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| [[File:circinus.galaxy.750pix.jpg|thumb|right|The [[Circinus Galaxy]], a Type II Seyfert galaxy]]
| | If you compare registry products there are a amount of elements to look out for. Because of the sheer amount of for registry cleaners available on the Internet at the moment it may be quite easy to be scammed. Something frequently overlooked is the fact that a few of these products may actually end up damaging a PC. And the registry they say they have cleaned can simply cause more issues with a computer than the ones you started with.<br><br>You are able to reformat a computer to make it run quicker. This will reset the computer to when you initially utilized it. Always remember to back up all files plus programs before carrying this out since this might remove a files from your database. Remember before we do this you require all the motorists and installation files plus this ought to be a last resort should you are searching for slow computer strategies.<br><br>Over time your disk may equally receive fragmented. Fragmentation causes your computer to slow down because it takes windows much longer to obtain a files place. Fortunately, a PC has a built in disk defragmenter. We can run this program by clicking "Start" - "All Programs" - "Accessories" - "System Tools" - "Disk Defragmenter". We might today have the way to choose which forces or partition we want to defragment. This action could take you several time thus it is advised to do this regularly thus because to avoid further fragmentation plus to accelerate your windows XP computer.<br><br>The 1328 error is a widespread issue caused by your program being unable to correctly procedure many changes for a program or Microsoft Office. If you have this error, it generally signifies which a computer is either unable to read the actual update file or a computer has difficulties with all the settings it's using to run. To fix this problem, we initially should change / fix any difficulties which a computer has with its update files, and then repair some of the issues which the system might have.<br><br>These are the results that the [http://bestregistrycleanerfix.com/tune-up-utilities tuneup utilities] found: 622 wrong registry entries, 45,810 junk files, 15,643 unprotected confidentiality files, 8,462 bad Active X products that have been not blocked, 16 performance features which were not optimized, plus 4 changes that the computer required.<br><br>The software finds these difficulties and takes care of them inside the shortest time possible. The difficult drive would equally cause difficulties sometimes, especially if you have 1 that is almost maxed. When the begin your machine, the are so many booting processes included plus having an almost full storage refuses to help a bit. You will constantly have a slow PC as there are a lot of factors inside the hard disk being processed at the same time. The best way to resolve this issue is to upgrade. This allows a PC several time to breath plus functioning quicker instantaneously.<br><br>The 'registry' is simply the central database which stores all the settings and options. It's a absolutely important part of the XP program, which means that Windows is regularly adding plus updating the files inside it. The problems happen when Windows really corrupts & loses certain of these files. This makes your computer run slow, because it attempts hard to find them again.<br><br>You can click here to locate out how to speed up Windows and grow PC perfomance. And you can click here to download a registry cleaner to help you clean up registry. |
| '''Seyfert galaxies''' are one of the two largest groups of [[active galactic nucleus|active galaxies]], along with [[quasar]]s. They have quasar-like nuclei (very luminous, distant and bright sources of electromagnetic radiation) with very high [[surface brightness]]es whose [[spectrum|spectra]] reveal strong, high-[[ionisation]] [[emission lines]],<ref name=Peterson1997 /> but unlike quasars, their host galaxies are clearly detectable.<ref name=Bulgaria />
