Active Galaxies Active galaxies (also known as Active Galactic Nuclei, AGNs for short) are defined as those galaxies which are observed to have either: (1) An exceptionally bright nuclear region whose spectra indicates a non- stellar origin for the intense radiation being emitted (2) Radio-emitting jets or blobs of gas, observed extending from the nuclear region out to thousands of light years. The energy output associated with active galaxies strongly implies that an extremely powerful “engine” must exist at the very centers of some galaxies. Active galaxies were initially discovered in the 1950s, by optical astronomers who noticed the unusual bright cores in a small number of galaxies, and by radio astronomers, who discovered immensely powerful radio sources which appeared to be located in very distant galaxies. Radio galaxies appear to be ellipticals with kiloparsec-length “jets” of plasma, ejected at near light-speed from the central engine. These radio jets are mapped using radio telescope interferometers, such as the VLA. Seyfert galaxies appear to be otherwise normal spirals with active nuclei. Quasars are the most luminous AGNs, shining with the power of 100s of our Galaxy. Quasars are so bright, that the host galaxy is usually extremely difficult to detect, lost in the glare of the quasar. The spectra of active galaxies are unlike those of normal galaxies. Astronomers do not see the absorption lines typical for stars, instead strong emission lines from ionized gas are common. Interpretation of these lines shows the gas clouds within the nuclear region are moving at thousands of kilometers per second. Origin of “Activity” The source of these energetic phenomena is believed to be a supermassive black hole at the center of the active galaxy. These monster black holes are millions to a few hundred million times more massive than the Sun. Power is generated by accretion of matter onto the black hole. Gas swirls around within an accretion disk, then is absorbed by the hole. Violent tangling of magnetic fields help propel jets of plasma out along the rotation axis of the accretion disk surrounding the black hole. The observed properties of all of the different types of AGNs appears to be explained by the power level of the central “engine” and the orientation of the active galaxy to our line-of-sight. Due to their extreme luminosity, active galaxies can be observed at enormous distances. They are excellent probes of the large scale structure of the Universe. The light from AGNs acts as a kind of lighthouse, illuminating the gas clouds along the line-of-sight to Earth. Only a small fraction of galaxies are “active”, perhaps 10 -15%. However, most galaxies may harbor a central black hole, which is “turned-off” most of the time. Supermassive black holes must have first formed early in cosmic time, as we observe some quasars only 1-2 billion years after the Big Bang.