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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) Cassiopeia A (Cas A) Cygnus A (Cyg A) Both Sgr A and Cas A are in Milky Way. Sgr A is the nucleus of our galaxy, Cas A was a supernova remnant Cyg A was another situation - no easy identification Galaxy Cygnus A This galaxy has a red shift corresponding to 6% the speed of light or 600 million light-years away 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). 21 cm radio image of Cygnus A source taken by VLA in 1994 Image of radio lobes spans about 500,000 light years 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. Cygnus A (3C 405) in a visible image 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 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. 3C 48 visible image : Believed by astronomers to be simply a very unusual star. 1962 another 3C273 “star” was The luminous discovered at jet 273 can be 3C seen and an This “star” also enlarged had image as well unidentifiable emission lines and also had a “jet” of bright gas streaming from one side of the “star” Then in 1963 Maarten Schmidt at CalTech identified the strange emission lines of 3C 273 as significantly red shifted lines of ordinary Hydrogen. 3C 273’s spectral lines are greatly redshifted This change implies a distance of 2 billion light years 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 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! Star-like Object 3C 48 This object that looks like a star must be enormously luminous - its redshift indicates it is 4 billion light years away!! 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! = 66 nm for the 410 nm line! If = 66 nm, then by Doppler Shift Eq. v c or written as v = c (3x108 m / sec)(66 x109 ) v 9 410 x10 Then v = 4.8 x 107 m/sec or 48,000 km/sec 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 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 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! 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. A quasar emits a huge amount of energy from a small volume Quasar 3C 279 Such rapid changes in brightness can only result from small objects (~ 5 light years) 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. When imaged by telescopes, Quasars appear very unimpressive...just points of light, as can be seen on the next image 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 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. This Quasar is the most distant object ever imaged. It is about 11 – 13 Billion light years distant. 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 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 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 Galaxies around quasars sometimes appear disturbed by collisions Radio galaxies contain active nuclei shooting out vast jets of plasma that emits radio waves Centaurus A - radio image superimposed on a visible image...note no light from radio lobes visible image Radio lobes 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 Radio galaxies don’t appear as quasars because dusty gas clouds block our view of accretion disk What is the power source for quasars and other active galactic nuclei? Accretion of gas onto a supermassive black hole appears to be the only way to explain all the properties of quasars This Quasar is the one of the most distant objects ever imaged. It is about 11 – 13 Billion light years distant. The current record for the most distant object imaged is the gravitationally lensed galaxy in this image ~ 13Bly 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 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” 21 cm radio image of Cygnus A source taken by VLA in 1994 Image of radio lobes spans about 500,000 light years Centaurus A - radio image superimposed on a visible image...note no light from radio lobes visible image Radio lobes 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 Xray. Some also emits “jets” of fast moving gas. • Core radiations often fluctuate rapidly ~ minutes. • Gas clouds moving very fast about core (104 km/sec. Energy Output Variation of Seyfert 3C 84 (Note the variations over about 1 year intervals) 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. Seyfert Galaxy NGC 5728 This galaxy has extraordinary infrared luminosity Seyfert galaxy NGC 1275 is actually two galaxies in collision. The HST image (right) shows about 50 small, bright, young globular clusters formed as a result of the collision. NGC 1275 is also a strong source of X rays and radio waves (bottom). Active Galaxy NGC 1275 (3C 84) HST Ground based image Rosat X-Ray Image Other Active Galaxies • BL Lacertae objects (BL Lacs) – featureless spectrum with a brightness that can vary by a factor of 15 times in a few months. BL Lacertae (900 Mly), brightness varies 15X in few months BL Lac objects appear to be giant elliptical galaxies with bright quasarlike nuclei. BL Lac objects contain much less gas and dust than Seyfert galaxies. Active galaxies lie at the center of most