The fact that most active galaxies are located at a very far distance indicates that they are young celestial bodies in the universe, because their light will take millions or even billions of years to reach the Earth.This led astronomers to believe that all galaxies may have experienced this stage of activity.[1]
Having violent activity or physical processGalaxy。also calledPerturbed galaxy。Including quasarsSeyfert galaxies , radio galaxy, scorpion tiger object, starburst galaxy, etc.10% of galaxies are active galaxies.The main characteristics of active galaxies are: there is a very small and bright nucleus in the central region of the galaxy, called active galactic nucleus;Strong non thermal continuum spectrum;There are wide emission lines in the spectrum.Some active galaxies have rapid light changes, with time scales ranging from hours to years.Some active galaxies have obvious eruptions, such as jets.The characteristics of active galaxies are mostly associated with active galactic nuclei.Some active galaxies, such as quasars and BL type objects in the constellation Scorpius, emit most of their radiation from galactic nuclei, while the radiation of other parts is almost invisible.The number of active galaxies is about 1% of the total number of normal galaxies, and their lifetime is about 100 million years.Human beings have little understanding of the nature of active galaxies, and the study of active galaxies has become one of the most active fields in galaxy astronomy and even in astrophysics.
Currently, active galaxies and active galactic nuclei are not strictly distinguished.
[Active galactic nucleus] A particularly bright nucleus in some galaxies, which is believed to be caused by matter falling onto a black hole with great mass.
Active galaxy
Active galaxies are galaxies that emit large amounts of energy from a central region called the nucleus.This gives such objects another name - active galactic nuclei, commonly referred to as AGNs.This term includes many high-energy galaxies with different names found at different times, including the Seyfert galaxy,Galaxy N, BL type objects and quasars in Scorpio.It is now believed that the energy of all these celestial bodies is provided by the process of accretion of matter by a supermassive black hole at the center of an active galaxy, which is basically the same.
When the matter of a galaxy falls into a black hole, the gravitational energy corresponding to its mass is released and transformed into electromagnetic radiation, including light, X-rays and radio waves.The efficiency of this process is extremely high, so that 10% or more of the mass of the inflow material is converted into energy according to Einstein's famous formula E=mc2.The mass of the central black hole can be as much as 100 million times the mass of the sun, which is exactly 0.1% of the mass of all bright stars in the galaxy surrounding it.It only needs to "devour" the mass of a star equal to 1~2 suns every year to provide the energy observed in the most powerful active galaxies.
The energy generated by renewable energy tends to shoot towards both sides of the galaxy, probably through the "pole" of the black hole.This energy cannot escape from other directions because it is blocked by accretion disks.The place where the emitted radiation interacts with the matter in the very vicinity of the galaxy can produce a thin jet, or the extension area of the emitted radio waves called the lobe.
Galaxies and galactic nuclei
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Generally, active galaxies and active galactic nuclei are not strictly distinguished.
Geomagnetism of the earth
Active galactic nucleus (AGN) is a kind of extragalactic galaxy with strong activity in the central core region.These galaxies appear to be more active than ordinary galaxies. They emit strong electromagnetic radiation in the whole band from radio waves to gamma rays. Their luminosity is about 1036-1041J/s. People call them active galaxies.The active galactic nucleus is the bright core of these galaxies, and its scale is usually about 1 light year, accounting for only a small part of the entire active galaxy.However, because its luminosity is much higher than that of the host galaxy, the active galactic nucleus usually refers to the entire active galaxy.
Since the discovery of quasars in the 1960s, many objects with similar characteristics have been found successively, all of which are alien systems, collectively called active galactic nuclei. The common point is that the spectrum has a very high red shift, indicating that the distance is far away from the cosmological scale, and the luminosity is high, far higher than that of ordinary galaxies.Further observations show that these objects tend to have rapid light changes, with light change time scales ranging from hours to days, indicating that their scales only account for a small part of the entire galaxy.In addition, the spectral range of active galactic nuclei is very wide, showing non thermal radiation spectrum, strong emission lines, and often accompanied by jet phenomenon.There are many kinds of active galactic nuclei discovered in recent decades, including the Sifo galaxy, quasars, radio galaxies, and BL type objects in the constellation Scorpius.For a long time, people have been puzzled about their mechanism and evolution, and have invested a lot of human and material resources in research, making AGN one of the hottest and most active research fields in astronomy since the 1990s.At present, it is widely accepted that AGN is composed of supermassive black holes and accretion disks.Based on theoretical and observational research, people have established the standard model of AGN, that is, there is a black hole in the center, and the surrounding matter is gravitationally falling, forming aAccretion disk。Because of the dissipation, the gas is heated to a very high temperature, and gradually falls to the center of the black hole, and forms a jet along the normal direction of the accretion disk.The observational characteristics of AGN mainly depend on the centerblack hole, the characteristics of accretion disk and the direction of line of sight.
