Astronomy
Active Galaxies
Weblecture
Quasars and Active Galaxies
In the 1990s, studies of "irregular, active" galaxies led astronomers to create several different classifications based on observations much like those we made with neutron stars.
Some objects appeared to be extremely distant, very high energy objects, with point light sources. These "starlike" objects where too bright to be individual stars, but because they had other stellar characteristics, they were classified as quasi-stellar objects, or quasars. Redshifts in their emission lines showed they were moving at very high speeds away from Earth and the Milky Way, and not part of our galaxy at all. One puzzling aspect of these objects is their low radiowave range emission levels.
Seyfert galaxies form another group of odd galaxies. These are spiral galaxies that have strong emission lines in their spectra, but relatively weak radio emissions, similar to quasars.
At the other end of the range, there are elliptical galaxies that have shifts like quasars but emit very strong radio signals. These radio galaxies often have ejection jets thousands of parsecs long. In some cases, we observe only one jet; in others, we can see pairs of jets.
Blazars form another group of objects similar to quasars in luminosity but lacking any absorption or emission lines that can be identified with particular elements. Such radiation is general the result of extreme magnetic field distortion of particle motion, resulting in synchrotron radiation emissions. Blazars have jets where materials appear to move with speeds greater than the speed of light, a phenomena called superluminal motion.
For several decades, astronomers tried to come up with models that would explain the development of such radically different galactic types: differences in formation, collisions after formation, disruptions from nearby galaxies were all proposed as possible mechanisms.
One of the characteristics all of these active objects share is variability. Luminosity can change radically over years to decades. Light variations depend on the size of the object and can be used to determine the diameter of the object emitting the changing luminosity levels. Rapid variations in luminosity for all of these objects led to the conclusion that they are all relatively small: the very active centers of galaxies. This important result helped astronomers realize that the objects were not actually as radically different as originally supposed, but that some of the apparent differences were the result of the orientation of the object to the Earth observers' line of sight. As with pulsating neutron stars, the same object has a different appearance depending on whether the rotational angle and jets point toward Earth, or at right angles to the plane of the object.
All active galactic nuclei appear to be "powered" by black holes. Dust and gas falling onto an accretion disk compress and heat up to temperatures twice as hot as the most energetic O and B stars, resulting in high luminosity and high-energy output. High pressures in the accretion disk also cause material to be ejected at right angles to the plane of the disk, forming material jets observed in active galaxies. The nucleus is usually surround by a doughnut of duss and gas that can hide the accretion disk. Whether we see a jet, a pair of jets, or no jets depends on the angle of the jets relative to the line of sight toward Earth. Whether we see the nucleus or the accretion disk also depends on the orientation of the object to Earth and the angle of the light-blocking dusty torus.
Another factor influencing our view of active galaxies is their age. The further away the galaxy, the longer it has taken light to reach us, and the earlier in time we see the galaxy. Nearby galaxies appear relatively stable, while further galaxies show evidence of active nuclei and the furthest galaxies are more likely to be quasars. This indicates that quasars are an early stage of galaxy formation, while active galactic nuclei are later, and more stable galactic energy output, like that of our own Milky Way, are even later stages in the galactic life cycle.
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