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| Seyfert galaxies account for about 10% of all galaxies<ref name=Maiolino /> and are some of the most intensely studied objects in [[astronomy]], as they are thought to be powered by the same phenomena that occur in quasars, although they are closer and less luminous than quasars. These galaxies have [[supermassive black hole]]s at their centres which are surrounded by [[accretion disc]]s of in-falling material. The accretion discs are believed to be the source of the observed ultraviolet radiation. Ultraviolet [[emission lines|emission]] and [[absorption lines]] provide the best diagnostics for the composition of the surrounding material.<ref name=Davidsen />
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| Seen in [[visible light]], most Seyfert galaxies look like normal [[spiral galaxies]], but when studied under other wavelengths, it becomes clear that the [[luminosity]] of their cores is of comparable intensity to the luminosity of whole galaxies the size of the [[Milky Way]].<ref name=Soper />
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| Seyfert galaxies are named after [[Carl Keenan Seyfert|Carl Seyfert]], who first described this class in 1943.<ref name=Seyfert />
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| == Discovery ==
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| [[File:Messier 77 spiral galaxy by HST.jpg|thumb|NGC 1068 (Messier 77), one of the first Seyfert galaxies classified]]
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| Seyfert galaxies were first detected in 1908 by [[Edward A. Fath]] and [[Vesto Slipher]], who were using the [[Lick Observatory]] to look at the [[spectrum|spectra]] of [[astronomical object]]s that were thought to be "[[spiral nebulae]]". They noticed that [[Messier 77|NGC 1068]] showed six bright [[emission lines]], which was considered unusual as most objects observed showed an [[absorption spectrum]] corresponding to [[star]]s.<ref name=OpenLearn />
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| In 1926, [[Edwin Hubble]] looked at the emission lines of NGC 1068 and two other such "nebulae" and classified them as [[Extragalactic astronomy|extragalactic objects]].<ref name=Hubble /> In 1943, [[Carl Keenan Seyfert]] discovered more galaxies similar to NGC 1068 and reported that these galaxies have very bright stellar-like nuclei that produce broad emission lines.<ref name=Seyfert /> A year after, [[Cygnus A]] was detected at 160 MHz,<ref name=Reber /> and detection was confirmed in 1948 when it was established that it was a discrete source.<ref name=Bolton /> Its double radio structure became apparent with the use of [[interferometry]].<ref name="Hanbury Brown" /> In the next few years, other [[radio sources]] such as [[supernova]] remnants were discovered. 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 (≈10<sup>9±1</sup> solar masses), and the duration of peak nuclear emissions is relatively short (>10<sup>8</sup> years).<ref name=Torres-Papaqui />
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| In the 1960-1970s, 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.<ref name=Walker /> Controversy existed over whether Seyfert redshifts were of cosmological origin.<ref name=Weedman1977 /> 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.<ref name=Weedman1977 /> In the same time period, research had been undertaken in order 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.<ref name=Peterson1973 /> At the time, it was believed that 1% of spiral galaxies are Seyferts.<ref name=deVancouleurs /> By 1977, it was found that very few Seyfert galaxies are ellipticals, most of them being normal or barred spiral galaxies.<ref name=Adams /> During the same time period, efforts have been made to gather [[spectrophotometric]] data for Seyfert galaxies. It became obvious that not all spectra from Seyfert galaxies look the same, so they have been subclassified according to the characteristics of their [[emission spectra]]. A simple division into types I and II has been devised, with the classes depending on the relative width of their [[emission lines]].<ref name=Weedman1973 /> 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 [[Seyfert galaxy#Classification|Classification]]).<ref name=Osterbrock1976 /><ref name=Osterbrock1993 /> Early surveys for Seyfert galaxies were biased in counting only the brightest representatives of this group. More recent surveys that count galaxies with low-luminosity and obscured Seyfert nuclei suggest that the Seyfert phenomenon is actually quite common, occurring in 16% ± 5% of galaxies; indeed, several dozen galaxies exhibiting the Seyfert phenomenon exist in the close vicinity (≈27 Mpc) of our own galaxy.<ref name=Maiolino />