classification
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Seyfert galaxy
Stellar explosion
Seiffert galaxy: The first active galaxy discovered, named after its discoverer, Carl Seiffert.Seiffert galaxy is a spiral or bar spiral galaxy with a very bright galactic nucleus. It has a strong high ionization emission line, a very wide spectral line, and strong and variable X-ray.Although they do not all radiate radio waves, the Seiffert galaxy is a strong source of infrared radiation.According to the width and shape of the transmission line, it can be divided into type I and type II.Type I Seiffert galaxies have wide emission lines, and their emission line spectra show that they are generated by hydrogen clouds rotating at high speed around the center;Type II only has narrow emission lines. Although there are hydrogen lines in its spectrum, there seems to be no such fast moving gas cloud.It can also be further divided into 1.5, 1.8, 1.9 and other types.[1]
a quasar
Quasars (QSO): Quasars are thought to be very similar to Seyfert galaxies - except that the core activity is more intense.They appear in the sky as points that glow like stars (hence the name quasar), but areSpectroscopeResearch can show that they are obviously not stars.According to the redshift in their spectral lines, most of them are located in extremely remote places, and they are one of the most distant objects known in the universe.Like Seiffert galaxies, they can be "radio noisy" (in this case called“Quasar radio source”）Or "radio static" (traditional QSO).The brightness of quasars can be 1000 times that of ordinary type systems.[1]
Radio galaxy
Radio galaxy: as its name indicates, this type of galaxyElectromagnetic spectrumThe radio band has the strongest radiation.Unlike point sources, radiation emanates from the huge radiation lobes on both sides of such galaxies.Most of them have two radiation sources, which are called double source radio galaxies.Usually elliptical galaxies.According to the width of emission lines, they can be divided into wide line radio galaxies (BLRG) and narrow line radio galaxies (NLRG).
BL Lacertae objects
BL type object in Scorpio: The galactic nucleus is very bright, and the short time exposure is very similar to the stars.The luminosity has a rapid change, the radio radiation has a strong polarization, and there is no absorption line or emission line in the spectrum, so its red shift can only be inferred from the spectrum of the host galaxy.
Optical cataclysmic quasar
Optical Cataclysmic Quasars (OVV): The luminosity has a rapid change and is often an intense radio source.Together with BL type objects in ScorpioFlamboyant variant（Blazar）。Glow variant is very close to quasars in many aspects, but it does not haveSpectral line。
Low ionizing nuclear radiation area
Low ionizing nuclear radiation area (LINER): the nuclear luminosity is relatively low, with low ionizing nuclear radiation area, and sometimes it is found to be a type 2 Seiffert galaxy with low luminosity.
Narrow line X-ray galaxies
Narrow line X-ray galaxy (NLXG): It has a high ionization emission line, similar to the Seiffert galaxy, but with low luminosity.Seyfert galaxy whose spectrum is thought to be extinguished by the dust in the galaxy.
Starburst galaxy
Starburst galaxy: It has a huge star forming region, with infrared luminosity higher than visible luminosity, and most of it is spiral galaxy.It belongs to active galaxy, but the relationship between it and active galactic nucleus is still uncertain.
Other information
In addition, there are N galaxies, Zweiki galaxies, high polarization quasars (HPQ), low luminosity active galactic nuclei (MAGN), hot stars (Warmer), etc.
According to the radiation of radio band, it can also be divided into radio quiet AGN and radio noise AGN.Among them, radio quiet active galactic nuclei include: low ionizing nuclear radiation regions, Seiffert galaxies, and some quasars; radio noise active galactic nuclei include radio noise quasars, flare variants (including BL type objects in Scorpio and optical cataclysmic quasars), radio galaxies, and so on.
