Published online by Cambridge University Press: 04 August 2010
AGN emit energy across the electromagnetic spectrum from radio waves to gamma rays. Observations in any single waveband give a very incomplete view of the relevant physical processes; higher energies generally probe smaller scales near the nucleus. AGN with compact radio cores show nonthermal emission from relativistic particles at many wavelengths. Many observed properties are altered by beamed emission from a relativistic jet. Low luminosity AGN without strong radio cores show thermal emission from cool dust in the far infrared, and from hot gas in the ultraviolet. Anisotropic obscuration leads to observed properties which are a function of orientation. A new window on AGN has been opened with the detection of high energy gamma rays.
The greatest challenge in research on Active Galactic Nuclei (AGN) is to understand the physical mechanisms behind the prodigious energy output of these distant sources. Progress has been slow because the continuum emission extends over at least eighteen decades in frequency (from 108 to 1026 Hz). Unfortunately, we operate our narrow bandwidth detectors in a broad bandwidth universe. Until the past decade, most of our information had come from two limited windows at radio and optical frequencies. Space missions and new detector technologies have opened up a variety of new wavebands, for example in the millimetre, far infrared, X-ray and γ-ray parts of the spectrum.