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| == Characteristics ==
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| [[File:Seyfert galaxy NGC4151 (GL-2002-001035).jpg|thumb|Optical and ultraviolet images of the black hole in the centre of NGC 4151, a Seyfert Galaxy]]
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| An [[active galactic nucleus]] (AGN) is a compact region at the centre of a galaxy that has a higher than normal [[luminosity]] over portions of the [[electromagnetic spectrum]]. A galaxy having an active nucleus is called an active galaxy. Active galactic nuclei are the most luminous sources of electromagnetic radiation in the Universe, and their evolution puts constraints on cosmological models. Depending on the type, their luminosity varies over a timescale from a few hours to a few years. The two largest subclasses of active galaxies are quasars and Seyfert galaxies, the main difference between the two being the amount of radiation they emit. 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.<ref name=Peterson1997 /><ref name=Popping /> Seyfert galaxies have extremely bright nuclei, with luminosities ranging between 10<sup>8</sup> and 10<sup>11</sup> solar luminosities. Only about 5% of them are radio bright; their emissions are moderate in gamma rays and bright in X-rays.<ref name=Massi /> Their visible and infrared [[Electromagnetic spectrum|spectra]] shows very bright [[spectral line|emission lines]] of [[hydrogen]], [[helium]], [[nitrogen]], and [[oxygen]]. These emission lines exhibit strong [[Doppler broadening]], which implies [[velocity|velocities]] from {{convert|500|to|4000|km/s|abbr=on}}, and are believed to originate near an [[accretion disc]] surrounding the central black hole.<ref name=Osterbrock2006 />
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| === Eddington luminosity ===
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| {{Main|Eddington luminosity}}
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| A lower limit to the mass of the central black hole can be calculated using the [[Eddington luminosity]].<ref name=Heinzeller /> This limit arises because light exhibits radiation pressure. Assume that a black hole is surrounded by a disc of luminous gas.<ref name=Yoshida /> 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.<ref name=Blandford /><ref group=note>The gravitational force ''F<sub>grav</sub>'' of the black hole can be calculated using:
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| :<math>F_{grav}=\frac{GM_{BH}m_p}{r^2}</math>
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| where ''G'' is [[gravitational constant]], ''m<sub>p</sub>'' is the [[proton mass]] and ''M<sub>BH</sub>'',''r'' are the mass and radius of the black hole respectively.
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| <br />
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| We derive the outward radiative force ''F<sub>rad</sub>'' as we do for stars assuming spherical symmetry:
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| :<math>F_{rad} =\frac{dp}{dt}=\frac{1}{c}\frac{dE}{dt}=\frac{1}{c}\sigma_t\frac{L}{4\pi r^2}</math>
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| where ''p'' is momentum, ''t'' is time, ''c'' is the [[speed of light]], ''E'' is energy, ''σ<sub>t</sub>'' is the Thomson cross-section and ''L'' is luminosity.
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| The luminosity of the black hole must be less than the Eddington luminosity ''L<sub>Eddington</sub>'', which is given when:
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| :<math>F_{rad} = F_{grav} \rightarrow L < L_{Eddington} </math> <math>= \frac{4\pi c G M_{BH} m_p}{\sigma_t} </math> <math>= 1.3 \times 10^{38} \frac{M_{BH}}{M_{solar}} \, erg/sec </math> <math>= 30000 \frac{M_{BH}}{M_{solar}} L_{solar}</math>
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| where ''M<sub>solar</sub>'' is the mass of the [[Sun]] and ''L<sub>solar</sub>'' is the solar luminosity.
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| 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.</ref>
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| [[File:Active Galactic Nucleus Model.gif|thumb|The image shows a model of an active galactic nucleus. The central black hole is surrounded by an accretion disc, which is surrounded by a torus. The broad line region and narrow line emission region are shown, as well as jets coming out of the nucleus.]]