Model
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Active galaxies (Figure 3)
The unified model of active galactic nuclei attempts to describe two or more active galactic nuclei with one model. Different types of active galactic nuclei are only due to different observation perspectives.The radio quiet and radio loud active galactic nuclei have their own unified models: the radio weak unified model and the radio strong unified model.The weak unified radio wave model believes that the Xifo type I galaxy is the active galactic nucleus itself with lower luminosity observed directly, while the Xifo type II galaxy is blocked by the obscuring ring around the accretion disk in the line of sight direction.If the luminosity of the active galactic nucleus is high, the direct observation is not the Siver I galaxy, but quasars.The unified model of radio wave intensity mainly focuses on radio wave strong quasars with high luminosity. They can be unified with narrow ray radio wave galaxies in a way similar to the weak unified model of radio wave, that is, radio wave galaxies are blocked by a masking ring, while quasars do not.If the angle between the line of sight and the jet is very smallScorpion tiger BLType quasar (blazar).
Introduction: The mystery of the huge energy of active galaxies and quasars has puzzled astronomers for decades. Now it is generally believed that their energy comes from the high-speed rotating supermassive black hole in the center of the galaxy.
After the 1920s, astronomers finally realized that there were countless galaxies of different sizes and shapes scattered in the vast space outside the Milky Way.However, until the middle of the 20th century, astronomers still believed that galaxies were quite quiet. Only a rare supernova burst out with the same brightness as the whole galaxy, could occasionally break through the deep silence of the universe.With the development of radio astronomy, astronomers have discovered radio sources in the center of our Milky Way Galaxy and strong radio sources in many galaxies.It is worth noting that these extragalactic radio sources emit much more energy in the radio band than the silver center, thus opening the prelude to understanding galactic activity.
Since then, the development of space astronomy has detected galactic activities in infrared and X bands, especially those related to the nuclei of some types of galaxies.In this way, astronomers realized that the activity of galaxies is quite common.However, the activity level of most galaxies (about 98%) is very low, such as our Milky Way, which we call normal galaxies.Only 2% of the galaxies have intense activity and are classified as active galaxies.
A normal galaxy is a group of celestial bodies formed by a large number of stars under gravitational binding. Most of its radiation is emitted by stars, and the radiation is mainly concentrated in the optical band.The radiation of active galaxies covers the entire electromagnetic band from radio to gamma rays.In addition, the energy emitted in radio, infrared, ultraviolet and X-ray bands is greater than that in optical bands, indicating that these radiation is emitted by a large number of non stellar materials.Under the special physical conditions in active galaxies, these substances are undergoing large-scale accretion, turbulence and explosion, and jet structures are observed near some active galaxies, apparently formed by matter ejected from galaxies.Active galaxies include many types, but there is no unified and exact classification so far. BL type objects in Scorpius, Seyfert galaxies and radio galaxies are some of the main types.
In 1929, a celestial body was discovered in the constellation Scorpio, which has very rapid light changes.Its apparent star magnitude fluctuates between 14 and 16 degrees, and it can be brightened occasionally to 13 degrees, which means that the brightness of the visible light band can vary by about 15 times, and the brightness can change by 10% to 32% in one day.At first, astronomers thought it was a variable star, which was called BL Scorpius according to the way of naming variable stars.Weak absorption lines are observed in the spectrum of BL Scorpio, which are generated by the nebula material around the object.From the measured red shift of spectral lines, the distance calculated according to Hubble's law is 590Mpc (1Mpc=3.26 × 106 light years).Astronomers now have strong evidence that it is an extragalactic object.Later, more than 300 objects with the same characteristics as BL Scorpius were discovered, which are commonly called BL Scorpius objects.The common features of the BL type objects in Scorpio are as follows: they are generally star shaped, and no structure can be seen, but some of them have weak envelopes;Brightness changes rapidly in radio, infrared and visible light bands, with time scales ranging from days to months;In the spectrum, there are neither absorption lines nor radiation rays, only a continuous radiation spectrum with no characteristics;Many are compact radio sources with strong radio radiation at the core.
