASA/ESA)
For more information, see
- ALMA Reveals Intense Magnetic Field Close to Supermassive Black Hole, ESO Press Release [16 April 2015]
- A strong magnetic field in the jet base of a supermassive black hole, Marti-Vidal et al. (2015), Science, 348, 311 [Swinburne student login]
- Active Galactic Nuclei at the half-century mark, talk by Bradley Peterson, April 2011
- Probing the jet base of the blazar PKS 1830−211 from the chromatic variability of its lensed images, Marti-Vidal et al. (2013) A&A, 558, A123
[Glen Mackie & Sarah Maddison]