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| === Emissions ===
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| 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.<ref name=Goad />
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| The narrow lines are believed to originate from the outer part of the AGN 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.<ref name=Peterson2004 /> 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.<ref name="Peterson et al" />
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| In the few radio-loud Seyfert galaxies that have been observed, the radio emission is believed to represent [[synchrotron radiation|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.<ref name=Haardt />
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| == Classification ==
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| [[File:A wanderer dancing the dance of stars and space.jpg|thumb|[[NGC 1097]] is an example of a Seyfert galaxy. A supermassive black hole with a mass of 100 million solar masses lies at the centre of the galaxy. The area around the black hole emits large amounts of radiation from the matter falling into the black hole.<ref name=PictureRef />]]
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| 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 both permitted and forbidden narrow lines. [[Forbidden mechanism|Forbidden lines]] are spectral lines that occur due to [[Atomic electron transition|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.<ref name=Britannica />
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| 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 [[H-alpha|Hα]] emission line has rarely, if ever, disappeared.<ref name=Carroll /> The origin of the differences between Type I and Type II Seyfert galaxies is not known yet. 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. It is believed by some that all Type II Seyferts are in fact Type I, where the broad components of the lines are impossible to detect because of the angle we are at with respect to the galaxy. 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 centre of the galaxy. By contrast, in Type II Seyfert galaxies, the active nuclei are obscured and only the colder outer regions located further away from the clouds' broad line emission region are seen. This theory is known as the "Unification scheme" of Seyfert galaxies.<ref name=Pradhan /><ref name=Singh /> However, it is not yet clear if this hypothesis can explain all the observed differences between the two types.<ref name=Pradhan />
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| === Type I Seyfert galaxies ===
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| [[File:SeyfertTypeISpectra.gif|thumb|right|Optical spectrum of the Type I Seyfert galaxy NGC 5548]]
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| Type I Seyferts are very bright sources of [[ultraviolet]] light and [[X-ray]]s 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 10<sup>4</sup> km/s.<ref name=Armitage /> 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, R<sub>BLR</sub>, 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.<ref name=Massi />
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| === Type II Seyfert galaxies ===
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| Type II Seyfert galaxies have the characteristic bright core, as well as appearing bright when viewed at [[infrared]] wavelengths.<ref name=Morgan /> Their spectra contain narrow lines associated with forbidden transitions, and broad lines associated with allowed strong dipole or intercombination transitions.<ref name=Pradhan /> In some Type II Seyfert galaxies, analysis with a technique called spectro-polarimetry (spectroscopy of [[polarised light]] component) revealed obscured type I regions. In the case of NCG 1068, nuclear light reflected of a dust cloud was measured, which led scientists to believe in the presence of an obscuring dust [[torus]] around a bright continuum and broad emission line nucleus. When the galaxy is viewed from the side, the nucleus is indirectly observed through [[Reflection (physics)|reflection]] by gas and dust above and below the torus. This reflection causes the [[Polarization (waves)|polarisation]].<ref name=Barthel />
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| === Type 1.2, 1.5, 1.8 and 1.9 Seyfert galaxies ===
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| In 1981, [[Donald Edward Osterbrock|Donald Osterbrok]] introduced the notations Seyfert 1.5, 1.8 and 1.9, where the subclasses are based on the optical appearance of the spectrum, with the numerically larger subclasses having weaker broad-line components relative to the narrow lines. For example, Type 1.9 only shows a broad component in the [[H-alpha|Hα]] line, and not in higher order [[Balmer series|Balmer lines]]. In Type 1.8, very weak broad lines can be detected in the [[H-beta|Hβ]] lines as well as Hα, even if they are very weak compared to the Hα. In Type 1.5, the strength of the Hα and Hβ lines are comparable.<ref name=Caltech />
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| === Other Seyfert-like galaxies ===
