Probing some of the most distant and energetic galaxies in the universe, NASA's Hubble Space Telescope has uncovered surprisingly varied and intricate structures of stars and gas that suggest the processes powering these so-called radio galaxies are more complex than previously thought.
The Hubble observations, made by a team of astronomers at Cambridge University, England, should shed light on the nature of active galaxies, that might be powered by immense black holes at their cores, and more generally, on galaxy evolution. The radio galaxies observed are so far away they existed when the universe was half its present age, and the light is only now reaching us.
The bizarre, never-before-seen details may be a combination of light from massive star forming regions, small satellite dwarf galaxies, and bow shocks caused by jets of hot gas blasted out of the galaxy's core by a suspected black hole.
The observations were made by Professor Malcolm Longair and Philip Best of the Cavendish Laboratory, Cambridge University, and Huub Rottgering of Leiden Observatory, The Netherlands, who have published images of three radio galaxies (3C368, 3C324 and 3C265) in the August 1, 1995 issue of the Monthly Notices of the Royal Astronomical Society.
The team is analyzing a sample of 28 radio galaxies that have been imaged by Hubble in visible light, by the Very Large Array Radio Interferometer at radio wavelengths, and by the United Kingdom Infrared Telescope.
A radio galaxy emits powerful radio waves along two opposite directions pointing out from the galaxy's core. The radio lobes usually extend far beyond the host galaxy. The suspected powerhouse behind the radio emission is a one-billion solar mass black hole in a galaxy's core. Gaseous jets, traveling at nearly the speed of light, blast out along the rotation axis of the spinning black hole. These jets bore through space like a narrow stream of water from a garden hose nozzle plowing through sand. When they are finally stopped by the intergalactic medium, a huge amount of energy is released in the form of radio waves.
Previous ground-based observations since 1987 have shown that, in visible light, distant radio galaxies have an unusual elongated structure - unlike the classic spiral and elliptical shapes in normal galaxies - that align to the twin lobes of radio emissions that are the trademark of such active galaxies. In the Hubble views, these shapes break up into a string of bright knots that might be regions where new stars are forming, or could be glowing clouds of gas. In one galaxy, the knots align to the axis of the jet, while in another case they do not, and instead cluster around the galaxy like smaller "satellite" galaxies.
One explanation for the alignment between the invisible jets and optical structures is that the jets trigger the formation of stars along their paths. However, some of the galaxies emit highly polarized light. Since this type of light is not produced by stars, other processes must be at work. A possible explanation is that the light from the galaxy's hidden active nucleus is scattered in our direction by dust or electrons.
Longair, Best and Rottgering propose that the remarkable structures seen in the Hubble images are different manifestations of activity associated with radio galaxies. They conclude that at least two mechanisms must be responsible for the alignment effect, with both scattering of nuclear light and star-formation playing a role. They also note that the period during which there is strong radio emission is quite short relative to the total lifetime of a galaxy, so different processes may dominate as the radio source ages. They are planning further observations to determine the relative importance of the different effects.
