Radio galaxy

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The radio galaxy Hercules A in a combined image in the visible spectrum ( Hubble space telescope ) and in the radio range ( VLA )

A radio galaxy, similar to the radio-noisy quasars and blazars , is an active galaxy , whose extraordinarily strong continuum synchrotron radiation can be observed in the radio wave range.

The observed radio emission is produced in this case by the interaction between the double-matter beams (engl. Jets ) of an active galactic core and the intergalactic medium , being close to the active galactic core by relativistic beaming can be modified.

In general, this creates emission areas that can extend far beyond the actual extent of the galaxy into the surrounding medium. The galaxy does not necessarily have to be observable as an active galaxy due to the time lag of the interaction . However, it is almost exclusively a massive elliptical galaxy .

Despite their extragalactic distance, some radio galaxies are among the apparently brightest and first discovered radio sources in the sky, named after the constellation in which they were found. Examples are Virgo A and Cygnus A . The radio galaxy closest to us is Centaurus A in the southern sky.

Since the brighter representatives of the radio galaxies are visible up to great distances, they represent a valuable tool in cosmology. For a long time they were the only galaxies in the distant universe accessible to precise observation ( redshifts greater than one). In addition, unlike quasars, the galaxy is not outshone by its core in visible light.

More recently, there has been increasing effort to understand how radio galaxies affect and are themselves affected by their surroundings. So is z. B. the outflow of radio galaxies in galaxy clusters and the associated heating of the surrounding gas is an important feedback mechanism to suppress the influx of cool gas in their center and thus generally the star formation rate in the central galaxies.

The emission of radio galaxies can be used to determine the intergalactic magnetic field carried into space by the matter wind. This requires assumptions about the electron density, i.e. the density of the surrounding medium.

Appearance

The radio emission of many radio galaxies comes from two mostly symmetrical emission regions ( radiolobes ), which are much larger than the visible galaxy with dimensions of up to several Mpc . Sometimes thin matter beams (engl. Jets ) recognizable connecting these regions with the galaxies core. The energy source of radio galaxies is a supermassive black hole in the galaxy's core, which on the one hand absorbs matter, but also accelerates ionized matter in its vicinity to speeds close to the speed of light, amplifies magnetic fields and emits matter beams. Both in the matter beams and in the areas where they hit the intergalactic medium, the observed radio radiation is synchrotron radiation .

In the mostly English-language scientific literature there is a diverse classification of radio galaxies into different types. The best known is the classification established by Bernie Fanaroff and Julia Riley in 1974 based on the ratio of the brightness of the radio lobes and the jet, now called FRI (class I), and FRII (class II) in honor of these.

Terminology

The term “radio galaxy” is sometimes misinterpreted because other galaxies - especially starburst galaxies  - can have detectable radio emissions, but are not referred to as radio galaxies.

literature

  • Andrzej G. Pacholczyk: Radio galaxies. Pergamon Press, Oxford 1977, ISBN 0-08-021031-7 .
  • Philip E. Hardee: Energy transport in radio galaxies and quasars. Astronomical Society of the Pacific, San Francisco 1996, ISBN 1-886733-21-X .
  • HJA Röttgering: The most distant radio galaxies. North-Holland, Amsterdam 1999, ISBN 90-6984-238-6 .
  • DS De Young: The Physics of Extragalactic Radio Sources. University of Chicago Press, Chicago 2002, ISBN 978-0226144153 .
  • Jeffrey Benett et al: Astronomy: The Cosmic Perspective. Springer, Munich 2010, ISBN 978-3-8273-7360-1 .

Individual evidence

  1. ↑ B. ~ R McNamara, P. ~ E. ~ J. Nulsen: Mechanical feedback from active galactic nuclei in galaxies, groups and clusters . In: New Journal of Physics . 14, No. 5, 2012, p. 055023. bibcode : 2012NJPh ... 14e5023M . doi : 10.1088 / 1367-2630 / 14/5/055023 .
  2. ↑ , Croston, et al .: The cluster environments of radio-loud AGN . In: IAU Symposium., Pp. 299-300. doi : 10.1017 / S1743921315002367

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