Quasar: Energy Monster Or Great Unknown?
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Quasar: energy monster or great unknown? Domingos Soares To begin with, the quasar is an \inhabitant" of this immense universe! A cosmic object, just like are the stars, the planets, the comets, the galaxies, etc. Quasars were discovered in 1960, in a very interesting way. They showed up to humans, for the first time, as sources of radio waves! When those sources were located on the sky, they had the appearance of a bluish star! But they are not stars, they only have the distinctive appearance of a star, that is, a luminous point. Its name originated from these characteristics. The word \quasar" is an acronym that refers to the expression \quasi-stellar radio source". The quasars are also called, more generally, QSOs. QSO are the initials of \quasi-stellar object". Let us go now to the history of the discovery of these objects. Many galaxies emit radio waves. The radio waves are electromagnetic waves, like the visible light, but with wavelengths in the order of centimeters or meters. The waves are, in general, similar to the waves on a stretched rope, which is forced to undulate. The wavelength is the distance between two successive wave peaks on the rope. This idea can be also applied to the electromagnetic waves, but in this case, the oscillations refer to a physical entity called elec- tromagnetic field. Our well-known radio receivers, which we use to listen to the music or the latest news, are actually true detectors of electromagnetic waves in the wavelength range of meters. The visible light has a very small wavelength. From the violet to the red color | the visible spectrum |, the wavelengths are in the order of thousandths of a millimeter! Incidentally, our eyes are excellent detectors of visible light, and the leaves of the plants are ex- ceptional detectors of visible sunlight, which they use for the photosynthesis of organic matter. 1 Now, in the end of the 1950s, astronomers had already observed many radio sources, using the newly-invented radio telescopes. The radio telescopes are very big metallic \dishes" with parabolic profiles | some are up to 20 meters in diameter | which focus the radio waves on the detectors located at the foci of the dishes. Most of the radio sources were galaxies, called, for this reason, radio galaxies. The radio emissions of the galaxies extend far beyond the boundaries of their visible light, often, in the form of two enormous lobes of emission, located at opposite directions with respect to the galaxy centers. The smaller these lobes are, the further away is the radio galaxy. It happens that there were many radio sources whose lobes simply did not exist! The radio emission was not extensive but came from one point. That seemed to indicate that these objects were very far away from us. The American radio astronomer Thomas Matthews selected ten of these radio sources and determined their positions on the sky. 2 Radio galaxy Centaurus A, located in the Centaurus constellation, where the star nearest us is. It is a very peculiar galaxy, where one sees a disk galaxy | a spiral seen edge-on | superimposed on an elliptical galaxy | in white on the image. Probably, the spiral galaxy \fell down" over the giant elliptical and both are in the merging process. The enormous radio emission lobes are represented in blue (Image: Jack O. Burns and David Clarke | radio | and National Optical Astronomy Observatories, United States | optical). It comes next the astronomer, also American, Allan Sandage (1926-2010), who have worked under the supervision of the famous Edwin Hubble (1889- 1953). He took it upon himself the task of locating the origins of the radio sources, that is, their visible counterparts on the sky. One of the sources selected by Matthews is called 3C 48, i.e., the forty-eighth radio source of the third catalog of the Cambridge Radio Observatory in England. Sandage pointed the 5-m diameter telescope of Mount Palomar Observa- tory to the position of the radio source, which sits at the Triangulum constel- 3 lation. He then marked, in the photograph that he obtained, the position of the radio source and, for his surprise, he verified that it coincided with a very faint star! But stars do not emit radio waves of that magnitude. Would this be a new category of radio source, a radio star? The next step by Sandage was to get a spectrum of the visible radiation emitted by the strange \star". A spectrum of radiation is the distribution of the intensity of the radiation along the several wavelengths. Stars have spectra of visible radiation rather characteristic, very distinctive. But the spectrum of that object proved to be completely different from any stellar spectrum ever observed! \That" was definitely not a star! The spectrum of an astronomical object allows the as- tronomer to identify, among other things, the chemical elements responsible for the radiation present in the spectrum. And Sandage, as an experienced astronomer, could not identify any feature of any known chemical element. It was certainly a new inhabitant of the Cosmos! Discovered because it emitted radio waves, of hitherto unknown origin. 