double radio sources NGC 1265 NGC 1265 is an active elliptical galaxy moving at a high speed through the intergalactic medium. v i s i b l e r a d i o v i s i b l e i n f r a r e d Some forms of “Peculiar” galaxies are also “Starburst” galaxies Many of these galaxies have large regions where intense star birth seems to be occurring. Examination of these (and other) types of galaxies can demonstrate the power of multi-spectral imaging for analyzing the properties of astronomical objects. As an example, we will look at M82, a galaxy in the constellation Ursa Major located 12 Million light years from us. It is a satellite galaxy of the much larger M81. Radio Image of M81 and its satellite M82. The image reveals the lanes of Hydrogen gas between these galaxies Visible radio A visible, long-exposure (~40 min.) from the Digital Sky Survey. The center is saturated to bring out the wispy halo around the galaxy. A true color image, short exposure <10 min. The diff. in color of the center and outer parts can be seen as well as dust/gas in galaxy A very short exposure (< 1 min.) clearly shows the bands of dust/gas throughout the galaxy. The red color of the center is due to scattering away the blue light by large dust/gas clouds. This is a composite image (4 different wavelengths combined) showing in much greater detail the dust/gas lanes throughout the galaxy. An interferometric 21 cm radio image ( the VLA in Az. and a UK radio telescope). The image shows the chaotic pattern of radiation emitted by the galaxy. This far infrared image shows the intense IR radiation from the immense dust/gas clouds in this galaxy. This galaxy is one of the brightest IR sources in the Universe. An ultraviolet image. UV is given off by large bright stars (must be young stars) indicating that this part of the galaxy is a region of intense starbirth activity. This x-ray image (ROSAT) shows that these emissions are localized to the center of this galaxy which is a very strong x-ray source Is there a single mechanism that can explain the intense energy emissions of Quasars, Radio Galaxies, BL Lacs, and Seyfert Galaxies? • small energy sources • ejecting jets of gas at tremendous speeds • radiates strongly at many wavelengths Giant Gas Clouds (surrounding the galaxy) Intergalactic gas jet Galaxy-M87 (which is actually quite large) Black Holes • A common choice is to assume the energy is emitted by the rapidly spinning accretion disk around a massive black hole. • Black Hole about size of earth’s orbit containing 100 Million or more solar masses • As long as it was “fed” more matter, radiation would continue Supermassive black holes lurk at the centers of some galaxies • High resolution spectroscopy allows astronomers to peak at the motion of gas near centers of galaxies • Some galaxies exhibit high-velocity jets of material leaving the center • Observations suggest that the centers of some galaxies are incredibly massive • All of this suggests the existence of supermassive black holes Image of center of NGC 4261 - disk is about 320 ly across radio visible Jets of matter ejected from around a black hole may explain quasars and active galaxies Jets of matter ejected from around a black hole may explain quasars and active galaxies From where you observe it might make all the difference ... Small black hole 4 - 6 MO perhaps created in supernova near center of the galaxy •Hole grows as dust/gas within galaxy falls into it •If large enough, the Black Hole could swallow entire stars and grow very massive, maybe millions of MO •If galaxy massive enough, or through encounters with other galaxies, could grow even more massive •As galaxy ages, available mass drops and activity diminishes Expect more such energetic cases in younger galaxies •Quasars are very distant, therefore we see them when very young •Black hole model is only consistent general explanation for most active galaxies •Consistency does NOT guarantee its correct! Model of the center of an Active Galaxy Gravitational lensing • Einstein’s General Relativity predicted that light rays can be bent by intense gravitational field. • Examination of distant quasars provided the first direct experimental evidence that such could occur in space. location of one image quasar location of second image Earth intervening galaxy First Observed Gravitational Lens : English Radio Telescope from 1972. Visual confirmation in 1979 HST Infrared Image of Einstein Ring Equivalent Radio Image showing multiple images of distant quasar Einstein Cross - gravitational lensing produces 4 images of a quasar 8 billion lys distance, the imaging galaxy is 400 million lys away Next slide shows 10 different gravitational lenses imaged by Hubble Space Telescope A gravitational lens distorts our view of things behind it A gravitational lens distorts our view of things behind it All methods of measuring cluster mass indicate similar amounts of dark matter Einstein rings - light from a distant quasar is bent into rings around the intervening galaxy A Gravitational Lensed Cloverleaf Measuring the timing variations