With ingenious devices scanning the heavens in spectrums beyond the visible, many marvels have been revealed in recent years, and some of the most awesome are the aptly named active galaxies.
Normal galaxies make up about 90% of all galaxies in the universe., but are a class of galaxies that are referred to as active galaxies. There are two main differences between active galaxies and normal galaxies:
- Total Luminosity – An active galaxy has a larger total luminosity (is the total amount of energy emitted by an astronomical object per unit time) than a normal galaxy of the same type.
- Spectrum – The luminosity from a normal galaxy is basically equal to the sum of the luminosities of its stars. However, the spectrum of an active galaxy shows that it gives off a lot of nonsteller radiation that is coming from a source other than the stars in the galaxy.
Radio galaxies spew jets of matter many light-years long from a relatively small nucleus called an AGN, an active galactic nucleus, brightly lit in X-ray and radio frequencies even though they may be optically dim. These radio galaxies show nuclear and extended radio emissions. They are usually elliptical and show two lobes of radio frequency emission that are aligned with the jets observed in the visible spectrum and may extend for millions of light years. These galaxies can live for a few hundred thousand years or up to a few million.
Seyfert galaxies are spinning discs with intensely active highly ionized nuclei. They were the earliest distinct class of AGN to be identified. They show optical range nuclear continuum emission, narrow and occasionally broad emission lines. They also occasionally show strong nuclear X-Ray emission including a weak small scale radio jet.
Elliptical N-galaxies have nuclei that vary in intensity, and a subspecies, known as BL Lacertae (BL Lac) objects, they are also called “Blazers” because they have active nuclei that are even more wildly fluctuating. This is probably as a result of being head-on into a jet.
Quasars (quasi-stellar radio sources) are a hundred to a thousand times brighter than normal galaxies and emit vast quantities of radiation. It is thought that they are the bright hearts of active galaxies, powered by supermassive black holes. With very high redshifts suggesting a very great distance, quasars are thought to be young galaxies from the earlier ages of our universe.
However, there are puzzles. Twin quasars have been observed on each side of younger, lower redshift active galaxies, suggesting that they might be connected, hence at the same distance. This raises the possibility that redshifts are not always caused by recession speed and so they might not be such a reliable measures of distance, although the effect could be caused by gravitational lensing. Additionally, some jets have extreme redshifts, appearing to be moving faster than light, breaking the cosmic speed limit. These seemingly super luminal emissions may be due to serious space-time distortions in the high energy zones dominated by the huge black holes from which they have burst.