NASA's Hubble Space Telescope has revealed an unusual and fascinating new optical jet in the nucleus of the elliptical galaxy NGC 3862.
"It appears that we are seeing a new class of phenomenon," says Dr. Philippe Crane, of the European Southern Observatory in Garching, Germany, who discovered the jet in images sent back by the European Space Agency's Faint Object Camera on board HST. "The jet-like feature was totally unexpected in this galaxy. This is typical of the kinds of discoveries that were expected from HST."
Crane and ESA's Faint Object Camera Instrument Definition Team made the observations.
NGC 3862, also known as 3C264, is a bright radio source and x-ray source. It is the sixth brightest galaxy in a rich cluster of galaxies known as Abell 1367, located at a distance of about 260 million light-years away in the constellation Leo.
Previous observations of NGC 3862 taken in radio wavelengths have revealed a jet-like structure which extends for a very long distance. Because the jet is only 0.6 arc seconds long (equivalent to the apparent width of a dime located ten miles away) it would have been very difficult to see from a ground-based observatory," says Crane. "The jet is also prominent in ultraviolet light. Both of these characteristics are especially well exploited by the Faint Object Camera."
NGC 3862 was observed with the FOC in high resolution (f196) mode on January 25, 1992 with two filters. One exposure, taken in yellow-green light, was expected to reveal, outside the nucleus, the distribution of the normal old stellar population in the galaxy. The second exposure, taken in the near ultraviolet light, was intended to reveal evidence for a young hot stellar population in the nucleus. To their surprise, the researchers found the optical jet.
Extragalactic jets are not well understood. They appear to transport energy in a confined beam out from the active nucleus of the host galaxy. Presumably super massive black holes are the powerhouses behind jets.
Extragalactic jets have been detected in radio wavelengths in many active galaxies, but only a few have been seen in optical light. Astronomers do not yet understand why some jets are seen in visible light and others are not. They also would like to understand the connection between radio and optical emissions.
The NGC 3862 jet does not easily fit into the standard model of jets. The new jet is markedly different from the optical jet seen in M87 (a galaxy previously studied in detail by HST) in several ways. The NGC 3862 jet is about 750 light-years long, compared to a length of 5,000 light-years for the M87 jet. The jet in M87 is brighter at redder wavelengths because it emits reddish synchrotron radiation (so-named because this form of light was first detected in laboratory particle accelerators) produced by high-speed electrons spiraling in the magnetic field which confines the jet. The NGC 3862 jet, however, is much brighter in ultraviolet light (UV), relative to visual light.
"Further observations will be needed to clarify the nature of the emission seen in this jet," says Crane. This new type of jet suggests that astronomers are seeing a new and unexpected phenomena in galactic nuclei.