eso8709 — Science Release
Hunting the Black Hole
16 June 1987
Of all exotic objects predicted by current theories, none are as elusive as the Black Holes. Despite great efforts, their existence has never been unambiguously proven by astronomical observations.
However, a team of French astronomers have now obtained strong evidence in favour of the presence of a black hole at the centre of a peculiar galaxy. Based on observations at the European Southern Observatory, Danielle Alloin, Catherine Boisson and Didier Pelat of the Paris Observatory (Meudon) find that a mass of about 70 million times that of the Sun is contained within a very small volume at the centre of the active galaxy Arakelian 120 . The centre is surrounded by a rapidly spinning, gaseous disk.
Black holes are thought to be created when matter coalesces into a vanishingly small volume. For instance, if a mass equal to that of the Earth were to form a black hole, its diameter would be less than 2 centimetres. A black hole is exceedingly dense and its gravitational pull is so great that even light cannot escape from its surface. It is therefore completely dark and it only manifests its presence by the gravitational attraction which it exerts on nearby objects.
According to recent theories, the matter around a black hole will arrange itself in a rapidly rotating disk. Enormous amounts of energy are liberated when matter at the inner edge of the disk falls into the black hole and disappears from sight.
Since black holes are invisible, their existence must be deduced by observing the phenomena that take place around them. Looking for likely sites, astronomers have been particularly interested in X-ray emitting binary stars in which a large amount of energy is liberated from a very small volume in space. There is growing evidence (but so far no definite proof) that black holes with masses a few times that of the Sun may exist in some binary star systems.
Black holes may also be expected to form in the nuclei of galaxies. For instance, recent radio and infrared observations of the innermost regions of the Milky Way Galaxy indicate the presence of a heavy black hole at the very centre. Furthermore, the nuclei of some galaxies, Active Galaxy Nuclei (eso8702) or AGN's, emit prodigious amounts of energy. The same phenomenon, but on a still larger scale, is found in quasars, now thought to be the extremely bright and energetic centres of very distant galaxies. The hypothetical black holes in quasars would be millions of times heavier than the Sun.
But even if the nucleus in a galaxy emits large amounts of energy, how do you prove that this energy comes from matter falling into a black hole ? As the black hole cannot be observed directly, the proof must necessarily be indirect. In specific terms, it must be shown that there is so much mass inside such a small volume, in other words that the density is so high, that a black hole is the only possible explanation. The observational problem is therefore to measure the mass in the smallest possible volume that can be distinguished at the centre of the galaxy.
This is exactly what the French team has done. First, they noted that the energy output from the nucleus in the AGN galaxy Arakelian 120 is variable on a relatively short time scale, alternating between states of “high" and “low" activity. It was an Armenian astronomer, M.A. Arakelian at the Bjurakan Observatory, who first found this peculiar galaxy in 1975. It is situated in the constellation Orion, just south of the celestial equator. From the measured radial velocity, 10000 km/sec, the distance is estimated at 500 million light-years.
The spectrum of Arakelian 120 shows bright lines of various elements, including hydrogen and iron. The shapes (profiles) of some of these lines depend on whether the nucleus is in the “high" or the “low" state. By subtracting spectra, which were obtained with the ESO 1.52 m telescope with an IDS spectrograph when the galaxy was in “high" state and with the Anglo-Australian 4 metre telescope when it was in “low" state, respectively, the astronomers were able to prove that the excess light at “high" state in some of the hydrogen lines comes from a rotating disk around the nucleus. The rotational velocity at the edge of this disk is about 2100 km/sec, and the disk appears to be inclined by 60° to the line of sight. Outside the disk there are many hydrogen clouds.
From continued monitoring, they noted a time delay of about 2 months between the moment when the nucleus changes from one state to the other and when the profiles of certain spectral lines that originate in the surrounding clouds begin to change. This delay therefore corresponds to the time it takes the light to traverse the disk and the radius of the disk cannot be larger than 2 light-months, or 10.000 Astronomical Units. One Astronomical Unit = 150 million kilometres, i.e. the distance from the Sun to the Earth. Note also that 2 light-months at the distance of Arakelian 120 only subtend an angle of about 60 microarcseconds, too small to be directly resolved with existing interferometric techniques. (An object measuring 12 cm and placed on the surface of the Moon, would be seen under the same angle.)
From the size of the disk and its rotational velocity, the mass inside the disk can now be calculated as 70 million solar masses. (The mass of the Sun is calculated in a similar way from the radius of the Earth's orbit and its velocity in this orbit.) This observation therefore shows that the density is very high at the centre of Arakelian 120. In fact, if the total mass were spherically distributed and belonged to individual stars, each with a mass like that of the Sun, then the mean distance between two stars would only be about equal to the distance between the Sun and Pluto.
Such an enormous stellar density is highly unlikely since a system like this would suffer from stellar collisions and would not be very stable. Thus, from the measured density and the total amount of energy emitted from the small volume, a much more plausible explanation is the presence of a black hole. Its mass may be a significant fraction of the total mass inside the perimeter of the disk and the radius is perhaps 0.5 Astronomical Units.
The three astronomers describe their detailed findings in an article which has been accepted for publication in the European journal Astronomy & Astrophysics.
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