Quasars and Other Active Galaxies

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Quasars and Other Active Galaxies

FIGURE 17-8 Seyfert Galaxy NGC 1566 This Sc galaxy is a Seyfert galaxy some 50 Mly (16 Mpc) from Earth in the southern constellation Dorado (the Goldfish). The nucleus of this galaxy is a strong source of radiation whose spectrum shows emission lines of highly ionized atoms. (Anglo- Australian Observatory)

3C-273 – The brightest Quasar around! If 100 light years away (~ Pollux in Gemini) it would be as bright as the Sun is to us now!

3 BILLION solar masses of material

Active galaxies are brighter, and emit more non-stellar radiation

In this chapter you will discover… 5-10% of galaxies unusually bright, called active galaxies The most distant objects we can see: quasars Supermassive black holes are central engines for radio galaxies, quasars, Seyfert galaxies, & BL Lac objects Distribution of Quasars is a key to the evolution of the Universe, and to the Big Bang Theory of its formation.

Active Galaxy Types Seyfert Galaxies: Spirals with very bright centers Radio Galaxies: Ellipticals with huge emissions of radio energy in “lobes” Quasars: Very tiny, distant objects All have very bright, active galactic nucleii

Active Galactic Nuclei Seyfert Galaxies spiral galaxies with incredibly bright, star-like center (nucleus) they are very bright in the infrared their spectra show strong emission lines Circinus

Active Galactic Nuclei Seyfert Galaxies The luminosity can vary by as much as the entire brightness of the Milky Way Galaxy!!

Radio Galaxies

FIGURE 17-1 Cygnus A (3C 405) Radio image produced from observations made at the Very Large Array. Most of the radio emissions from Cygnus A come from the radio lobes located on either side of the peculiar galaxy seen in the inset, a Hubble Space Telescope image. Each of the two radio lobes extend about 160,000 light-years from the optical galaxy and contain a brilliant, condensed region of radio emission. Inset: At the heart of this system of gas lies a strange-looking galaxy that has a redshift that corresponds to a recessional speed of 5% of the speed of light. According to the Hubble law, Cygnus A is therefore 635 million light-years from Earth. Because Cygnus A is one of the brightest radio sources in the sky, this remote galaxy’s energy output must be enormous. (R. A. Perley, J. W. Dreher, J. J. Cowan, NRAO; inset: William C. Keel, Robert Fosbury) Cygnus A Radio Image 635 Million light years away, and still one of the brightest radio sources in the entire sky!

Active Galactic Nuclei Radio Galaxies | galaxies – usually giant ellipticals - which emit large amounts of radio waves the radio emission come from lobes on either side of the galaxy; not the galaxy itself

Radio Galaxy Lobes These lobes are swept back because the galaxy is moving through an intergalactic medium.

FIGURE 17-10 Head-Tail Source NGC 1265 This active elliptical galaxy is moving at a high speed through the intergalactic medium. Because of this motion, the two tail jets trail the galaxy at its head, giving this radio source a distinctly windswept appearance. (NRAO)

X-ray/Radio Image of Centaurus A X-ray is blue; radio is red

FIGURE 17-9 Peculiar Galaxy NGC 5128 (Centaurus A) This extraordinary radio galaxy is located in the constellation Centaurus, 11 million light-years from Earth. At visible wavelengths a dust lane crosses the face of the galaxy. Superimposed on this visible image is a false-color radio image (green) showing that vast quantities of radio radiation pour from matter ejected from the galaxy perpendicular to the dust lane, along with radio emission (rose-colored) along the dust lane, and X-ray emission detected by NASA’s Chandra X-ray Observatory (blue). The X rays may be from material ejected by the black hole or from the collision of Centaurus A with a smaller galaxy. Inset: This X-ray image from the Einstein Observatory shows that NGC 5128 has a bright X-ray nucleus. An X-ray jet protrudes from the nucleus along a direction perpendicular to the galaxy’s dust lane. (X ray: NASA/CXC/M. Karovska et al.; radio 21-cm: NRAO/VLA/J. Van Gorkom/Schminovich, et al.; radio continuum: NRAO/VLA/J. Conden, et al.; optical: Digitized Sky Survey U.K. Schmidt Image/STScI; inset: X ray—NASA/CXC/Bristol U./M. Hardcastle; radio—NRAO/VLA/Bristol U./M. Hardcastle)

