Seyfert galaxy
These galaxies have supermassive black holes at their centers which are surrounded by accretion discs of in-falling material.
[6] Seyfert galaxies were first detected in 1908 by Edward A. Fath and Vesto Slipher, who were using the Lick Observatory to look at the spectra of astronomical objects that were thought to be "spiral nebulae".
[7] In 1926, Edwin Hubble looked at the emission lines of NGC 1068 and two other such "nebulae" and classified them as extragalactic objects.
By the end of the 1950s, more important characteristics of Seyfert galaxies were discovered, including the fact that their nuclei are extremely compact (< 100 pc, i.e. "unresolved"), have high mass (≈109±1 solar masses), and the duration of peak nuclear emissions is relatively short (> 108 years).
A few direct measurements of the actual sizes of Seyfert nuclei were taken, and it was established that the emission lines in NGC 1068 were produced in a region over a thousand light years in diameter.
[15] Confirming estimates of the distance to Seyfert galaxies and their age were limited since their nuclei vary in brightness over a time scale of a few years; therefore arguments involving distance to such galaxies and the constant speed of light cannot always be used to determine their age.
[15] In the same time period, research had been undertaken to survey, identify and catalogue galaxies, including Seyferts.
Beginning in 1967, Benjamin Markarian published lists containing a few hundred galaxies distinguished by their very strong ultraviolet emission, with measurements on the position of some of them being improved in 1973 by other researchers.
[18] During the same time period, efforts have been made to gather spectrophotometric data for Seyfert galaxies.
A simple division into types I and II has been devised, with the classes depending on the relative width of their emission lines.
[19] It has been later noticed that some Seyfert nuclei show intermediate properties, resulting in their being further subclassified into types 1.2, 1.5, 1.8 and 1.9 (see #Classification).
[20][21] Early surveys for Seyfert galaxies were biased in counting only the brightest representatives of this group.
[22] An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a higher than normal luminosity over portions of the electromagnetic spectrum.
Active galactic nuclei are the most luminous sources of electromagnetic radiation in the Universe, and their evolution puts constraints on cosmological models.
In a typical Seyfert galaxy, the nuclear source emits at visible wavelengths an amount of radiation comparable to that of the whole galaxy's constituent stars, while in a quasar, the nuclear source is brighter than the constituent stars by at least a factor of 100.
[24] Their visible and infrared spectra show very bright emission lines of hydrogen, helium, nitrogen, and oxygen.
These emission lines exhibit strong Doppler broadening, which implies velocities from 500 to 4,000 km/s (310 to 2,490 mi/s), and are believed to originate near an accretion disc surrounding the central black hole.
The exact geometry of the emitting region is difficult to determine due to poor resolution of the galactic center.
[31] For 35 AGN, reverberation mapping has been used to calculate the mass of the central black holes and the size of the broad line regions.
The highest energy photons are believed to be created by inverse Compton scattering by a high-temperature corona near the black hole.
However, the spectra of Type II Seyfert galaxies show only narrow lines, both permitted and forbidden.
There are a few cases where galaxies have been identified as Type II only because the broad components of the spectral lines have been very hard to detect.
Specifically, in Type I Seyfert galaxies, we observe the central compact source more or less directly, therefore sampling the high velocity clouds in the broad line emission region moving around the supermassive black hole thought to be at the center of the galaxy.
When the galaxy is viewed from the side, the nucleus is indirectly observed through reflection by gas and dust above and below the torus.
[50] Another very interesting subclass are the narrow-line Type I galaxies (NLSy1), which have been subject to extensive research in recent years.
This suggests that active galaxies occurred in the early Universe and, due to cosmic expansion, are receding away from the Milky Way at very high speeds.
Quasars are the furthest active galaxies, some of them being observed at distances 12 billion light years away.
[41] Here are some examples of Seyfert galaxies: We derive the outward radiative force Frad as we do for stars assuming spherical symmetry:
where p is momentum, t is time, c is the speed of light, E is energy, σt is the Thomson cross-section and L is luminosity.
This derivation is a widely used approximation, but when the actual geometry of accretion discs is taken into account, it is found that the results can differ considerably from the classical value.
