Several galaxies emit a radio power more than one hundred times larger
than the others, and for this reason they are called "radiogalaxies". The
first radiogalaxy, Cygnus A, was discovered in the thirties in the Cygnus
constellation: within this spectral band, it radiates one million times
more than our Galaxy.
The development first of radiotelescopes,
and then of radiointerferometers,
allowed to identify other strong radio sources, among which M82, M87, Centaurus
A.
The study of the radio emission showed that, in many radio galaxies,
it comes from two extended regions, called
radiolobes, that is two
jets of gas, that are several light years long, and symmetric with respect
to the galaxy center. Their extension can be an enormous one, up to 15
million light years, such as in the 3C 236 radiogalaxy.
The emitting region of a radiogalaxy is a few hundred kiloparsec
wide, on average. The power emitted is not thermal, i.e. it is not due
to stellar emission. It is produced by the motion of relativistic electrons
(that is, moving at speeds close to that of light) in a very strong magnetic
field. These electrons emit a so-called "synchrotron" radiation, when they
are decelerated in the magnetic field.
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These radiogalaxy images (HST) have been combined with radio maps
(blue lines) obtained with a very large baseline radiointerferometer.
The complicated structures of gas and stars lead to think that the mechanism
which feeds radiogalaxies is more complex than previously considered. These
structures are caused both by the formation of new stars, and by shock
waves generated by hot jets of gas coming from the center of these galaxies
(likely due to a central black hole). Left, the galaxy 3C265. The line
corresponds to the axis of the radio emission, which is not coincident
with that of the optical emission.
At the center, 3C324. The central regions of the galaxy are obscured by a large band of dust. Right, 3C368, one of the better known radiogalaxies. The bright regions can be stars or dust. As for other radiogalaxies the gas jets, emitted by a likely central black hole, could have induced the formation of new stars along the direction of the jets. (HST) |
It is thought that in the nucleus of radiogalaxies there exists a mechanism
able to expel jets of matter and to produce strong magnetic fields. It
could be likely a black hole, just as for other active galaxies. These
jets of gas, after traveling thousands of light years, interact with intergalactic
medium and slow down, thus forming the characteristic lobe structures.
High resolution investigations revealed also a compact radio source,
coincident with the galactic nucleus.
Besides extended radiogalaxies, there are also compact radio sources,
where the radio emission is constrained in the galactic nucleus. The size
of the region is about one light year. These are also the dimensions of
the central engine of an active nucleus. The optical counterparts of this
kind of radio sources are often quasars or Lacaertides.
For the extended radiosources, the age of the radiolobes can be determined
from the estimated emission speed of the gas from the nucleus, and taking
into account the lobes' size. It is usually comprised between 1 and 100
million years. This is also the time interval during which the central
engine was active and produced the lobes. The emission of a radiogalaxy
does not last for a long time, unless the electrons are continuously replaced.
It is then thought that the radiogalaxy phase is just a fraction of a galaxy's
lifetime.
