Seyfert << `say` fuhrt >> galaxy is a type of galaxy with an extremely bright nucleus, or center. Seyfert galaxies are a type of active galactic nucleus (AGN), a class of astronomical objects that also includes blazars and quasars . Astronomers believe that all such objects are powered by a supermassive black hole at their core. A black hole is an object with a gravitational pull so strong that nothing—not even light—can escape it.
Characteristics.
Astronomers classify Seyfert galaxies into two main types based on differences in the electromagnetic spectrum that they emit. The electromagnetic spectrum is the continuous range of electromagnetic waves, including visible light, given off by an object, arranged in order of wavelength. Wavelength is the distance between successive wave crests. Type I Seyfert galaxies possess broad emission lines in the range of visible light. An emission line is a bright line visible at a particular location on the spectrum. It represents powerful electromagnetic energy given off at specific wavelengths. Emission lines correspond to the presence of certain chemical elements abundant in the object. Type II Seyfert galaxies produce narrow emission lines. In addition, the electromagnetic waves emitted by some Type II Seyfert galaxies are polarized—that is, the electromagnetic radiation vibrates in a simple, regular pattern. Most astronomers think that a torus (thick disk) of dust orbiting the supermassive black hole polarizes the light. When the galaxy is viewed from the side, gas and dust scatter the light emitted by the AGN. This scattering causes the observed polarization.
Scientists determine the distance of a galaxy by measuring its redshift . The redshift is a stretching of the wavelengths of light or other electromagnetic waves given off by an astronomical object as it moves away from Earth. The more distant an object is, the greater its redshift. Seyfert galaxies are not as bright as quasars or blazars. They also typically display smaller redshifts, meaning they are generally closer to Earth than are other AGN’s. The light from one of the closest confirmed Seyfert galaxies traveled 13 million light-years to reach Earth. A light-year is the distance light travels in a year, about 5.88 trillion miles (9.46 trillion kilometers). Because light takes time to travel, the relative closeness of Seyferts means that they formed more recently than quasars and blazars. Many astronomers believe Seyfert galaxies may represent an early, less active, stage of AGN development.
Like other AGN’s, a Seyfert galaxy can vary significantly in apparent magnitude (brightness as seen from Earth) over the course of years, hours, or even minutes. Many astronomers have proposed that the different types of AGN’s may not actually be different types of objects. Rather, they believe the observed differences among AGN types may simply be due to variations in the angle and distance at which they can be seen from Earth. For example, a blazar may be an AGN emitting light directly toward Earth, whereas a quasar may be an AGN where the light emitted is observed from Earth at an angle.
History.
Seyfert galaxies were first identified as a unique class of celestial objects in 1908 by the American astronomer Edward Fath. He was observing the electromagnetic spectra of objects called “spiral nebulae.” Astronomers thought at that time that these objects were clouds of gas and dust within our own galaxy, the Milky Way . Fath noticed that a particular object, now known as NGC 1068, showed six bright emission lines. Most stars and nebulae in the Milky Way have spectra that are dominated by dark gaps, called absorption lines, in their spectra.
In 1926, the American astronomer Edwin Hubble studied the emission lines of NGC 1068 and two other such spiral nebulae. Based on their redshifts, Hubble determined that the objects were moving away from Earth far too quickly for them to be located in the Milky Way. Instead, he proposed that these objects were galaxies themselves. Until that point, most astronomers thought that all stars and other celestial objects were part of our own galaxy.
The American astronomer Carl K. Seyfert discovered more galaxies similar to NGC 1068 in 1943. Astronomers named the objects Seyfert galaxies in his honor. They remained poorly studied for 20 years, however, until the discovery of quasars in the 1960’s stimulated astronomers’ interest in Seyfert galaxies.