Superbright Elliptical Galaxy BL-Lac Objects Superbright Elliptical Galaxy FIGURE 17-12 BL Lacertae This photograph shows fuzz around BL Lacertae (arrow). The redshift of this fuzz indicates that BL Lacertae is about 900 Mly (280 Mpc) from Earth. BL Lac objects appear to be giant elliptical galaxies with bright quasar-like nuclei, much as Seyfert galaxies are spiral galaxies with quasar-like nuclei. BL Lac objects contain much less gas and dust than Seyfert galaxies. (T. D. Kinman, NOAO/AURA)

Quasars FIGURE 17-2 Quasar 3C 48 For several years, astronomers erroneously believed that this object is simply a peculiar, nearby star that happens to emit radio waves. Actually, the redshift of this star like object is so great that, according to the Hubble law, it must be roughly 4 billion light-years away. (Alex G. Smith, Rosemary Hill Observatory, University of Florida) A peculiar Star-like object, emitting lots of radio waves? But not with a stellar spectral fingerprint!

Quasar Spectra Star-like objects spectra that look nothing like a star Faint Hydrogen lines… VERY red-shifted!

FIGURE 17-4 Spectra of 3C 273 The visible and infrared spectra of 3C 273 are dominated by four bright emission lines caused by hydrogen. This radiation is redshifted nearly 16% from its rest wavelengths.

Quasar Observations emit light at all wavelengths A hot dense source? occasionally VERY strong radio sources Associated with jets from galaxies in clusters

FIGURE 17-3 Quasar 3C 273 This combined X-ray and infrared view shows the star like object associated with the radio source 3C 273 and the luminous jet it has created. The jet is also visible in the radio and visible parts of the spectrum. By 1963, astronomers determined that the redshift of this quasar is so great that, according to the Hubble law, it is nearly 2 billion light-years from Earth. (S. Jester, D. E. Harris, H. L. Marshall, K. Meisenheimer, H. J. Roser, and R. Perley)

Quasars Brightness varies in time! FIGURE 17-7 Brightness of 3C 279 This graph shows variations in the brightness of the quasar 3C 279. Note the especially large outburst observed in 1937. These data were obtained by carefully examining old photographic plates in the files of the Harvard College Observatory.

Quasars Brightness varies in time! FIGURE 17-7 Brightness of 3C 279 This graph shows variations in the brightness of the quasar 3C 279. Note the especially large outburst observed in 1937. These data were obtained by carefully examining old photographic plates in the files of the Harvard College Observatory.

Quasars Show enormous redshifts  VERY far away by Hubble’s Law Show extreme variability  VERY small, in scales of a light-hours to light years ….and so…. Quasars must be some of the most powerful objects we know of in the universe!

Quasar Distribution Seen at greatest distances (earliest history of the universe!) Not seen nearby…. But… Quasar behavior in some nearer clusters FIGURE 17-5 Spectrum of a High-Redshift Quasar The light from this quasar, known as PKS 2000-330, is so highly redshifted that spectral emission lines normally in the far-ultraviolet (L and L) are seen at visible wavelengths. Note the many deep absorption lines on the short-wavelength side of L. These lines, collectively called Lyman-alpha forest, are believed to be created by remote clouds of gas along our line of sight to the quasar. Hydrogen in these clouds absorbs photons from the quasar at wavelengths less redshifted than the quasar’s L emission line.

FIGURE 17-6 History of Quasar Formation The greater the redshift of a quasar, the farther it is from Earth and the farther back in time we are seeing it. By observing the number of quasars found at different redshifts, astronomers can calculate how the density of quasars has changed over time.

A theoretical model quasar Must account for observations: Small Size Enormous energy output across spectrum Source of Jets Similar behavior in galaxies in clusters Some radio synchrotron emission (indicating magnetic field) Full spectrum emission

A quasar model… Supermassive Black Hole Engine Formed as Galaxies are born… Pulling in gas, dust, and stars into huge accretion disk

A quasar model… Generating jets of X-ray radiation for millions of years “Quiet down” as fuel diminishes “Re-ignited” during collisions & mergers of galaxies in clusters

Quasar Energy Source? The energy is generated from matter falling onto a supermassive black hole… 1.2 x 109 M for NGC 4261 3 x 109 M for M87 …which is at the center (nucleus) of the galaxy.

