Active Galaxies and Quasars

Transcription

1 Active Galaxies and Quasars Radio Astronomy Grote Reber, a radio engineer and ham radio enthusiast, built the first true radio telescope in 1936 in his backyard. By 1944 he had detected strong radio emissions from sources in the constellations Sagittarius, Cassiopeia, and Cygnus. Two of these sources, named Sgr A and Cas A, happen to lie in our Galaxy; they are the galactic nucleus and a supernova remnant. 1

2 Radio Astronomy The nature of the third source, Cyg A, proved more elusive. The mystery only deepened in 1951, when Walter Baade and Rudolph Minkowski used the 200-inch telescope to discover a dim, strange-looking galaxy at the position of Cyg A. An Unusual Object When Baade and Minkowski photographed the spectrum of Cyg A, they found a number of bright emission lines. By contrast, a normal galaxy has only absorption lines, due to the atmospheres of the member stars. To have emission lines, something must be exciting the atoms. Furthermore, the wavelengths of the emission lines are all shifted by 5.6% the speed of light (z = = ; v = 16,000 km/s). 2

3 An Unusual Object If Cyg A participates in the same Hubble flow as clusters of galaxies, then this recessional velocity corresponds to 230 Mpc (750 million light years). Yet despite its tremendous distance, radio waves from Cyg A can be picked up by amateur astronomers using backyard equipment. Its radio luminosity is 10,000,000 times that of an ordinary galaxy. What type of object is extremely luminous in the radio and extremely distant? A Radio Star Cygnus A was not the only unusual radio source detected. In 1960, Allan Sandage used the 200-inch telescope to discover a star at the location of a radio source called 3C 48. Because ordinary stars are not strong radio sources, 3C 48 must be something unusual. Like Cyg A, its spectrum showed emission lines, but astronomers were not able to identify the chemical elements that produced them. 3

4 A Second Radio Star Many astronomers thought 3C 48 was just a strange star. Then a second star was discovered 3C 273. This object was even more unusual, with a luminous jet protruding from one side. Like 3C 48, its visible spectrum contained a series of emission lines that no one could explain. Identification In 1963, Maarten Schmidt realized that four of the brightest emission lines in 3C 273 were positioned relative to one another the same way as the first four Balmer lines of hydrogen. However, these lines were all shifted to much longer wavelengths. 4

5 Identification Schmidt obtained a redshift of z = (15% c = 44,000 km/s). No star could be moving this fast and remain in our Galaxy, so 3C 273 must be outside the Milky Way. Distances According to the Hubble Law, the redshift of 3C 273 implies it is about 620 Mpc (2 billion light years) away. To be seen at such a great distance means 3C 273 must be an extraordinarily powerful source of both visible light and radio radiation. 5

6 Distances Upon learning how Schmidt deciphered the spectrum of 3C 273, Jesse Greenstein and Thomas Matthews, found they could identify the hydrogen spectral lines of 3C 48 as having an even larger redshift, with z = 0.367, which puts 3C 48 twice as far away as 3C 273 (~1300 Mpc = 4 billion light years). Quasars Because of their strong radio emission and starlike appearance, 3C 48 and 3C 273 were dubbed quasi-stellar radio sources, which was shortened to quasars. After the first quasars were discovered by their radio emission, many similar, high-redshift, star-like objects were found that emit little or no radio radiation. These radio-quiet quasars were called quasi-stellar objects, or QSOs, to distinguish them from the radio emitters. Today, the term quasar is used to include both types. (Only about 10% of quasars are radio-loud like 3C 48 and 3C 273.) 6

7 General Characteristics More than 10,000 quasars are now known. They all look rather like stars, and all have large redshifts, ranging from z = 0.06 to at least z = 6.42! Most quasars have redshifts of 0.3 or more, which implies that they are more than 1000 Mpc (3 billion light years) away. Relativistic Redshifts A value of z > 1 does not mean that a quasar is receding from us faster than the speed of light. At very high speeds, the relationship between redshift and recessional velocity must be modified by the Special Theory of Relativity. z = ( - o ) / o v ( z + 1 ) = c ( z + 1 )

8 Example z = ( - o ) / o v ( z + 1 ) = c ( z + 1 ) Let z = 4 v ( ) = = = c ( ) v = c Light Travel Time Light takes time to travel across space, so when we observe a very distant object, we are seeing it in the remote past. This means that for very distant objects, the relationship between redshift and distance from the Earth depends on how the Universe has evolved over time. Because the Universe is expanding, if you could watch the motions of widely separated clusters of galaxies over millions of years, you would see them gradually moving away from one another. 8

9 Look-Back Time Because there are no quasars with small redshifts, there are no nearby quasars. The nearest one is 250 Mpc (= 800 million light years) from Earth. Think of distances as look-back time. The absence of nearby quasars means that there have been no quasars for nearly a billion years. Quasars were a common feature in the distant past (= far away from us = early Universe), but there are none in the present-day Universe. Extraordinarily Luminous Quasars must be extraordinarily luminous if they can be seen from the Earth despite their enormous distances. Indeed, quasars are among the most distant objects ever seen. A quasar s luminosity can be calculated from its apparent brightness and distance using the Inverse-Square Law. A bright quasar is a thousand times more luminous than the entire Milky Way Galaxy. In the 1950 s to 1980 s, many astronomers thought the Hubble Law could not be linear at these distances and that the quasars were not extremely far away. Over time, more data were collected with larger telescopes. Today it is firmly believed that the distances and Hubble Law are valid. 9

