The Radio Galaxy PKS 0521-36

NASA's Hubble Space Telescope has obtained the most detailed and highest resolution optical images of the radio galaxy PKS 0521-36. The observations were made with the European Space Agency's Faint Object Camera.

The galaxy PKS 0521-36, which is at a distance of about one billion light years from us, is known to be a source of powerful radio emission. It has a quasar-like nucleus that is very bright, both at optical and radio wavelengths. This galaxy belongs to a small group of radio sources which have "jets" that have been seen at both radio and at optical wavelengths. These jets show that high energy material is streaming out from the nuclei at high velocity, often close to the speed of light.

Although the existence of optical jets in galaxies has been known for a long time – the prototype, the jet of the galaxy M 87 was first observed in 1918 – very few other examples of optical jets have been found. The discovery in the 1970's that radio jets are widespread nevertheless established the fact that jets play a fundamental role in understanding the nature of active galaxies. Astronomers believe that they provide the route for energy to be carried from the powerful nucleus out to very large distances from the galaxy.

PKS 0521-36 is one of only a handful of radio galaxies that are known to possess jets visible in both the radio and optical portions of the spectrum.

The emission is understood to be due to so-called "synchrotron radiation" which is produced by fast electrons moving in a magnetic field. The beam is produced in a very powerful source located at the center of the galaxy – possibly a black- hole. Exactly how this occurs is not yet known. Optical emission in the jet, or beam, is very localized, and arises only in parts of the beam where electrons are being accelerated, perhaps due to instabilities at the edges of the flow. For this reason, by studying the optical structure of the jet, astronomers hope to learn much about how it works.

The Faint Object Camera image shows the galaxy with the jet protruding outside it. In ground-based images, the whole optical jet is embedded within the galaxy and to make it evident, the star light from the galaxy has to be removed or subtracted by computer processing. A similar procedure with the Faint Object Camera data shows the jet in great detail all the way into the nucleus of the galaxy. The jet is about 30,000 light years in length. This data also shows "hot spots", which are areas of enhanced emission – probably where acceleration of electrons is actually occuring.

Taken together, the radio and optical data will help astronomers to determine the nature of the energy source and the acceleration mechanism for these jets. Is the central "monster" responsible for the enormous amount of energy observed in these jets a black-hole or some other complex mechanism? Does the acceleration occur continuously along the jet or in a small number of acceleration sites? How does the jet stay so well-defined and narrow over such a very large distance?

Astronomers hope that by using the Faint Object Camera to study PKS 0521-36 and other objects like it, they can untangle the complex processes giving rise to the "jet" phenomenon.