Quasar Energy Source? Matter swirls through an accretion disk before crossing over the event horizon. Gravitational energy lost like E = mc2 10 – 40% of this is radiated away Process is very efficient

A quasar model… Works to explain quasars and… Even Supermassive Black holes are TINY Accretion Disk generates thermal spectrum, jets, magnetic fields Highly variable as mass is pulled in and… Works for active galaxies, too!

Theory FIGURE 17-16 Supermassive Black Holes as Engines for Galactic Activity (a) In the accretion disk around a supermassive black hole, in-swirling gas heats and expands. Pulled inward, compressed, and heated further, some of it is eventually expelled perpendicular to the disk in two jets. (b) The giant elliptical galaxy NGC 4261 is a double-radio source located in the Virgo cluster, about 100 million light-years from Earth. An optical photograph of the galaxy (white) is combined with a radio image (orange and yellow) to show both the visible galaxy, which does not emit much radio energy, and its jets, which do. Inset: This Hubble Space Telescope image of the nucleus of NGC 4261 shows a disk of gas and dust about 800 ly (250 pc) in diameter, orbiting a supermassive black hole. (b: NASA; inset: ESA) Observation

FIGURE 17-11 Binary Head-Tail Source This combined radio and X-ray image of 3C 75 shows the head-tail sources emanating from supermassive black holes in a pair of galaxies that are in the process of merging. The black holes are separated by 25,000 light-years and are 300 million light-years away from Earth. (X-ray: NASA/CXC/D. Hudson, T. Reiprich, et al. [AIFA])

FIGURE 17-18 Focusing Jets by Magnetic Fields The hot, ionized accretion disk (red-yellow) around the black hole rotates and creates a magnetic field that is twisted into spring-shaped spirals above and below the disk. Some of the accretion disk’s gas falling toward the black hole is overheated and squirted at high speeds into the two tubes created by the magnetic fields. The fields keep the gas traveling directly outward from above and below the disk, thus creating the two jets.

Quasar Jet Formation Magnetic fields twisted Pull charged particles out of disk & accelerate like slingshot Particles bound to magnetic field; focused in a beam

FIGURE 17-19 Orientation of the Central Engine and Its Jets BL Lacertae objects, quasars, double-radio sources, and active galaxies appear to be the same type of object viewed from different directions. If one of the jets is aimed almost directly at Earth, we see a BL Lac object. If the jet is somewhat tilted to our line of sight, we see a quasar. Tilted farther and we see an active galaxy. If the jets are nearly perpendicular to our line of sight, we see a double-radio source. The central region of the system is shown in Figures 17-16a and 17-18.

Model Quasar Accounts for Other Observations, too Orientation determines what we see: if beams points at us, see a quasar if not, molecular clouds/dust of galaxy block view of nucleus we see a radio galaxy or Seyfert lobes are where jets impact intergalactic medium

Hubble space telescope shows us that quasars do live in galaxies…they are Active Galactic Nuclei!

If the theory is right --- all galaxies start with Black Holes! FIGURE 17-15 Sombrero Galaxy (M104) This spiral galaxy in Virgo is nearly edge-on to our Earth-based view. Spectroscopic observations indicate that a billion-solar-mass black hole is located at the galaxy’s center. You can see the bright region created by stars and gas that orbit the black hole. (NASA and the Hubble Heritage Team [STScI/AURA])

Evidence Quasars are distant? Hubble’s Law Association with Galaxies in clusters Gravitational Lensing FIGURE 17-20 Gravitational Lensing of Quasars Image from the Hubble Space Telescope that shows the gravitational lensing of a quasar in the constellation of Pegasus. The quasar, about 8 billion light-years from Earth, is seen as four separate images that surround a galaxy that is only 400 million light-years away. This pattern is called an Einstein cross. The diffuse image at the center of the Einstein cross is the core of the intervening galaxy. The physical effect that creates these multiple images is the same as that seen for galaxies, as depicted in Figure 16-30. (NASA/ESO)

One Quasar, Five Images Gravitational lensing by galaxies in the intervening cluster brings five images of a distant quasar in our direction. (The fifth image is behind the large, orange galaxy.) The cluster of galaxies is 7 billion light-years away, whereas the quasar is 10 billion light-years distant. Three images of the same galaxy are also visible. (ESA, NASA, K. Sharon [Tel Aviv University] and E. Ofek [Caltech])

Essay Questions to Know! What are active galaxies? How do active galaxies produce their energy? How do we know? What are quasars, and where are they found? What do they tell us about the universe?