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Chapter 17

Chapter 17. Quasars and Active Galaxies and Other Ultahigh Energy Sources. What are quasars?. Radio Astronomy began in 1936 when amateur astronomer Grote Reber built a crude radio telescope in his back yard. By 1944 Reber had detected 3 very strong radio sources Sagittarius A ( Sgr A )

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Chapter 17

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  1. Chapter 17 Quasars and Active Galaxies and Other Ultahigh Energy Sources

  2. What are quasars?

  3. Radio Astronomy began in 1936 when amateur astronomer Grote Reber built a crude radio telescope in his back yard. By 1944 Reber had detected 3 very strong radio sources Sagittarius A (Sgr A) Cassiopeia A (Cas A) Cygnus A (Cyg A)

  4. Both Sgr A and Cas A are in Milky Way. Sgr A is thenucleus of our galaxy, Cas A was a supernova remnant Cyg A was another situation - no easy identification

  5. Galaxy Cygnus A This galaxy has a red shift corresponding to 6% the speed of light or 600 million light-years away

  6. 3C 405 refers to the object number 405 in the Third Cambridge Catalogue of radio sources When its overall radio intensity was determined, it was found to be more intense (about 1011 times) than an entire galaxy - such as M31 (Andromeda Galaxy).

  7. 21 cm radio image of Cygnus A source taken by VLA in 1994 Image of radio lobes spans about 500,000 light years

  8. Using Palomar 200” telescope a visible spectrum of Cyg A was taken. The observed redshifts gave a speed of 17,000 km/sec which implied (using Hubble’s Law) that the source was 750 Mly away.

  9. Cygnus A (3C 405) in a visible image

  10. It was astonishing that a source stronger than an entire galaxy such as M31 could be so far away....… This implied that Cyg A was an extraordinary object. Astronomers began to examine other 3C objects

  11. 1960 Alan Sandage at Palomar discovered a “star” at the location of source 3C 48. This “star” was very unusual in the fact that it had emission lines that could not be identified. It was also a strong radio emitter while normal stars are not strong radio sources.

  12. 3C 48 visible image : Believed by astronomers to be simply a very unusual star.

  13. 1962 another “star” was discovered at 3C 273 This “star” also had unidentifiable emission lines and also had a “jet” of bright gas streaming from one side of the “star” 3C273 The luminous jet can be seen and an enlarged image as well

  14. Then in 1963 Maarten Schmidt at CalTech identified the strange emission lines of 3C 273 as significantly red shifted lines of ordinary Hydrogen.

  15. 3C 273’s spectral lines are greatly redshifted This change implies a distance of 2 billion light years

  16. 3→2 4→2 5→2 6→2 7→2 8→2 9→2 transition Name Hα Hβ Hγ Hδ Hε Hζ Hη Wavelength (nm) 656.3 486.1 434.1 410.2 397.0 388.9 383.5 Hβ (lab) = 486.1 nm Hβ (meas) = 565 nm Δλ = 78.9 nm v = [Δλ/λ]c = 4.87 x 107 m/sec D = v/Ho = 4.87 x 104 km/sec ÷ 25 km/sec/Mly = 1,948 million light years ~ 2 billion lyrs

  17. The observed red shift for 3C 273 corresponded to a distance of 2 Billion light years. A similar analysis was done for object 3C 48 and the results indicated a distance of 4 billion light years!

  18. This object that looks like a star must be enormously luminous - its redshift indicates it is 4 billion light years away!! Star-like Object 3C 48

  19. Since it would be absolutely impossible to see even the brightest possible star at a distance this large, these objects could not be “stars” and were dubbed QUASARS (quasi-stellar-radio-objects). In fact, most Quasars are NOT strong radio sources! But the name has stuck!

  20.  = 66 nm for the 410 nm line!

  21. If  = 66 nm, then by Doppler Shift Eq. Then v = 4.8 x 107 m/sec or 48,000 km/sec

  22. By Hubble’s Law v = HoD where Ho = 75 km/sec/Mpc This gives a distance of D = v/Ho D = (48,000 km/sec) / 75 km/sec/Mpc D = 640 Mpc or 2090 Mly or 2.1 Billion Light Yrs

  23. Quasars look like stars but have huge redshifts object with a spectrum much like a dim star highly red shifted enormous recessional velocity huge distance (ala Hubble’s Law) must be enormously bright to be visible at such a great distance Quasi-stellar object - QSO or Quasar

  24. Galaxies are bright and very big.... The Milky Ways shines with the light of about 10 Billion suns. The largest elliptical galaxies have brightnesses about 10 or 100X this brightness. Quasars have brightnesses this large and larger!