In 1943, the American astronomer C.K. Seyfert found six spiral galaxies with unusually wide emission lines in the spectrum. Later, it was confirmed that this was a class of active galaxies, named Seyfert Galaxy.They are spiral galaxies with exceptionally bright cores, which almost occupy all the light emitted by the galaxy. They are easily mistaken for stars on the short exposure film. The long exposure film reveals that there is a hazy vortex structure around the core.The galactic nucleus is full of ionized gas with a mass of 10 to 103 solar masses and an ion density of 107/cm3 to 109/cm3. The gas moves randomly at a great speed with a speed of 103 km/s.This speed may be caused by violent eruptions.Seyfert galaxies have stronger radio radiation and infrared radiation than normal spiral galaxies. Some Seyfert galaxies have detected X-ray radiation.
Since the 1940s, radio astronomers have discovered tens of thousands of radio sources.At first, the strongest sources were represented by the constellation name followed by a capital Latin letter.For example, the strong radio source in Cygnus is called Swan A, and the strong radio source in Virgo is called Virgo A.Most radio sources are located beyond the river, and about 1/3~1/2 of them have been identified as galaxies.
Galaxies with strong radio radiation (higher than 1034 watts) are called radio galaxies.Their radiation power in radio band is not only much higher than that of normal galaxies, but also much higher than their radiation power in optical band.Most of these galaxies are elliptical galaxies.The main types are double source type and dense type.The typical double source radio galaxy has a small radio source near the center of the galaxy, and two large radio sources far away from the galaxy itself.The two sources, or radio lobes, may be 105 seconds to 107 seconds apart and 104 seconds to 106 seconds wide.Sometimes multiple pairs of valves can be seen.The structure of these sources shows that they point to the center of the galaxy, which is actually the matter ejected from the galaxy.Each radio lobe is a high-energy electron cloud with a magnetic field. The radio lobes are far away from the central galaxy, and their front faces the vast galactic space, compressing a huge amount of galactic material, causing violent collisions, and forming hot spots in the front.The observation of X-ray detection satellites shows that they are also strong X-ray sources.The energy stored by a typical radio petal is about equal to the energy radiated by all stars in the Milky Way in 100 million years!
In 1948, radio source Swan A was discovered. In 1954, it was proved to be an extragalactic galaxy with an apparent brightness of 16.It is 734 million light years away from the Milky Way, and its radiation power is about 107 times stronger than that of the Milky Way. It is the strongest known extragalactic radio source and the first discovered radio galaxy.Centaur A is the nearest radio galaxy with a distance of 11 million light years. Although it is not as powerful as Swan A, it is similar in other aspects.They are typical examples of dual source radio galaxies.The radio radiation region of compact radio galaxies is usually very small, not larger than the range of optical images of galaxies on the negative film.Some even do not exceed a few light years.M87 is a giant elliptical galaxy, located near the center of Virgo Cluster, and is the optical counterpart of radio source Virgo A.Its diameter is 500000 light years, 65 million light years from the earth, and its angle in the sky is more than half a degree (about the size of the full moon).It is a typical compact radio galaxy.
In 1960, astronomers found that the optical counterpart of radio source 3C48 was a stellar object with an apparent magnitude of 16, surrounded by very dark nebulous matter.It is puzzling that there are several completely strange spectral lines in the spectrum.In 1962, a 13th class "star" was found at the position of radio source 3C273.Astronomers are also puzzled by the unusual spectral lines in their spectra.
In 1963, someone finally recognized the true face of 3C 273 spectral lines. It turned out that they were spectral lines of hydrogen atoms, but they experienced a large red shift, making it difficult to identify spectral lines.Following the clue of red shift, we analyzed the spectrum of 3C 48 and found that its red shift was even greater.Assuming that the redshift is caused by the Doppler effect, 3C 273 and 3C 48 have great retrogression speeds, which are 1/6 and 1/3 of the speed of light respectively.Astronomers name such celestial bodies, which look like stars in optical photos, but whose essence is quite different, as quasi stellar radio sources.Further observation and research revealed another class of celestial bodies, which are also very similar to stars in shape and have a large red shift, but do not have radio radiation. They are called radio quiet quasars.Later, both were called quasars.