for changes in the different images in a gravitationally lensed image has provided a powerful experimental confirmation of the validity of Einstein’s General Theory of Relativity. Dark matter map. Optical view of a small galaxy cluster (center of image). Analysis of the distortions this galaxy produces on images of more distant galaxies allows an estimate of the presence of dark matter around the galaxy. Lensing simulation Does dark matter really exist? Knowing the orbital speed, one can calculate the force of gravity necessary...which in turns tells how much mass is necessary to keep the sun in orbit This mass is about 1011 M0 inside the solar system’s orbit force of gravity solar system Consider a galaxy like our own....... Assuming most stars are smaller than the sun, gives about 400 billion stars inside the orbit of the sun. Measuring the mass of a galaxy is done as was discussed earlier, using the sun’s motion in the Milky Way. If we can measure the velocities of stars or gas within a galaxy (optical or radio wavelength Doppler shifts), then the mass can be estimated using Kepler’s Laws and the Law of Gravity. When this is done, the calculated mass ALWAYS exceeds the observable mass…..DARK MATTER! The existence of Dark matter was predicted in the 1930s by astronomer Fred Zwicky (who also predicted the existence of neutron stars) Zwicky had an eccentric personality and his ideas were not accepted by the astronomy community despite the careful detail with which the work was done. No one else was willing to work on the idea to independently confim/deny his results. The existence of Dark Matter is further suggested by looking at Galaxy Rotation Curves for all spiral galaxies, as we shall see next. In many galaxies the calculated mass exceeds the visible mass by a factor of 10. What can be seen constitutes only ~ 10% of the actual mass found in the galaxy. Using the measured doppler shifts, one can determine the rotation velocities of different parts of a galaxy forming a: galaxy rotation curve Using this data and a modified Kepler’s Third Law, one can estimate the mass of a galaxy If all the mass were uniformly distributed in the disk Rotation_merrygoround.htm If all the mass were concentrated in the center of the disk Actual curve for the Milky Way Galaxy Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter Dark Matter associated with a Spiral galaxy like ours Elliptical galaxies pose a problem for such rotation curve measurements since they have little hydrogen gas and do not produce detectable 21 cm radiation. The velocities of individual stars are disorganized and randomly oriented (unlike stars in spiral arms). The Doppler shifts measured are collective averages of many stars. The widths of the Doppler shifted peaks is determined by the speeds of the individual stars. The faster the stars, the broader the peaks These galaxies also have dark matter Doppler broadening peaks for Elliptical galaxies Our Options 1. Dark matter really exists, and we are observing the effects of its gravitational attraction 2. Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter Because gravity is so well tested, most astronomers prefer option #1 What might dark matter be made of? How dark is it? … not as bright as a star. Two Basic Options • Ordinary Dark Matter (MACHOS) – Massive Compact Halo Objects: dead or failed stars in halos of galaxies • Extraordinary Dark Matter (WIMPS) – Weakly Interacting Massive Particles: mysterious neutrino-like particles Two Basic Options • Ordinary Dark Matter (MACHOS) – Massive Compact Halo Objects: dead or failed stars in halos of galaxies • Extraordinary Dark Matter (WIMPS) – Weakly Interacting Massive Particles: mysterious neutrino-like particles The Best Bet MACHOs occasionally make other stars appear brighter through lensing MACHOs occasionally make other stars appear brighter through lensing … but not enough lensing events to explain dark matter Why Believe in WIMPs? • There’s not enough ordinary matter. WMAP results puts ordinary matter at 4% of universe • WIMPs could be left over from Big Bang • Models involving WIMPs may explain how galaxy formation works Thought Question What would you conclude about a galaxy whose rotational velocity rises steadily with distance beyond the visible part of its disk? A. B. C. D. Its mass is concentrated at the center It rotates like the solar system It’s especially rich in dark matter It’s just like the Milky Way What is the evidence for dark matter in clusters of galaxies? We can measure the velocities of galaxies in a cluster from their Doppler shifts The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars! Clusters contain large amounts of X-ray emitting hot gas Temperature of hot gas (particle motions) tells us cluster mass: 85% dark matter 13% hot gas 2% stars Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass Cluster CL0025+1654 4.5 Bly Dark matter is Blue Isolated neutron star -1st detected by its x-ray emissions has been imaged by the Hubble telescope, ~ 18 lys and traveling at about 100 km/sec. The End!