  25. Light Variations Quasars have been observed to fluctuate in brightness with periods ranging from a few years to a few hours. Recall that this light variation places an upper limit on the size of the quasar’s energy source. If they are as distant as Hubble’s Law indicates then some mechanism must be producing energies greater than 100s of galaxies in a region about the size of our solar system.

  26. A quasar emits a huge amount of energy from a small volume Such rapid changes in brightness can only result from small objects (~ 5 light years) Quasar 3C 279

  27. Quasar Brightness • Most galaxies cannot be seen beyond 4 Billion light years • With a few of the very brightest barely visible at 8 Billion light yr. • Since more distant quasars are clearly visible • They must be much brighter than even the brightest galaxies.

  28. When imaged by telescopes, Quasars appear very unimpressive...just points of light, as can be seen on the next image

  29. This “dot of light” has a luminosity of 1000 Milky Way galaxies. It looks unimpressive because it is located at about 2 Billion light years distance The Quasar 3C48 we saw earlier is located at 4.2 Billion light years

  30. Today thousands of quasars have been observed with redshifts corresponding to speeds as high as 92% of the speed of light, giving distances of 10 - 13 Billion light years (ALL quasars lie at least 800 Million light years from the Milky Way). Since looking OUT in distance = looking BACK in time..... When we look at quasars we are seeing objects as they existed when the universe was very young.

  31. This Quasar is the most distant object ever imaged. It is about 11 – 13 Billion light years distant.

  32. If the center of a galaxy is unusually bright we call it an active galactic nucleus or (AGN) Quasars are the most luminous AGN Examples Active Nucleus in M87

  33. The highly redshifted spectra of quasars indicate large distances From brightness and distance we find that luminosities of some quasars are >1012 LSun Variability shows that all this energy comes from region smaller than solar system

  34. Thought Question What can you conclude from the fact that quasars usually have very large redshifts? A. They are generally very distant B. They were more common early in time C. Nearby galaxies might hold dead quasars D. Galaxy collisions might turn them on

  35. Galaxies around quasars sometimes appear disturbed by collisions

  36. Radio galaxies contain active nuclei shooting out vast jets of plasma that emits radio waves

  37. Centaurus A - radio image superimposed on a visible image...note no light from radio lobes visible image Radio lobes

  38. An active galactic nucleus can shoot out blobs of plasma moving at nearly the speed of light Speed of ejection suggests that a black hole is present

  39. Radio galaxies don’t appear as quasars because dusty gas clouds block our view of accretion disk

  40. What is the power source for quasars and other active galactic nuclei?

  41. Accretion of gas onto a supermassive black hole appears to be the only way to explain all the properties of quasars

  42. This Quasar is the one of the most distant objects ever imaged. It is about 11 – 13 Billion light years distant.

  43. The current record for the most distant object imaged is the gravitationally lensed galaxy in this image ~ 13Bly

  44. Active Galaxies bridge the energy gap between ordinary galaxies and quasars Many different types of so-called “active” or “peculiar” galaxies have been discovered in the last 30 years Besides Quasars, 3 interesting types are: Radio Galaxies Seyfert Galaxies Peculiar galaxies (pec) appear to be blowing themselves apart

  45. Radio Galaxies • Very bright in radio emissions (usually ellipticals) • Emissions come from core and gas “lobes” • Radio spectrum is “synchotron radiation” • high speed electrons moving in magnetic fields • Electron emission often in “jets”

  46. 21 cm radio image of Cygnus A source taken by VLA in 1994 Image of radio lobes spans about 500,000 light years

  47. Centaurus A - radio image superimposed on a visible image...note no light from radio lobes visible image Radio lobes

  48. Seyfert Galaxies • Spiral galaxies (mostly) with abnormally “bright” nucleus • Highly energetic core that is very small and emits more energy than entire milky way. With emissions in radio, optical, infrared, uv and X-ray. Some also emits “jets” of fast moving gas. • Core radiations often fluctuate rapidly ~ minutes. • Gas clouds moving very fast about core (104 km/sec.

  49. Energy Output Variation of Seyfert 3C 84 (Note the variations over about 1 year intervals)

  50. Seyfert NGC 1566 (50 Mly) This galaxy’s luminosity varies over a range of 700 million L0 in a few weeks, and has a strong source of radiation showing emission lines of highly ionized atoms.

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