Seyfert galaxy

The Circinus Galaxy, a Seyfert 2 galaxy

Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars. They have a quasar-like nucleus (very luminous, distant bright source of electromagnetic radiation) but, unlike quasars, the host galaxy is clearly detectable. Seyfert galaxies have nuclei with very high surface brightness whose spectra reveal strong, high-ionisation emission lines. They account for about 10% of all galaxies and are some of the most intensely studied objects in astronomy, as they are thought to be closer and less luminous versions of the same phenomena occurring in quasars. These galaxies have a supermassive black hole at the centre which is surrounded by an accretion disc of in-falling material. The accretion disk is believed to be the source of the observed ultraviolet radiation. Ultraviolet emission and absorption lines provide the best diagnostics of the composition of the surrounding material.

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 core is of comparable intensity to the luminosity of a whole galaxy the size of the Milky Way.

Seyfert galaxies are named after Carl Seyfert, who first described this class in 1943.

Discovery

NGC 1068 (Messier 77), one of the first Seyfert galaxies classified

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". They noticed that NGC 1068 showed six bright emission lines, which was considered unusual as most objects observed showed an absorption spectrum corresponding to stars.

In 1926, Edwin Hubble looked at the emission lines of NGC 1068 and two other such "nebulae" and discovered that they were in fact extragalactic objects and concluded they are different galaxies from our own. 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. A year after, Cygnus A was detected at 160 MHz, and detection was confirmed in 1948 when it was also established that it was a discrete source. Its double radio structure became apparent with the use of interferometry. In the next few years, other radio sources, like remnants of supernovae, were discovered. By the end of the 1950s, more characteristics of Seyfert galaxies were discovered, such as the fact that their nuclei are unresolved.

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. Accurate measurements of the distance to Seyfert galaxies and their age were limited due to the way their nuclei vary in brightness over a time scale of a few years, so arguments involving distance to such galaxies and the constant speed of light cannot always be used to determine their age. In the same time period, research has 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. At the time, it was believed that 1% of spiral galaxies are Seyferts, and by 1977 it was found that very few Seyfert galaxies are ellipticals, most of them being normal or barred spiral galaxies. 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've been subclassified solely on the properties shown by the emission lines on their spectra. A simple division into types I and II has been devised, with the classes depending only on the relative width of the emission lines. It has been later noticed that some Seyfert nuclei show intermediate properties, hence why they have been further subclassified into types 1.2, 1.5, 1.8 and 1.9 (see Classification).

Characteristics

Optical and ultraviolet images of the black hole in the centre of NGC 4151, a Seyfert Galaxy

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 that the constituent stars by at least a factor of 100.

Seyfert galaxies have extremely bright nuclei, with luminosities ranging between 10⁸ and 10¹¹ solar luminosities. Only about 5% of them are radio bright, their emissions are moderate in gamma rays and bright in X-rays. Their visible and infrared spectra shows 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 mass of the central black hole can be calculated using the Eddington luminosity. Its gravitational force can be calculated using

$F_{grav}={\frac {GM_{BH}m_{p}}{r^{2}}}$

The outward radiative force is equal to

$F_{rad}={\frac {dp}{dt}}={\frac {1}{c}}{\frac {dE}{dt}}={\frac {1}{c}}\sigma {\frac {L}{4\pi r^{2}}}$

In the case of a black hole $F_{rad}<F_{grav}$, therefore

${\frac {1}{c}}\sigma {\frac {L}{4\pi r^{2}}}<{\frac {GM_{BH}m_{p}}{r^{2}}}$

The image shows a model of an active galactic nucleus. The central black hole is surrounded by an accretion disk, 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.

The actual luminosity of the black hole is less than the Eddington luminosity.

$L<L_{eddington}={\frac {4\pi cGM_{BH}m_{p}}{\sigma }}=1.3\times 10^{38}{\frac {M_{BH}}{M_{solar}}}$

In the above derivation, F_rad is the outward radiative force, F_grab is the gravitational force of the black hole, p = momentum, t = time, c = speed of light, E = energy, L = luminosity, r = radius of black hole, G = gravitational constant, M_BH = mass of black hole, L_Eddington is the Eddington luminosity, m_p is the proton mass, M_solar is the mass of the Sun and σ is the Stefan–Boltzmann constant. Therefore, by knowing the Eddington luminosity and using the mass of the Sun, an approximate value for the mass of the central black hole at the centre of an active galaxy can be obtained.

The emission lines seen on the spectrum of a Seyfert galaxy may come from the surface of the accretion disk 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 disk 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.

