How To Feed A Black Hole ?

Fig.1: The barred galaxy NGC1097. Colours show the total radio emission at 3.5cm wavelength. The small lines show the orientations of the magnetic fields and thus also of the gas flow. Radio image: VLA (Rainer Beck and Vladimir Shoutenkov, MPIfR Bonn) Full size image available through contact

Do black holes wipe up all material from their sourroundings? No, they don't! If we would squeeze the mass of the sun to the size of a black hole (about 3km), all planets would still move on stable orbits. New radio observations of the galaxy NGC1097, produced at the Max Planck Institute for Radio Astronomy (Bonn, Germany) and published in NATURE on Jan.28 1999, show how a black hole in the centre of a galaxy can be fed with gas.

In 1996, a team of astronomers from Germany, Russia and Australia started to observe the radio emission from the 20 brightest barred galaxies. According to the sizes and positions of these galaxies on sky, they used the 100-m Effelsberg telescope of the Max Planck Institute for Radio Astronomy, the Very Large Array (Socorro/USA) and the Australia Compact Array (Narrabri/Australia). Remarkable results concerning the barred galaxy NGC1097 (at a distance of about 50 million light years) has just been published in the magazine NATURE.

Barred galaxies are characterized by an oval distribution of stars. In case of NGC1097, the bar has an extension of about 60.000 light years (Fig.1). The asymmetric gravitation of such a galaxy causes highly elliptical orbits of stars and gas. This leads to a "traffic jam" in the bar, a so-called shock front.

Numerical computer models showed that the shocked gas does not proceed in the same direction with strongly reduced speed (as in a traffic jam), but finds a fast diversion along the bar towards the inner galaxy. Such models are now basically confirmed by the new data.

Fig.2: Southern half of the barred galaxy NGC1097. Overlay of an optical photograph with contour lines of the polarized radio emission at 3.5cm wavelength. The small lines show the orientations of the magnetic fields and thus also of the gas flow. The small circle in the right bottom corner gives the angular resolution of the radio telescope. Optical photograph: Cerro Tololo (Halton Arp, MPE Garching) Radio image: VLA (Rainer Beck and Vladimir Shoutenkov, MPIfR Bonn) Full size image available through contact

The gas in the bar is cold and forms molecules which radiate only weakly in the radio range. The new observations reveal a new, elegant method to investigate the gas and its motion: Polarized radio emission gives strengths and orientations of the magnetic fields which are swept along with the fast gas stream. The map of magnetic field lines (see lines in Fig.1 and Fig.2) is also a map of the velocity flow.

In NGC1097 the shock front in the bar becomes directly visible for the first time. The length of the shock front reaches several 10.000 light years (Fig.2). At the shock front the direction of the gas flow jumps abruptly by almost 90 degrees. However, in contrast to model expectations, the shock front is located in the middle of the bar, not at its edge. This discrepancy is probably due to the magnetic fields.

Gas and magnetic fields in NGC1097 stream inwards along the bar with about 100km/s and accumulate in a ring of about 5.000 light years diameter around the galaxy's centre. Some of the gas is transformed into stars. Fig.3 shows the light from this ring seen by the Hubble Space Telescope with high resolution.

The centre of NGC1097, similar to many other galaxies, probably contains a black hole. The black hole itself is of course invisible, but infalling gas very close to the black hole is observable via its strong radio and X-ray emission.

The German X-ray satellite ROSAT was also pointed to NGC1097 to search for hot gas. As expected, most of the hot gas is located in the immediate vicinity of the centre, but the astronomers also detected X-rays from an extended hot halo around the whole galaxy. This gives evidence for a hot wind from the centre outwards, just opposite to the flow of the cold gas.

Every black hole has to be fed regularly with fresh material (gas or stars), otherwise the radiation from its vicinity quickly decays. A central ring, however, is a bad resource because the gas and stars in the ring rotate on stable circular orbits. Before mass can fall into the black hole, it has to be decelerated by a mechanism not known before.

Fig.3: The central gas ring in NGC1097. Overlay of the high-resolution optical image obtained with the Hubble Space Telescope (HST) with contour lines of the total radio emission and orientations of the magnetic field. Optical photograph: HST (Aaron Barth, Cambridge/USA) Radio image: VLA (Rainer Beck and Vladimir Shoutenkov, MPIfR Bonn) Full size image available through contact

The new radio observations offer a possible solution to this problem: The magnetic field in the central ring (Fig.3) is shaped like a spiral and, different from the flow in the bar, runs across the motion in the ring. The resulting magnetic force on the gas is sufficient to deflect a gas mass comparable to the mass of the sun towards the centre per year -- enough to feed the black hole.

Until now, the supplying material to feed black holes was thought to be attracted from outside, for example infalling small galaxies ("cosmic cannibalism"). The new observations show that at least barred galaxies do not need such rare events, but are able to feed their black holes themselves.

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Original publication: R.Beck, M.Ehle, V.Shoutenkov, A.Shukurov, D.Sokoloff: Magnetic field as a tracer of sheared gas flow in barred galaxies, Nature Vol. 397, 28 Jan. 1999, page 324-327.

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