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| 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.<ref name=Ho /> An example of such a galaxy is M104 in the Virgo constellation, also known as the [[Sombrero galaxy]].<ref name=Heckman /> 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.<ref name=Starling /> Another very interesting subclass are the narrow line Seyfert I galaxies (NLSy1), which have been subject to extensive research in recent years.<ref name=Osterbrock1985 /> They have much narrower lines than the broad lines from classic Seyfert I galaxies, steep hard and soft X-ray spectra and strong Fe[II] emission.<ref name=Boller /> Their properties suggest that NLSy1 galaxies are young AGNs with high accretion rates, suggesting a relatively small but growing central black hole mass.<ref name=Mathur2005 /> There are theories suggesting that NLSy1s are galaxies in an early stage of evolution, and links between them and ultraluminous infrared galaxies or Seyfert II galaxies have been proposed.<ref name=Komossa />
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| == Evolution ==
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| The majority of active galaxies we observe are very distant and show large [[Doppler shift]]s. This suggests that active galaxies occurred in the early Universe and, due to [[cosmic inflation]], are receding away from us at very high speeds. Quasars are the furthest active galaxies, some of them being observed at distances 12 billion light years away. Seyfert galaxies are much closer than quasars.<ref name=Goddard /> Because light has a finite speed, looking across large distances in the Universe is equivalent to looking back in time. Therefore, the observation of active galactic nuclei at large distances and their scarcity in the nearby Universe suggests that they were much more common in the early Universe,<ref name=UTennessee /> implying that active galactic nuclei could be early stages of [[galactic evolution]]. This leads to the question about what would be the local (modern-day) counterparts of AGNs found at large redshifts. It has been proposed that NLSy1s could be the small redshift counterparts of quasars found at large redshifts (z>4). The two have many similar properties, for example: high [[Metallicity|metallicities]] or similar pattern of emission lines (strong Fe [II], weak O [III]).<ref name=Mathur2000 /> Some observations suggest that AGN emission from the nucleus is not spherically symmetric and that the nucleus often shows axial symmetry, with radiation escaping in a conical region. Based on this observations, models have been devised to explain the different classes of AGNs as due to their different orientations with respect to the observational line of sight. Such models are called unified models. Unified models explain the difference between Seyfert I and Seyfert II galaxies as being the result of Seyfert II galaxies being surrounded by obscuring toruses which prevent us from seeing the broad line region. Quasars and [[blazar]]s can be fit quite easily in this model.<ref name=Halliday /> The main problem of such an unification scheme is trying to explain why some AGN are radio loud while others are radio quiet. It has been suggested that these differences may be due to differences in the spin of the central black hole.<ref name=Armitage /> | |
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| == Examples ==
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| [[File:Mrk 1513.png|thumb|Seyfert galaxy MRK 1513]]
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| The table below lists a few representative Seyfert galaxies from the [[Markarian galaxies|Markarian catalog]].<ref name=SeyfertList />
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| {| class="wikitable sortable"
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| ! Name !! Other names !! Longitude !! Latitude !! Right ascension !! Declination
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| | Mark 205 || MRK 0205 || 185.4338322 || 75.3106237 || {{RA|12|21|44.120}} || {{DEC|+75|18|38.25}}
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| | Mark 231 || MRK 0231 || 194.0593100 || 56.8736767 || {{RA|12|56|14.2344}} || {{DEC|+56|52|25.236}}
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| | Mark 266 || NGC 5256 || 204.573720 || 48.276093 || {{RA|13|38|17.69}} || {{DEC|+48|16|33.9}}
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| | Mark 270 || NGC 5283 || 205.2739946 || 67.6723111 || {{RA|13|41|05.759}} || {{DEC|+67|40|20.32}}
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| | Mark 279 || MRK 0279 || 208.2643618 || 69.3082128 || {{RA|13|53|03.447}} || {{DEC|+69|18|29.57}}
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| | Mark 335 || MRK 0335 || 1.5813306 || 20.2029144 || {{RA|00|06|19.519}} || {{DEC|+20|12|10.49}}
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| | Mark 530 || NGC 7603 || 349.7359060 || 0.2439521 || {{RA|23|18|56.617}} || {{DEC|+00|14|38.23}}
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| | Mark 590 || NGC 0863 || 33.6398442 || -0.7666930 || {{RA|02|14|33.562}} || {{DEC|-00|46|00.09}}
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| | Mark 686 || NGC 5695 || 219.3421784 || 36.5678087 || {{RA|14|37|22.123}} || {{DEC|+36|34|04.11}}
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| | Mark 744 || NGC 3786 || 174.9272970 || 31.9092853 || {{RA|11|39|42.551}} || {{DEC|+31|54|33.43}}
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| |}
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| == See also ==
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| * [[Low-ionization nuclear emission-line region]]