4 The astronomer Allan Sandage, who, amongst other contributions to astronomy, identified the first quasar, the radio source 3C 48. To complete the discovery, enter the scene the Dutch astronomer, living in the United States: Maarten Schmidt. He studied another quasar of the Matthews list, 3C 273, and discovered an extraordinary thing. When he ana- lyzed the visible spectrum of 3C 273, he noticed that if the wavelengths of the light were shifted by about 16% to larger wavelengths, he could identify the familiar radiation emitted by the hydrogen atom! This radiation appeared in the form of emission spectral lines. And more, it fitted as a glove in the standard model of cosmology, which states that the universe is expanding. The expansion of the universe makes the radiation of distant objects to have an increased wavelength, proportion- ally to the object distance. Such an observational fact had been discovered by Edwin Hubble in the late 1920s. But in order to have a shift of 16%, it would mean that 3C 273 should be at a fabulous distance from us! And the idea was immediately applied to 3C 48, with a more dramatic 5 result: if the spectrum was shifted by 37% towards greater wavelengths, the spectrum would crystal-clearly reveal the same lines as the hydrogen atom. According to the standard cosmology, it should be even further away than 3C 273. In this regard, we say in the astronomical jargon that the spectrum of the quasars were \redshifted", because in the visible spectrum the red color has the largest wavelength. Also according to the standard cosmology | the Big Bang cosmology |, 3C 48 must be at 5 billion light-years and 3C 273 at approximately 3 billion light-years! At these distances, they are true \energy monsters", so that their optical and radio wave brightnesses may be explained. If they are really at these distances, the brightest quasars may have an intrinsic brightness of more than 100,000 times the brightness of a galaxy like the Milky Way! In order to have an idea of the magnitude of quasar distances, the galaxies relatively close to us as, for example, the Virgo cluster galaxies, have spectra with redshifts of about 0,5%, being located at about 20 million light-years. How could such a large amount of energy be generated in quasars? There is not yet a complete theory firmly established. The prevailing theory involves the existence of another strange object: a black hole. The black hole is an object, predicted by the theory of gravitation of Albert Einstein (1879- 1955) | the General Relativity Theory |, whose main characteristic is the existence of a gigantic gravitational attraction in its immediate vicinity. The attraction tends to infinity in its center. Its size is conventionally given by the distance from its center, where the gravitational attraction is so great that it is able of \holding" the light itself, eventually trying to escape from it. It is a cosmic object of a theoretical character, still without definitely proven existence. The general idea, investigated by astronomers, is that there is a black hole at the center of the quasar that \swallows" the nearby matter. Such a black hole must have dimensions similar to the solar system and mass equivalent to hundreds of millions of times the mass of our Sun! Very well, the matter located in the vicinity of this black hole falls down towards the black hole and acquires a very high speed. The processes of mutual interaction of the falling material, at speeds close to the speed of light, are those that generate the observed radiation of quasars, which emit not only in visible light and in the radio range, but in X-rays and other wavelengths as well. In general, quasars are located at the center of a host galaxy. Since they are very far away and their brightnesses are so great, the host galaxies are not clearly detected on the observations. For example, it was later found that 3C 6 273 sits at the center of an elliptical galaxy. The host galaxies provide the material to be \swallowed" by the quasar's black hole. This whole scenario is made from the point of view of the standard cosmology and is still the subject of investigation, but it is accepted by most of astrophysicists and cosmologists. In this framework, the quasars are probably linked to the processes of formation of the galaxies that we see in the nearby universe.