B~CKHOLES------disk structure never seen before. Seeing and believing The search for a proof of the existence of supermassive black holes has long fol­ Jules P. Halpern lowed two complementary paths. The first method is the one described above, which THE most productive year yet for the black hole. Theoretical predictions of the now appears to have reached fruition. The pursuers of black holes has been crowned line profile corresponding to this picture second method is an older one. It involves by a long-awaited discovery that repre­ have been discussed optimistically for the observation and modelling of galactic 3 sents the closest we have come to actual\Y past six years or so . Advances in X-ray rotation and random velocities using the seeing one. On page 659 of this issue , spectroscopy put into practice by the starlight spectrum in the centres of Tanaka et al. report that a severely dis­ Japanese-American satellite ASCA, and nearby, non-active galaxies such as torted emission line, seen in the X-ray dedicated long exposure times, made Andromeda and NGC3115, in order to spectrum of a Seyfert galaxy, is of exactly possible the corresponding observational deduce the presence of dark, gravitating 1 the right shape to be coming from the discovery reported by Tanaka et a/. • mass that cannot be accounted for by the innermost regions of an accretion disk Both the inclination angle of the accre­ amount of starlight ~resent. The existence around a supermassive black hole. The tion disk to the line of sight, and its radius of black holes of 10 -109 solar masses has 4 bright, 'active' nucleus of a Seyfert galaxy relative to the radius of the black hole's been inferred in this way . More recently, is thought to be powered by the this stellar dynamics method has gravitational potential energy of been applied in a simplified form material spiralling slowly inwards to the observations of maser emis­ through an accretion disk, into a sion with radio interferometers5 black hole of more than a million and visible line emission with the 6 solar masses. The shape of an Hubble Space Telescope , using emission line from such a structure the apparently circular orbits of - in this case the Ko: fluorescent these sources to demonstrate the line of iron at an energy of 6.4 ke V need for supermassive black holes (wavelength 1.94 angstroms) - in regions as small as one light year provides the most direct evidence across at the centres of active for the enormous velocity galaxies. approaching the speed of light, The two methods are orthogon­ and other manifestations of the al in the following sense. The strong gravitational field that con­ X-ray emission line offers the trols the appearance of the disk in opportunity to 'see' a black hole the immediate neighbourhood of through the unique relativistic the black hole's 'event horizon'. effects of strong gravity; but be­ If one could view an accreting cause the emission arises from gas, black hole in visible light, it would which is susceptible to forces other look something like the accom­ than gravity, it cannot prove rigor­ 2 panying figure . The rotating ously that a black hole is responsi­ accretion disk, rather than ble. The stellar dynamics method, appearing as the symmetric annu­ Simulated picture of an accretion disk around a black hole, on the other hand, is immune from lus around a dark centre that it from ref. 2, viewed at an angle of 20° above the disk plane. the effects of non-gravitational really is, looks distorted. The The sense of rotation is left side approaching and right side forces. However, it demonstrates gravity of the black hole bends the receding. The observed temperature. shown in false colour. the existence of a supermassive rays of light passing around it. ranges from <3x106 K (orange) to >1.2x107 K (violet). black hole without seeing it, be­ Also, the side of the disk moving cause it operates in a region far towards the observer looks brighter than event horizon, are determined by fitting larger than any in which general relativity the side moving away, a result of the the spectrum to a simple model. One could produce observable effects that are so-called headlight effect for rapidly mov­ might try to dream up alternative explana­ distinct from ordinary newtonian gravity. ing sources oflight. With colour vision, we tions for the peculiar shape of the line that Perversely, we can either see a black hole, would see the Doppler shift, with the do not involve a black hole, but I agree or prove it exists, but not both! Which approaching side of the disk blue, and the with the authors that the detailed prop­ method is more satisfying is ultimately a receding side red. erties of this particular spectrum are diffi­ matter of taste. Probably because of this We will probably never see this picture cult to rationalize in any other way. Just fundamental complementarity, practi­ directly because, at the great distances of about the only quantity that has not been tioners of either approach hardly ever the galaxies, the accretion disk, which is measured by this observation is the mass acknowledge the others. Perhaps they will comparable in size to the Solar System, is of the black hole, for the simple reason -starting now. D too small to be resolved. Nevertheless, the that material falling in has the same veloc­ iron Ko: line seems to be confined to the ity no matter what the black hole's mass is. Jules P. Halpern is in the Department of inner accretion disk where conditions are It is probably 107 solar masses, and future, Astronomy, Columbia University, 538 West uniquely suited to its production. Spec­ more sensitive instruments should be able 120th Street, New York, New York 10027, troscopic measurements of its profile (in­ to measure it because time variability of USA. tensity versus wavelength) can be even the line profile can reveal the absolute more revealing of the black hole's extreme size of the disk, which is proportional to 1. Tanaka, Y. eta!. Nature375, 659-661 (1995). properties than a visible image, because the mass of the black hole and poss­ 2. Fukue,J. &YokoyamaT. Publsast. Soc.Japan40, 15-24 the spectrum contains information about ibly only a few light-minutes in radius. (1988). 3. Chen, K., Halpern, J.P. & Filippenko, A. V.Astrophys. J. the velocity of the material in the disk as With the acquisition of higher quality 339, 742-751(1989). well as the time dilation (gravitational data, it may even be possible to learn 4. Kormendy,J. &Richstone, D.Astrophys.J. 393,559-578 (1992). redshift) that causes the atoms' clocks to whether or not the black hole itself is ro­ 5. Miyoshi, M. eta/. Nature373, 127-129 (1995). run slower in the gravitational field of the tating, not to mention details of accretion 6. Harms, R. J. eta/. Astrophys. J. Lett. 435, L35-L38 (1994). NATURE · VOL 375 · 22 JUNE 1995 633