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| == Notes ==
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| {{Reflist|group=note}}
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| == References ==
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| {{reflist|2|refs=
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| <ref name=Adams>
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| {{cite journal |title=A Survey of the Seyfert Galaxies Based on Large-Scale Image-Tube Plate |journal=The Astrophysical Journal Supplement |first=Thomas F. |last=Adams |volume=33 |pages=19-34 |year=1977 |bibcode=1977ApJS...33...19A |doi=10.1086/190416}}</ref>
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| <ref name=Armitage>
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| {{cite web |url=http://jila.colorado.edu/~pja/astr3830/lecture27.pdf |title=Astrophysics 2, lecture 27: Active galaxies - the Unified Model |work=ASTR 3830 Lecture Notes |publisher=[[University of Colorado Boulder]] |last=Armitage |first=Phil |year=2004 |accessdate=10 November 2013}}</ref>
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| <ref name=Barthel>
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| {{cite book |chapterurl=http://ned.ipac.caltech.edu/level5/ESSAYS/Barthel/barthel.html |chapter=Active galaxies and quasistellar objects, interrelations of various types |title=The Astronomy and Astrophysics Encyclopedia |publisher=[[Wiley-Interscience]] |first=Peter |last=Barthel |editor-first=Stephen P. |editor-last=Maran |year=1991 |isbn=978-0-471-28941-8}}</ref>
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| <ref name=Blandford>
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| {{cite web |url=http://ned.ipac.caltech.edu/level5/ESSAYS/Blandford/blandford.html |title=Active Galaxies and Quasistellar Objects, Accretion |publisher=NASA/IPAC Extragalactic Database |first=Roger D. |last=Blandford |accessdate=6 December 2013}}</ref>
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| <ref name=Boller>
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| {{cite journal |title=Soft X-ray properties of narrow-line Seyfert 1 galaxies |journal=[[Astronomy and Astrophysics]] |first1=T. |last1=Boller |first2=W. N. |last2=Brandt |first3=H. |last3=Fink |volume=305 |page=53 |year=1996 |arxiv=astro-ph/9504093 |bibcode=1996A&A...305...53B}}</ref>
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| <ref name=Bolton>
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| {{cite journal |title=Observations on the Variable Source of Cosmic Radio Frequency Radiation in the Constellation of Cygnus |journal=Australian Journal of Scientific Research A |first1=J. G. |last1=Bolton |first2=G. J. |last2=Stanley |volume=1 |pages=58-69 |year=1948 |bibcode=1948AuSRA...1...58B}}</ref>
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| <ref name=Britannica>
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| {{cite web |url=http://www.britannica.com/EBchecked/topic/213039/forbidden-lines |title=Forbidden lines |work=[[Encyclopædia Britannica]] |year=2013 |accessdate=27 November 2013}}</ref>
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| <ref name=Bulgaria>
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| {{cite web |url=http://www.astro.bas.bg/~petrov/galaxies_files/agn.html |title=Active Galaxy Nuclei |publisher=Bulgarian Academy of Sciences/Institute of Astronomy |editor-first=G. T. |editor-last=Petrov |year=2004 |accessdate=9 December 2013}}</ref>
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| <ref name=Caltech>
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| {{cite web |url=http://ned.ipac.caltech.edu/level5/Glossary/Essay_seyfert.html |title=Seyfert galaxies |publisher=California Institute of Technology |accessdate=10 October 2013}}</ref>
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| <ref name=Carroll>
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| {{cite book |title=An Introduction to Modern Astrophysics |publisher=[[Addison-Wesley]] |first1=Bradley W. |last1=Carroll |first2=Dale A. |last2=Ostlie |edition=2nd |pages=1085-1086 |year=2006 |isbn=0-321-44284-9}}</ref>
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| <ref name=Davidsen>
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| <ref name=Yoshida>
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| {{cite web |url=http://www.ppl.phys.chiba-u.jp/lecture/radiation/node2.html |title=The Eddington Limit |publisher=Department of Physics, Chiba University |first=Shigeru |last=Yoshida |accessdate=7 December 2013}}</ref>
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| }}
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| == External links ==
| |
| {{Commons category|Seyfert galaxies}}
| |
| * [http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html Active Galaxies and Quasars] at NASA.gov
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| * [http://spider.seds.org/spider/ScholarX/seyferts.html Seyfert Galaxies] at SEDS.org
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| * [http://xmm.esac.esa.int/external/xmm_science/gallery/public/level2a.php?p=0&cat=5&subcat=3 Seyfert Galaxies] at ESA.int
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| {{Galaxy}}
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| [[Category:Seyfert galaxies| ]]
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| [[Category:Active galaxy types]]
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