Seyfert galaxy
Seyfert galaxies are one of the two largest groups of
Seyfert galaxies account for about 10% of all galaxies
Seen in
Seyfert galaxies are named after
Discovery
Seyfert galaxies were first detected in 1908 by
In 1926,
In the 1960s and 1970s, research to further understand the properties of Seyfert galaxies was carried out. 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.
Characteristics
An
Eddington luminosity
A lower limit to the mass of the central black hole can be calculated using the Eddington luminosity.[27] This limit arises because light exhibits radiation pressure. Assume that a black hole is surrounded by a disc of luminous gas.[28] Both the attractive gravitational force acting on electron-ion pairs in the disc and the repulsive force exerted by radiation pressure follow an inverse-square law. If the gravitational force exerted by the black hole is less than the repulsive force due to radiation pressure, the disc will be blown away by radiation pressure.[29][note 1]
Emissions
The emission lines seen on the spectrum of a Seyfert galaxy may come from the surface of the accretion disc itself, or may come from clouds of gas illuminated by the central engine in an ionization cone. The exact geometry of the emitting region is difficult to determine due to poor resolution of the galactic center. However, each part of the accretion disc has a different velocity relative to our line of sight, and the faster the gas is rotating around the black hole, the broader the emission line will be. Similarly, an illuminated disc wind also has a position-dependent velocity.[30]
The narrow lines are believed to originate from the outer part of the active galactic nucleus, where velocities are lower, while the broad lines originate closer to the black hole. This is confirmed by the fact that the narrow lines do not vary detectably, which implies that the emitting region is large, contrary to the broad lines which can vary on relatively short timescales. Reverberation mapping is a technique which uses this variability to try to determine the location and morphology of the emitting region. This technique measures the structure and kinematics of the broad line emitting region by observing the changes in the emitted lines as a response to changes in the continuum. The use of reverberation mapping requires the assumption that the continuum originates in a single central source.[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.[32]
In the few radio-loud Seyfert galaxies that have been observed, the radio emission is believed to represent synchrotron emission from the jet. The infrared emission is due to radiation in other bands being reprocessed by dust near the nucleus. The highest energy photons are believed to be created by inverse Compton scattering by a high temperature corona near the black hole.[33]
Classification
Seyferts were first classified as Type I or II, depending on the emission lines shown by their spectra. The spectra of Type I Seyfert galaxies show broad lines that include both allowed lines, like H I, He I or He II and narrower forbidden lines, like O III. They show some narrower allowed lines as well, but even these narrow lines are much broader than the lines shown by normal galaxies. However, the spectra of Type II Seyfert galaxies show only narrow lines, both permitted and forbidden. Forbidden lines are spectral lines that occur due to electron transitions not normally allowed by the selection rules of quantum mechanics, but that still have a small probability of spontaneously occurring. The term "forbidden" is slightly misleading, as the electron transitions causing them are not forbidden but highly improbable.[35]
In some cases, the spectra show both broad and narrow permitted lines, which is why they are classified as an intermediate type between Type I and Type II, such as Type 1.5 Seyfert. The spectra of some of these galaxies have changed from Type 1.5 to Type II in a matter of a few years. However, the characteristic broad
Type I Seyfert galaxies
Type I Seyferts are very bright sources of ultraviolet light and X-rays in addition to the visible light coming from their cores. They have two sets of emission lines on their spectra: narrow lines with widths (measured in velocity units) of several hundred km/s, and broad lines with widths up to 104 km/s.[41] The broad lines originate above the accretion disc of the supermassive black hole thought to power the galaxy, while the narrow lines occur beyond the broad line region of the accretion disc. Both emissions are caused by heavily ionised gas. The broad line emission arises in a region 0.1–1 parsec across. The broad line emission region, RBLR, can be estimated from the time delay corresponding to the time taken by light to travel from the continuum source to the line-emitting gas.[24]
Type II Seyfert galaxies
Type II Seyfert galaxies have the characteristic bright core, as well as appearing bright when viewed at
Type 1.2, 1.5, 1.8 and 1.9 Seyfert galaxies
In 1981,
Other Seyfert-like galaxies