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. The basic idea behind this technique is to study 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. 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.

In the few radio-loud Seyfert galaxies that have been observed, the radio emission is believed to be 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.

Classification

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.

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 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.

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 Hα emission line has rarely, if ever, disappeared. 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. However, it is not yet clear if this hypothesis can explain all the observed differences between the two types.

Type I Seyfert galaxies

File:SeyfertTypeISpectra.gif

Optical spectrum of the Type I Seyfert galaxy NGC 1275

Type I Seyferts are very bright sources of ultraviolet light and X-rays, as well as the visible light coming from their core. They have two sets of emission lines on their spectra: narrow lines with a width (measured in velocity units) of several hundred km/s and broad lines, with widths up to 10⁴ km/s. The broad lines originate above the accretion disk of the supermassive black hole thought to power the galaxy, while the narrow lines occur beyond the broad line region of the accretion disk. 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_BLR, 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.

Type II Seyfert galaxies

Type II Seyfert galaxies have the characteristic bright core, as well as appearing bright when viewed at infrared wavelengths. Their spectra contain narrow lines associated with forbidden transitions, and broad lines associated with allowed strong dipole or intercombination transitions. 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 by gas and dust above and below the torus. This reflection causes the polarisation.

Type 1.2, 1.5, 1.8 and 1.9 Seyfert galaxies

In 1981, Donald Osterbrok introduced the notations Seyfert 1.5, 1.8 and 1.9, where the subclasses are based only 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 Hα line, and not in higher order Balmer lines. In Type 1.8, very weak broad lines can be detected in the 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.

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 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 surprisingly 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. An example of such a galaxy is M104 in the Virgo constellation, also known as the Sombrero galaxy A galaxy that is both a LINER and a Type 1 Seyfert is NGC 7213, a galaxy that is fairly close compared to other AGNs. Another very interesting subclass are the narrow line Seyfert I galaxies (NLSy1), which have been subject to extensive research in recent years. 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. Their properties suggest that NLSy1 galaxies are young AGNs with high accretion rates, suggesting a relatively small but growing central black hole mass. 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.

Evolution

The majority of active galaxies we observe are very distant and show a large amount of Doppler shift. 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 away active galaxies, some of them being observed at distances 12 billion light years away. By measuring and comparing redshifts, we find that Seyfert galaxies are much closer. Because light has a finite speed, looking at 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. This suggests that active galactic nuclei could be early stages of galactic evolution. This leads to the discussion about what are 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 metallicities or similar pattern of emission lines (strong Fe [II], weak O [III]. 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. The unification of Seyfert I and Seyfert II galaxies is relatively easy to make, the main idea being that an obscuring torus surrounding the galaxy is preventing us from seeing the broad line region in Seyfert II galaxies. Quasars and blazars can also be fit quite easily in this model. The main problem of such an unification scheme is trying to explain why some AGN are radio loud while others are radio quiet. Attempts to explain this suggest differences in the spin of the central black hole.

Examples

Seyfert galaxy MRK 1513

Below is a table of recently analysed Seyfert galaxies.

Name	Other names	Longitude	Latitude	Right ascension	Declination
Mark 205	MRK 0205	185.4338322	75.3106237	12h 21m 44.120s	+75° 18′ 38.25″
Mark 231	MRK 0231	194.0593100	56.8736767	12h 56m 14.2344s	+56° 52′ 25.236″
Mark 266	NGC 5256	204.573720	48.276093	13h 38m 17.69s	+48° 16′ 33.9″
Mark 270	NGC 5283	205.2739946	67.6723111	13h 41m 05.759s	+67° 40′ 20.32″
Mark 279	MRK 0279	208.2643618	69.3082128	13h 53m 03.447s	+69° 18′ 29.57″
Mark 335	MRK 0335	1.5813306	20.2029144	00h 06m 19.519s	+20° 12′ 10.49″
Mark 530	NGC 7603	349.7359060	0.2439521	23h 18m 56.617s	+00° 14′ 38.23″
Mark 590	NGC 0863	33.6398442	-0.7666930	02h 14m 33.562s	−00° 46′ 00.09″
Mark 686	NGC 5695	219.3421784	36.5678087	14h 37m 22.123s	+36° 34′ 04.11″
Mark 744	NGC 3786	174.9272970	31.9092853	11h 39m 42.551s	+31° 54′ 33.43″

See also

• Low-ionization nuclear emission-line region

External links

• Active Galaxies and Quasars at NASA.gov
• Seyfert Galaxies at SEDS.org
• Seyfert Galaxies at ESA.int