Radio Galaxies (2) ...
Optical jets from radio galaxies are much harder to find than radio jets.
The one from M87 has been known for
some time (it was first observed optically in 1918),
but more recently faint optical jets in other radio galaxies have been found. The adjacent image shows
such optical jets from four radio galaxies.
In some cases
the jets in radio galaxies have been studied
also at other wavelengths such as X-ray and infrared.
Lobed Radio Galaxies
In contrast to core-halo radio galaxies like M87 where the radio emission is
confined to a region comparable in size with the optical galaxy, lobed radio galaxies can be
enormous compared with the corresponding optical galaxy.
A montage of radio lobes associated with some active galaxies is shown in the following
figure,
and we have seen two previous examples of this class, 3C219 and NGC 4261.
Such lobes often span a region ten times the size of the optical galaxy (millions of light
years or a significant fraction of a megaparsec) .
The largest known radio lobes are for the galaxy 3C236. Its dual radio lobes
span a distance of about 3 Mpc, which is larger than the average separation between galaxies in
a group (for example, this is about 4 times larger than the distance between the Milky Way and
the Andromeda Galaxy in our Local Group).
Although radio lobes are spectacular at RF wavelengths, they generally are invisible at optical
wavelengths.
Thus the true nature of a radio galaxy is largely hidden if we are restricted to
observing at visible wavelengths. The typical radio luminosity of the radio frequency emission
from radio galaxies ranges from about 10 percent of the total luminosity at all wavelengths of
a normal galaxy like the Milky Way up to 1000 times the total
Milky Way luminosity. Large radio
galaxies are truly spectacular sources of energy, but one has to look in the right portion of
the spectrum to realize this.
