Cosmic Radio Emissions

the radio-frequency radiation of galactic and metagalactic objects. Sometimes the radio-frequency radiation of the sun and planets are also included.

Cosmic radio emissions were discovered in 1931 by the American radiophysicist K. Jansky at a wavelength of about 15 m. Despite the extremely low resolution of the antenna of the radio telescope designed by him, Jansky subsequently proved that the radio-frequency radiation that he had observed originated from the vicinity of the Milky Way. In the 1940’s, radio astronomy, a new branch of astronomy, arose in connection with the rapid development of radar technology. This branch, which interacts closely with astrophysics, significantly augmented the results of astrophysical studies of cosmic objects. Using a radio interferometer, the British researchers J. Hey, J. W. Phillips, and S. Parsons in 1946 detected isolated “discrete” sources of cosmic radio emissions.

The radio astronomical instruments of the early 1970’s provide the possibility of observing about 1 million such sources. The flux of radio emissions from the weakest sources is millions of times weaker than that from the brightest known sources. The majority of weak sources lie outside our galaxy, in the metagalaxy. Several hundred have been identified with galaxies. Most of the unidentified sources are apparently associated with galaxies and quasars.

Our galaxy is also a source of cosmic radio emissions: in the belt of the Milky Way, regions with increased intensity of emissions can be observed. Most metagalactic sources of cosmic radio emissions are much more powerful than the Milky Way Galaxy. While our galaxy radiates approximately 10³⁸ erg/sec (about 10^–6 of its total radiation in the optical band), some metagalactic sources radiate as much as 10⁴⁵ erg/sec, which is close to the power of their optical radiation. Such objects, which are called radio galaxies, are generally vast, spheroidal, extremely massive stellar systems. Interference observations show that the regions of the optical and radio-frequency radiation of metagalactic objects do not coincide in space: the latter are usually localized in two clouds that are situated symmetrically with respect to the optical center and located tens of thousands of parsecs from it. In a number of cases, a source of extremely small angular dimensions (≪1¹¹), the flux of radio emission which changes rapidly with time, is observed in the optical center of a radio galaxy. This attests to the continuing activity of the galactic nuclei, which eject the matter from which clouds emitting radio-frequency radiation are formed.

The theory of the radiation of radio sources was proposed in 1950 by the Swedish scientists H. Alfvén and N. Herlofson and was worked out in detail by the Soviet scientists V. L. Ginzburg and I. S. Shklovskii. According to the theory, many of whose predictions have been fully confirmed by subsequent observations, cosmic radio emissions arise during the motion of fast, so-called relativistic, electrons in magnetic fields (synchrotron radiation). The application of the theory to specific metagalactic sources shows that they contain numerous relativistic particles whose total energy may be as high as 10⁶⁰ erg. This is comparable to the energy of the galaxy’s gravitational bond. These particles are generated in the vicinity of the galactic nuclei and are ejected from there during explosions.

In 1965 the “relict” radiation of the metagalactic background was detected in the cm band. It is characterized by a Planckian spectrum with a temperature of about 3° K. It was named relict because its quanta were radiated by the universe at an early stage of its development, at which time neither galaxies nor stars existed. The universe during this period was a hydrogen plasma with a temperature of 4000° C.

Galactic sources of cosmic radio emission are observed in addition to metagalactic sources. These—primarily special nebulas—are the remnants of outbursts of supernovas (for example, the Crab Nebula). In this case, the radiation is also synchrotron. Moreover, sources of thermal emission are observed in the Milky Way Galaxy as well as in the closest galaxies, such as the Magellanic Clouds. These sources are interstellar clouds of ionized gas and ordinary galactic nebulas. The spectrum of this radiation differs from synchrotron radiation, and “thermal” sources are primarily observed at comparatively short wavelengths.

In 1967, S. J. Bell and others (Great Britain) detected an entirely new type of radio source, called pulsars. It was soon determined that pulsars are strongly magnetized, rapidly rotating neutron stars that were formed after the explosions of supernovas.

All sources of cosmic radio emission mentioned above are characterized by a continuous spectrum. Moreover, individual spectral radio lines are observed in many cases, both in radiation and in absorption. The hydrogen line at a wavelength of 21 cm is the most important of these lines. The existence of this line was theoretically first predicted by the Dutch scientist H. van de Hulst in 1944. It was discovered in 1951 by the American astronomers H. Ewen and E. Purcell, and observations of it have become an inexhaustible source of information for various astronomical studies. In 1949, Shklovskii predicted a new class of interstellar molecular lines, in particular, the OH line at a wave-length of 18 cm. This line was finally discovered in 1963. In 1966 new extremely bright sources of radio-frequency radiation were discovered at this wavelength. The radiation of these sources is maserlike in nature. Still more intensive maser cosmic sources were soon discovered at a wavelength of 1.35 cm in the water vapor line.

At present (1970’s), radio astronomical techniques have detected more than ten interstellar molecules, including such polyatomic molecules as ammonia, alcohol, and formic acid. In 1962 the Soviet astronomer N. S. Kardashev substantiated the possibility of detecting in the radio-frequency band the lines of highly excited atoms of interstellar hydrogen, which were dis-covered soon after. Observations of these lines are extremely useful in analyzing the physical conditions in the interstellar medium.

At the end of the 1960’s, the first results were obtained of observations of superlong wave (with wavelengths on the order of km) cosmic radio-frequency radiation from artificial earth satellites and of submillimeter cosmic radio emissions. The expansion of the spectral range increases still further the possibilities of radio astronomy.