Astrophysics

The MAD world of black holes

An analysis of optical and radio observations has revealed how powerful jets are launched from the centres of active galaxies, where supermassive black holes accrete matter through magnetically arrested disks, or MADs. See Letter p.126

Although the light given off by most galaxies is due to stars and glowing gas, some galaxies have extremely bright centres, or nuclei, with luminosities about 100,000 times greater than those of normal galaxies. In such active galactic nuclei, energy is liberated when matter spirals inwards and is captured by a supermassive black hole — billions of times more massive than the Sun — sitting at the galactic centre. In about 10% of these active nuclei, some of the in-spiralling matter is pushed into two jets of matter and radiation that shoot out in opposite directions at close to the speed of light (Fig. 1). However, the forces causing these outflows have remained unknown. In this issue, Zamaninasab et al.1 (page 126) report direct evidence that the jets are launched on their journey by a kind of gigantic electromagnetic generator, in which magnetic fields in the vicinity of the black hole are twisted by the black hole's spin, with the energy of this spin being transformed into the energy of the jets' outward motion2.

Figure 1: Jets emerging from the centre of an active galactic nucleus.
figure1

a: ESO/WFI (Optical); MPIfR/ESO/APEX/A. Weiss et al. (Submillimetre); NASA/CXC/CfA/R. Kraft et al. (X-ray)

a, Composite image of the optical (true colour), X-ray (blue) and radio (orange) emission of the giant elliptical galaxy NGC 5128, showing its powerful radio jets extending far beyond the optical galaxy. b, Zamaninasab et al.1 have found evidence that jets emerging from the centres of active galaxies are launched from deep in the galactic nucleus (square in a) by twisted magnetic fields2, and that these fields are dynamically important in the vicinity of the galaxy's central black hole and surrounding accretion disk.

It has long been suspected that this mechanism might explain how jets in active galactic nuclei (AGN) are produced. However, direct observational evidence has been elusive, because the scales on which this mechanism should operate are tiny compared with the smallest scales that can be observed directly using the highest-resolution technique available, which are typically about 1 parsec (3 × 1016 metres) from the central black hole. This scale might seem huge, but at the extremely large distances of AGN (billions of parsecs), it translates to a tiny angular distance as observed on the sky.

Images of such small scales can be obtained using very-long-baseline interferometry (VLBI), an elegant technique in which radio telescopes around the world observe in synchrony to imitate a single radio telescope with a diameter the size of Earth. The larger the telescope used, the finer the detail that can be seen; accordingly, VLBI yields radio images with phenomenally high resolution, equivalent to being able to peer across the Atlantic Ocean from the western edge of Europe and identify a coin held by someone standing on the eastern coast of the United States.

Unfortunately, however, the scales imaged with VLBI are still tens to thousands of times larger than those on which the powerful jets of AGN are launched. Zamaninasab et al. have found a clever way to bridge this gap, by considering the magnetic flux in the jets — essentially, the product of the magnetic field pointing along the jet and the jet's cross-section. The magnetic flux near the black hole cannot be measured directly, but should be proportional to the luminosity of the matter in the accretion disk surrounding the black hole. The disk's luminosity can be estimated from observations of optical lines in the spectrum of the AGN.

The authors have found a tight linear correlation between the estimated accretion-disk luminosities for 76 AGN and the magnetic fluxes in their jets measured with VLBI. This correlation demonstrates that the magnetic flux near the black hole is proportional to the magnetic flux far down the jets, as would be expected if the electromagnetic jet-launching mechanism referred to above is at work. The authors' results thus provide direct observational evidence that this is the case.

Theoretical simulations3,4 of accretion disks have shown that, under certain conditions, the magnetic flux in the vicinity of the black hole naturally reaches a maximum equilibrium value. When this happens, forces exerted by the magnetic field dominate in the inner part of the disk. Disks for which this is true are called magnetically arrested disks, or MADs3,4. Zamaninasab et al. find that the derived slope of the linear correlation between the accretion-disk luminosity and the jet magnetic flux is precisely the value predicted for such disks, strongly suggesting that this MAD scenario is operating in the hearts of AGN.

These results indicate that the jets of AGN are launched electromagnetically by magnetic fields twisted by the black hole's spin, that these magnetic fields have a dominant role in determining the dynamics of the disk and jets in the vicinity of the central black hole, and that this may remain true at least out to VLBI scales, several parsecs from the black hole. It will therefore be important to consider the influence of the magnetic field, for example, when inferring the properties of the central black hole and accretion disk from high-resolution studies made with millimetre-wavelength, ground-based VLBI5,6,7, or with 'space VLBI', in which one or more antennas orbiting Earth are used with ground antennas8,9.

Zamaninasab and colleagues' findings also radically change the way astronomers view the jets emanating from the centres of AGN. These jets are not just outflows of matter carrying tremendous amounts of energy, but are also intrinsically magnetic structures. Many of their properties are probably determined by the magnetic fields embedded in them and travelling outwards with them. The twisting of the central magnetic fields that launches the jets should give rise to helical jet magnetic fields, which may be manifest in the jets' magnetic-field structure and morphology10,11. Because a fundamental relationship exists between magnetic fields and electrical currents, jet outflows should be regarded as systems of magnetic fields and currents. This is essential if we are to understand these enormous structures: how they propagate, why they remain so narrow as they traverse enormous distances, and how they interact with the material through which they are moving.

As the jets travel beyond their host galaxy and into intergalactic space, effects other than magnetic forces will probably also come into play, making the jets and the surrounding gas more turbulent and reducing the magnetic field's effects. Further detailed studies of the jets of AGN and their magnetic fields, from VLBI scales out to the ends of the jets many thousands of parsecs from the central black hole, should help to determine whether such a transition occurs, and where.

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Correspondence to Denise Gabuzda.

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Gabuzda, D. The MAD world of black holes. Nature 510, 42–43 (2014). https://doi.org/10.1038/510042a

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