Magnetic Turbulence in the Hearts of Clusters of Galaxies

Scientists from the Max-Planck-Institute for Astrophysics successfully detected and measured magnetic turbulence in the central gas of a galaxy cluster. Strengths and lengths of the magnetic eddies support novel theories about the highly complex life within the hearts of clusters of galaxies, where gas and a massive Black Hole cyclically exchange matter and energy.

Fig. 1: The centre of the Hydra cluster of galaxies. Two radio-emitting gas bubbles ejected by the central Black Hole represented in green are ploughing through the hot gas represented in purple. Credits: X-ray: NASA/CXC/SAO; Radio: Greg Taylor (NRAO).

Fig. 2: The new theory about the flow of matter and energy in hearts of galaxy clusters. Within the dense inner regions of the galaxy cluster (purple), the gas is cooling and falls through the central galaxy (yellow) into its central massive Black Hole (black). The Black hole reacts to the infalling cooled gas by ejection of ultra-hot, radio-emitting jets of gas which inflate two radio bubbles (green). The bubbles rise and stirr turbulent motions in the gas of the galaxy cluster (red arrows). The gas motion heats up the cooled gas and delays the gas from falling into the Black Hole. At the same time, the gas eddies impart turbulence into the magnetic fields which can be observed by Faraday rotation.

Fig. 3: The polarised radio radiation of the gas bubbles (green ellipses) is rotated by the magnetic fields (green line) in the gas of the galaxy cluster (red) by the Faraday effect. This rotation is observable by multi-frequency measurements with radio telescopes and allows the construction of maps of the Faraday rotation. These maps show the projection of the existing magnetic fields located in front of the radio bubbles (shaded blue-grey). An analyis of these maps allows one to draw conclusions about the structure of the magnetic fields.

Fig. 4: Faraday rotation map of the northern radio bubble in the Hydra cluster of galaxies. Blue regions indicate magnetic fields oriented towards Earth while red regions indicate magnetic fields pointing away from Earth. The distribution of scales in the Faraday structures gives information about the typical lengths of the magnetic field eddies in the galaxy cluster (the map was produced with the PACERMAN code).

Fig. 5: Power spectrum of the magnetic fields in the Hydra cluster of galaxies (green data points) from the analysis of the map shown in Fig. 4. A power spectrum shows how strong magnetic eddies of a certain length are. The large spatial eddies are at left and the small spatial eddies are at right in the graph. Especially strong are the eddies which are about 10,000 light years (3 kpc) large. This fits well to their production by the radio bubbles which are 3 — 10 times larger. On smaller scales, the magnetic field follows a Kolmogorov-spectrum (red line) which is typical for turbulence. This is the first direct detection of magnetic turbulence in clusters of galaxies.

The hot (10 million degrees) gas in the centres of galaxy clusters emits its heat as X-ray radiation allowing the mapping of this gas with X-ray telescopes (Fig. 1). As soon as the gas has cooled down it falls into the central galaxy of the galaxy cluster due to gravity. In its centre a massive Black Hole of a few billion solar masses exists which injests most of the infalling gas, thereby getting more massive. At the same time, enormous amounts of energy are released at the Black Hole by this process and enormous amounts of ultra-hot, radio-emitting gas are ejected. The ejected gas forms bubbles in the galaxy cluster (Fig.1 and 2) which rise quickly within the surrounding cluster gas due to their small density. The surrounding cluster gas rapidly moves downwards passing the bubbles. The opposing gas movements lead to turbulence in small eddies eventually providing heat for the cooling gas in the galaxy cluster. This heating-up prevents all of the gas from catastrophically collapsing rapidly into the central galaxy. But the gas collapse is delayed only for a cosmic moment and then the turbulence decays, the heating power decreases and the cooling of the gas by radiation of X-rays dominates again. Another heart beat starts in the galaxy cluster: gas clouds again fall into the Black Hole and a new cycle begins...

So much to the novel theory about the complex life within the hearts of clusters of galaxies. What about the confirmation of this theory by observation? The cooling gas can be observed by X-ray satellites, the radio emission from the bubbles provides impressive pictures for terrestrial radio telescopes (Fig. 1) and the gigantic Black Holes can be inferred from other observations. But is there really turbulence in the cooling gas which eventually heats it up again? Direct evidence was missing until now.

An indirect proof was provided through the detection of magnetic turbulence in the gas of the Hydra cluster of galaxies by Corina Vogt and Torsten Enßlin at the Max-Planck-Institute for Astrophysics. The movements of the gas in galaxy clusters should stretch, wrap up and amplify the magnetic fields interwoven within the gas. The theory predicts that the magnetic eddies are somewhat weaker and smaller compared to the gas eddies. Therefore, a measurement of the magnetic eddies provides information about the turbulence in the gas.

The measurement of the magnetic turbulence was realised by a statistical analysis of a so-called Faraday rotation map. Faraday rotation is the rotation of the polarisation direction of the radio emission when passing through magnetised gas. The strength of the rotation is dependent on the strength of the magnetic fields along the path of the radio radiation. This Faraday rotation can be measured by multi-frequency observations with radio telescopes. These maps yield a two dimensional image of the three dimensional magnetic fields between the source of the radio emission and Earth (Fig. 3). Observations of the radio bubbles in the Hydra cluster of galaxies therefore allow the mapping of the magnetic fields residing within the cluster (Fig. 4). This map already gives an impression of the chaotic magnetic field structure. But are the magnetic fields as turbulent as expected by the theory of gas motion?

A detailed, statistical analysis of the Faraday rotation map shows that the typical length scales and the typical magnetic field strengths match the expected but unobserved gas turbulence (Fig. 4). The strongest magnetic eddies reach about 10,000 light years and are, as expected, smaller than the radio bubbles which are about 30,000 to 100,000 light years in size. The strength of the magnetic eddies are also, as expected, one hundred thousandth of the Earth's magnetic field (the latter being about one Gauss). Furthermore, the statistics of the small magnetic field eddies follow a Kolmogorov-spectrum which is universally valid for gas turbulence. The statistics indicate to which degree the strength of the turbulent eddies lessens with decreasing scales of the eddies (Fig. 5). Practically, the Kolmogorov-spectrum is the fingerprint of turbulent gas motions. The discovery of the Kolmogorov-spectrum as revealed by the statistics of magnetic eddies means that the turbulence suspected in the gas of the Hydra cluster of galaxies is actually present there and thus, provides the surrounding gas with heat even as heat escapes from the gas by X-ray emission cooling. - A first confirmation of the new theory about flows of energy and matter in the hearts of galaxy clusters.

Torsten Enßlin, Corina Vogt

Further information:

C. Vogt, T.A. Enßlin: linkPfeilExtern.gifA Bayesian view on Faraday rotation maps - Seeing the magnetic power spectra in galaxy clusters, 2005, A&A, 67, 434.

T.A. Enßlin, C. Vogt: linkPfeilExtern.gifMagnetic turbulence in cool cores of galaxy clusters, 2005, A&A, submitted.