In addition to the Seyfert progression from Type I to Type II (including Type 1.2 to Type 1.9), there are other types of galaxies that are very similar to Seyferts or that can be considered as subclasses of them. Very similar to Seyferts are the low-ionisation narrow-line emission radio galaxies (LINER), discovered in 1980. These galaxies have strong emission lines from weakly ionised or neutral atoms, while the emission lines from strongly ionised atoms are relatively weak by comparison. LINERs share a large amount of traits with low luminosity Seyferts. In fact, when seen in visible light, the global characteristics of their host galaxies are indistinguishable. Also, they both show a broad line emission region, but the line emitting region in LINERs has a lower density than in Seyferts.[48] An example of such a galaxy is M104 in the Virgo constellation, also known as the Sombrero Galaxy.[49] A galaxy that is both a LINER and a Type I Seyfert is NGC 7213, a galaxy that is relatively close compared to other AGNs.[50] Another very interesting subclass are the narrow line Type I galaxies (NLSy1), which have been subject to extensive research in recent years.[51] They have much narrower lines than the broad lines from classic Type I galaxies, steep hard and soft X-ray spectra and strong Fe[II] emission.[52] Their properties suggest that NLSy1 galaxies are young AGNs with high accretion rates, suggesting a relatively small but growing central black hole mass.[53] There are theories suggesting that NLSy1s are galaxies in an early stage of evolution, and links between them and ultraluminous infrared galaxies or Type II galaxies have been proposed.[54]
Evolution
The majority of active galaxies are very distant and show large
Examples
Here are some examples of Seyfert galaxies:
- Circinus Galaxy, which has rings of gas ejected from its center
- relativistic jetspanning more than a million light years in length
- Cygnus A, the first identified radio galaxy and the brightest radio source in the sky as seen in frequencies above 1 GHz
- Messier 51a (NGC 5194), the Whirlpool Galaxy, one of the best known galaxies in the sky[59]
- Messier 64(NGC 4826), with two counter-rotating disks that are approximately equal in mass
- Messier 66 (NGC 3627), a part of the Leo Triplet
- Messier 77 (NGC 1068), one of the first Seyfert galaxies classified[60]
- Messier 81 (NGC 3031), the second brightest Seyfert galaxy in the sky after Centaurus A
- Messier 88 (NGC 4501), a member of the large Virgo Cluster and one of the brightest Seyfert galaxies in the sky
- Messier 106 (NGC 4258), one of the best known Seyfert galaxies,[61][62] which has a water vapor megamaser in its nucleus seen by 22-GHz line of ortho-H2O[63]
- NGC 262, an example of a galaxy with an extended gaseous H I halo[64]
- NGC 1097, which has four narrow optical jets coming out from its nucleus
- NGC 1275, whose central black hole produces the lowest B-flat note ever recorded[65]
- NGC 1365, notable for its central black hole spinning almost the speed of light[66]
- NGC 1566, one of the first Seyfert galaxies classified[60]
- NGC 1672, which has a nucleus engulfed by intense starburst regions
- NGC 1808, also a starburst galaxy
- NGC 3079, which has a giant bubble of hot gas coming out from its center
- NGC 3185, member of the Hickson 44 group
- NGC 3259, also a strong source of X-rays
- NGC 3783, also a strong source of X-rays
- NGC 3982, also a starburst galaxy
- NGC 4151, which has two supermassive black holes in its center
- NGC 4395, an example of a low surface brightness galaxy with an intermediate-mass black hole in its center
- NGC 4725, one of the closest and brightest Seyfert galaxies to Earth; it has a very long spiraling cloud of gas surrounding its center seen in infrared
- NGC 4945, a galaxy relatively close to Centaurus A
- NGC 5033, has a Seyfert nucleus displaced from its kinematic center
- NGC 5548, an example of a lenticular Seyfert galaxy
- ultraluminous infrared galaxy(ULIRG)
- NGC 6251, the X-ray brightest low-excitation radio galaxy in the 3CRR catalog[67]
- NGC 6264, a Seyfert II with an associated AGN
- NGC 7479, a spiral galaxy with radio arms opening in a direction opposite to the optical arms
- NGC 7742, an unbarred spiral galaxy; also known as the Fried Egg Galaxy
- IC 2560, a spiral galaxy with a nucleus similar to NGC 1097
- SDSS J1430+2303, a Seyfert I, predicted to host a supermassive black hole binary very close to the point of merger
-
Seyfert galaxy Messier 51
-
Seyfert galaxy Messier 88
-
Seyfert galaxy Centaurus A
See also
- Low-ionization nuclear emission-line region – Type of galactic nucleus
Notes
- ^
The gravitational force Fgrav of the black hole can be calculated using:
where G is theproton massand MBH, r are the mass and radius of the black hole respectively.
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.The luminosity of the black hole must be less than the Eddington luminosity LEdd, i.e.,
which is given when:where M☉ is the mass of the Sun and L☉ is the solar luminosity.Therefore, given the observed luminosity (which would be less than the Eddington luminosity), an approximate lower limit for the mass of the central black hole at the center of an active galaxy can be estimated. 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.
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External links
- Active Galaxies and Quasars at NASA.gov
- Seyfert Galaxies at SEDS.org
- Seyfert Galaxies at ESA.int