What are quasars?This poses a difficult problem for astronomers.In the decades since their discovery, the debate has not completely subsided.The debate focused on the cause of the redshift of quasars' spectral lines.Most people hold the view of "cosmological redshift", that is, quasars are located in the deep universe far away from the Milky Way. The farther away they are, the greater the redshift.If quasars are indeed so far away, there is another problem, that is, how to explain their huge energy output.The emission power of quasars is 102~104 times larger than that of ordinary spiral galaxies. What's more surprising is that the area of emission energy is very small, and its diameter is only on the order of light sky or even light time.It was a puzzle at that time that quasars could release such huge energy in such a small volume.Another view is that quasars are objects ejected from the Milky Way or other extragalactic galaxies, and they gain a great speed in the ejection. The greater the speed, the greater the red shift.
Comparing quasars with active galaxies such as BL type objects in the constellation Scorpius, Seyfert galaxies and radio galaxies, it is found that there are many similar observational characteristics, especially the understanding of radio galaxies, which is enough for astronomers to realize that quasars are different manifestations of the same phenomenon.In addition, since the 1980s, a large number of cosmic high-energy phenomena have been observed and understood, and the energy problem of quasars can also be reasonably explained.
On the premise that the redshift is a cosmological redshift, the redshift from large to small means that the celestial body is from young to old.In terms of redshift, quasars are the largest, followed by BL type objects in Scorpius and Seyfert galaxies, and radio galaxies are the smallest.Thus, we can roughly draw an evolutionary sequence: quasars, BL type objects in the constellation Scorpius, Seyfert galaxies, and radio galaxies, ending in normal galaxies.
As mentioned above, active galaxies (including quasars) only account for about 2% of the total number of galaxies. It can be seen that galaxies have evolved from birth to "maturity", and the active phase lasts a short time in their lifetime.From this point of view, quasars are the infancy of normal galaxies.So quasars are extremely active galactic nuclei.They are surrounded by galaxy disks.General quasars are too far away, so the galactic disk appears very weak, and the angular diameter is too small to be observed.In fact, for some recent quasars, such as 3C 273, evidence of the existence of galactic disks has been found.
The intense activity of active galaxies and quasars originates from the small core in the center.So, compared with the whole galaxy, it is difficult to explain the huge energy released in the small volume of the galactic nucleus.This problem has puzzled astrophysicists for a long time.It is now generally accepted that almost every large normal galaxy contains a supermassive black hole at its center.This hypothesis has been supported by increasing observational evidence.Astronomers once used the Very Large Array Radio Telescope (VLA) in New Mexico, United States, in combination with the Very Long Baseline Interferometry Array (VLBA) to make a complete survey of 100 neighboring galaxies, and found that at least 30% of the samples showed a small and dense central radio source with the unique characteristics of quasar phenomenon.
In addition, the cosmic background is also filled with faint X-ray glow, which spreads all over the sky.Unlike the microwave background radiation, which is the legacy of the Big Bang, the photon energy in the X-ray haze is too high to be generated in the early universe.Moreover, the microwave background radiation presents a basically uniform continuous distribution, and this kind of X-ray radiation distributed throughout the day is the contribution of countless discrete sources.The Chandra X-ray Observatory in the United States is equipped with a grazing X-ray imaging telescope, formerly known as an advanced X-ray astrophysical satellite, which was launched by the space shuttle in 1999.It was renamed in memory of the late Indian American Nobel Prize winner Subramanian Chandraseka.It has once exposed a selected sky depth, and can decompose at least 80% of the X-ray glow into a single point light source.This extrapolation to the whole sky shows that the total number is about 70 million.Then we tracked some of these objects and detected their radiation in other bands, and concluded that some are quite normal galaxies.They have dust shrouded cores that radiate X-rays - a sign of a central black hole.
It is now generally believed that such a huge energy source is due to the fact that there is a high-speed rotating supermassive black hole hidden in the center of the galaxy, whose mass is at least 107 times the mass of the sun.The black hole, with its huge gravity, attracts the surrounding matter to spiral and fall towards it, forming an accretion disk around it.The gas in the disk is compressed and heated. When the temperature exceeds 1 billion K, a strong radiation field will be formed, causing high-energy plasma jets to jet from the core to the poles perpendicular to the disk at a speed close to the speed of light.