10 Seyfert Galaxies The first missing links between quasars and ordinary galaxies were discovered before quasars themselves. In 1943, Carl Seyfert made a systematic study of spiral galaxies with bright, star-like nuclei that seem to show signs of intense and violent activity. Like quasars, the nuclei of these galaxies have strong emission lines. These galaxies are now known as Seyfert Galaxies. Seyfert Galaxies Visible X-ray A few percent of the most luminous spiral galaxies are Seyfert galaxies. More than 700 are known. The brightest Seyfert galaxies are as luminous as faint quasars. There is no sharp dividing line between the properties of Seyferts and those of quasars. Like radio-quiet quasars, Seyferts tend to only have weak radio emissions. 10

11 Radio Galaxies While Seyferts resemble dim, radio-quiet quasars, certain elliptical galaxies, called radio galaxies, because of their strong radio emission, are like dim, radio-loud quasars. The first of these peculiar galaxies was discovered in 1918 by Heber Curtis. His short-exposure photograph of the giant elliptical M87 revealed a bright, star-like nucleus with a protruding jet. Nonthermal Radiation The particular type of nonthermal light emitted from the jet is synchrotron radiation (like that of the beams of pulsars). Synchrotron radiation is produced by relativistic electrons traveling in a strong magnetic field. As the electrons spiral around the magnetic field, they emit radiation. The presence of synchrotron radiation coming from M87 s jet indicates that relativistic particles are being ejected from the galaxy s nucleus and encountering a magnetic field. 11

12 Double Radio Sources Radio galaxies generally consist of two radio lobes. The lobes usually span a distance that is 5 to 10 times the size of the parent galaxy, which is almost always an elliptical galaxy. They are sometimes referred to as Double Radio Sources. The spectra of radio galaxies show the characteristics of synchrotron radiation. They should have jets of relativistic particles. Some appear to have a head of concentrated radio emission, with a weaker tail trailing behind it. Active Galaxies Because of the many properties they share, quasars, blazars, Seyfert galaxies, and radio galaxies are collectively called Active Galaxies. The activity of such a galaxy comes from an energy source at its center. Hence, astronomers say that these galaxies possess active galactic nuclei (AGN). All of these objects show brightness variations some as short as 3 hours. These fluctuations place strict limits on the maximum size of a light source, because an object cannot vary in brightness faster than light can travel across that object. For example, an object that is one light-year in diameter cannot vary significantly in brightness over a period of less than one year. 12

13 Variation Limits Imagine an object that is one-light year across. Suppose the entire object emits a brief flash of light. Photons from the nearest part arrive first. Photons from the middle arrive six months later. Finally, light from the far side arrives a year after the first photons. Although the object emitted a sudden flash of light, we observe a gradual increase in brightness that lasts a full year. The flash is stretched out over an interval equal to the light travel time across the object s diameter. Supermassive Black Hole Cores The most widely accepted model at the present time is that AGNs derive their energy output from an enormous black hole (billion solar masses) at the center of what would otherwise be a normal galaxy. Given such a massive black hole, then relatively modest amounts (10 solar masses per year) falling onto the black hole would be adequate to produce as much energy as a thousand normal galaxies. 13

14 Unified Model Accretion onto a supermassive black hole is the most likely explanation of the immense energy output of active galactic nuclei. The Unified Model explains different types of active galaxies as just being different views of one type of object. In this model, at the heart of an active galaxy is a supermassive black hole surrounded by an accretion disk. Viewing Angle The unified model offers a single, simple explanation for all types of active galaxies. The main difference between double radio sources, radio-loud quasars, and blazars is the viewing angle of the black hole central engine. 14

15 Unified Model According to Kepler s Third Law, the inner regions of this accretion disk would orbit the hole more rapidly than would the outer parts. Thus, the inner parts would rub against the outer gas. This friction would cause the gases to lose energy and spiral inward toward the black hole. Unified Model As the gases move inward, they are compressed and heated to high temperatures. This causes the accretion disk to glow, thus producing the brilliant luminosity of an AGN. Any variations in the density of the gas will cause the luminosity to fluctuate. 15

16 Radio Jets Quasars and other active galaxies emit jets that extend far beyond the limits of the parent galaxy. The radio radiation is synchrotron emission, which indicates that strong magnetic fields and electrons moving at speeds approaching that of light are present. It is presumed that the jets originate from the vicinity of the black hole. Jets are formed by matter in the accretion disk moving inward toward the black hole. Some of this matter will not fall into the black hole but will feed the jets. In other words, some of the material will be blown out into space along the rotation axis of the black hole in a direction perpendicular to the plane of the accretion disk. Why No Nearby Quasars The accretion-disk idea also helps to explain why there are no nearby quasars. Over time, most of the available gas and dust surrounding a quasar s central engine is accreted onto the black hole. The central engine becomes starved of new infalling matter to act as fuel, and the quasar becomes less active. With the passage of time, a quasar might turn off altogether. As a result, there are essentially no quasars in the presentday Universe. 16

17 Evolution of Quasars The energy output of radio galaxies is about the same as that of Seyferts. These two types of galaxies bridge the gap between quasars and normal galaxies. Seyfert galaxies may be the nearby relics of radio-quiet quasars, whereas Radio galaxies may be the remnants of radio-loud quasars. Thus, as quasars have become extinct, they may have evolved into Seyfert and radio galaxies. Quasars Seyfert galaxies Normal spirals Quasars Radio galaxies Normal ellipticals 17