What are active galaxies?

Active galaxies are galaxies with centres that emit more energy than can be explained by their stars alone. This excess radiation comes from an active galactic nucleus (AGN), a compact region in the core of the galaxy powered by a supermassive black hole containing between millions and billions of times the mass of the Sun. This supermassive black hole consumes gas and dust from the surrounding galaxy, forming a superheated accretion disk of tortured material. Friction and compression in the disk heat the material to millions of degrees, causing them to emit radiate across the electromagnetic spectrum, from radio waves to gamma rays. The luminosity of an AGN can outshine the host galaxy by many times, with the brightest examples, known as quasars reaching outputs equivalent of trillions of Suns, from a region no larger than the solar system. 

The appearance of active galaxies varies depends on the accretion rate, the presence of relativistic jets and the viewing angle. In many cases, strong magnetic fields channel some of the infalling material into narrow jets of relativistic particles perpendicular to the accretion disk. These jets can extend for hundreds of thousands of lightyears from the host galaxy. These jets also produce intense radio emission through synchrotron radiation. Differences in orientation are responsible for distinct subclasses, blazars have jets pointed straight at the Earth, Seyfert galaxies are typically closer, with broad emission lines, with the central region outshining the rest of the galaxy. There also radio galaxies that display extended lobes, caused by the jets interacting with the intergalactic medium. 

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The Unified Model proposes that most AGN types share the same core structure, a supermassive black hole, an accretion disk, an obscuring torus of gas and dust, and jets that may or may not be present. The observed differences are merely because of the orientation. Active galaxies were more common in the early universe, peaking between 10 and 11 billion years ago when galaxy interactions supplied an abundance of gas. Today, only between five and ten per cent of galaxies show significant activity as gas supplies have diminished. Quasars, that were common in the early universe are rare in the local universe, although, Seyfert galaxies demonstrate lower-level activity. The central black holes cooevolve with their host galaxies, with feedback from the AGNs influencing the rate of star formation.