The Giant Elliptical Galaxy M87 (Active Galaxies And Radio Galaxies)
M87 is the biggest and brightest galaxy in the Virgo cluster. It is the nearest giant elliptical (at a distance of 15Mpc) in which there is clear evidence of activity, both within the nucleus and well out¬side it. The first direct evidence came with the discovery by H.D.Curtis in 1918 of a remarkable optical jet in the galaxy . It was found much later that M87 is also a powerful radio source and that a considerable fraction of the total radio emission comes from the jet. It is frequently referred to as Virgo A and 30 274, the names given to it by radio astronomers. The radio emission is clearly synchrotron radiation, and this emission is now known to be partially polarized. Optical studies have shown that the light from the jet is also probably synchrotron radiation.
Synchrotron radiation in caused when electrons move across magnetic field lines. If the electrons are extremely energetic (1011 or 1012eV) they radiate at optical wavelengths. However at these wavelengths the electrons lose their energy relatively quickly (within a few hundred years in the jet of M87). This jet is probably about 2000 pc long, so particles travelling with the speed of light will take around 6500 years to travel the whole length of the jet. If all the energy that gives rise to the jet was generated in the nucleus of M87, no particles could have enough energy left to radiate optically near the end of the jet. This shows us that the electrons in the jet must actually be given the energy which they eventually radiate while they are moving along the jet. How this might happen is difficult to guess, and astronomers are a long way from understanding how the jet of M87 is energized.
The nucleus of M87 is itself remarkable. It contains a very compact non-thermal high-frequency radio source which is less than three light-months & across. Within 300 pc of the centre of M87 are found clouds with velocities which may be as high as 900 kms-1. In addition to the radio structures associated with the jet (and those on the opposite side of the nucleus of M87 where a fainter jet has been reported), there is a low-brightness halo of radio emission.