The Anatomy of Black Holes - Page 6
Astronomers now believe that they have proof that black holes of all sizes once ruled the Universe. The Chandra X-ray Observatory has provided the deepest X-ray images ever recorded, and those pictures have delivered a unique look at the past 12 billion years of black holes. Overall, the observations show that black holes were commonplace and active in the early Universe.
The "Chandra Deep Field" observations, combined with data from the Hubble Space Telescope and ground-based telescopes, reveal that there were at least 200 million supermassive black holes (and an even larger number of smaller black holes) in the early Universe. This conclusion is based on extrapolating the number of black holes found in a small region of the sky and applying it to the entire sky. The observations also reveal a distant quasar, shrouded in a cloud of gas and dust, some 12 billion light-years away. Further observations of the quasar, and others like it, will reveal how dense clouds of gas formed galaxies, with massive black holes at their centers.
The closest possible black hole to us will be a stellar-mass black hole. A stellar-mass black hole requires a massive progenitor. The nearest such star is tens of light-years away. The event horizon of such a back hole is at most a few tens of kilometers in diameter. Thus, the angular size of this hypothetical black hole is 0.00000001 seconds of arc. Bottom line: a black hole floating alone in space would be hard, if not impossible, to see. Nevertheless, there is now a great deal of observational evidence for the existence of both stellar-mass and supermassive black holes. How has this happened?
If a black hole passes through a cloud of interstellar matter, or is close to another "normal" star, the black hole can accrete matter into itself. As the matter is pulled towards the black hole, it gains kinetic energy, heats up, and is squeezed by tidal forces. As it gets hotter, its peak radiation moves progressively through the ultraviolet, X-ray, and gamma-ray regimes. In fact, we expect X-ray emission to occur just before the matter crosses the Schwarzschild radius, and can use this radiation to probe the most extreme environments of gravity, density, temperature, and velocity. However, it is not as simple as this may sound.
To search for X-ray emission from black hole binary systems was first suggested in 1966 by Igor Novikov and Yakov Zel’dovich - not long after the discovery of the first cosmic X-ray sources. Ever since, a "signature" in the emission properties of a source to classify it as a black hole (versus a white dwarf or neutron star) has been sought. We know that the mass calculated for the condensed star must exceed 3 solar masses in order to be considered as a black hole; we know that a characteristic "double-horn" shape will be introduced into the spectrum of a black hole due to a gravitational redshift; we know that the X-ray emission from a black hole should be highly variable in time. But are any of these methods a foolproof way of identifying black holes to the exclusion of any other type of celestial object? The answer is not yet clear. Several objects, starting with Cygnus X-1, have been tentatively identified as black holes via such methods, and while some scientists believe these absolutely are black holes, other scientists still wait for confirming evidence.
