Quasars

1. Quasars • In 1963 Martin Schmidt was trying to understand some unidentified lines in the optical spectra from a star that had a strong radio signal • He realized that the lines were Balmer lines that were normally in the UV that had been shifted to the visible by the stars veleocity of recession which was about 15% of the speed of light • This “star” turned out to be very distant and not a star at all • Quasi-stellar object - quasar • Thousands of quasars have been found and they all show very large redshifts • The largest shows / = 5 • 94% of the speed of light! Lecture 22

2. The Luminosity Puzzle • The very large redshifts of quasars means that they are very far away and because we can see them they must be very luminous • The quasars also seemed to vary in luminosity over a period of months • This luminosity variation suggested that the quasars were large objects Lecture 22

3. Distances to Quasars • To figure out how far away quasars are astronomers looked for quasars associated with galaxies that to which they could measure the distance • This is difficult because quasars outshine entire galaxies by a lot • Then astronomers used previous techniques to measure the distance to the galaxy and hence the quasar Lecture 22

4. Seyfert Galaxies The central region of Seyfert galaxy NGC 1068 • Active galaxies produce abnormal amounts of energy, mostly in their centers • Active galactic nuclei, AGN • One example is Seyfert galaxies • Seyferts are spiral galaxies • Seyfert galaxies produce emission lines rather than absorption lines indicating hot gas clouds • Seyfert galaxies have luminosity variations on the scale of months like quasars and have point-like bright centers that are brighter than the sum of individual stars Lecture 22

5. Active Elliptical Galaxies • Elliptic galaxies are also observed to have active nuclei • Elliptic galaxy M87 has such an active center • Jets of ionized gas are visible coming from the center Lecture 22

6. The Power Behind Quasars • Astronomers are now convinced that quasars have massive black holes at their centers • We can say a black hole exists if we can demonstrate that there is a very massive object such that no star or cluster could account for that mass • We can determine the mass of an object using Kepler’s law as before • We simply measure the period of an object orbiting the quasar and calculate the mass Lecture 22

7. The Mass of the Quasar in M87 • The period of material orbiting the center of M87 was calculated by measuring the redshift of material circling the center • The rotation speed was measured to be 550 km/s • Using Kepler’s law we get a mass of about 2.5 billion Msun • There is evidence for black in the center of many galaxies • Once formed, these black holes continue to absorb material and grow Lecture 22

8. Radio Jets • Material falling into a black hole forms an accretion disk • Models show that these accretion disks can lead to jets along the axis of the disk • These jets glow with radio, light, and x-rays Lecture 22

9. Evolution of Quasars • When we see a distant object we see it as it was long ago • If we see more quasars far away, there must have been more quasars long ago • The theory is that quasars are black holes with enough fuel around them to make bright accretion disks • This theory leads to the conclusion that quasars should have formed early in the history of the universe • This theory leads to the conclusion that quasars should have formed early in the history of the universe Lecture 22

10. Gravitational Lenses and Quasars • The quasars brilliance and immense distance makes it ideal for the study of deep space • Gravitational lensing was first discovered using quasars in 1979 when identical images of a quasar were observed • On the right one can see four identical optical images of a quasar (top) and an “Einstein ring” of a quasar made with radio waves using the VLA Lecture 22

11. The Distribution of Galaxies in Space • Looking at distant galaxies is like looking back in time • When we look at astronomical objects we find they are seldom alone • The question arises: do galaxies cluster also? • Hubble used the 100 inch Palomar telescope to sample the sky in 1283 places • He found that number of galaxies visible is about the same • He found that the number of galaxies increased with faintness • More evidence for a constant density of galaxies Lecture 22

12. The Cosmological Principle • The universe is the same everywhere • The universe appears to be isotropic and homogeneous • Without the cosmological principle, we could not make progress in understanding the universe around us • Hubble had simply counted the number of galaxies • Recently astronomers have measure the distances of thousands of galaxies and have built up a picture of the distribution of galaxies Lecture 22

13. The Local Group • The Milky Way is a member of a small group of galaxies called the Local Group containing more than 40 members Lecture 22

14. Neighboring Groups and Clusters • Galaxies form clusters • Rich galaxy cluster have thousands of galaxies • The nearest rich galaxy cluster is called the Virgo Cluster • An much larger galaxy cluster is the Coma cluster with a diameter of 10 million LY • Large galaxy clusters such as Coma have few spirals in the center but have many ellipticals Lecture 22

15. Superclusters and Voids • Galaxy clusters form superclusters • Among the superclusters are giant voids • The Milky Way is located in the Local Supercluster • One conclusion we can draw is the space is mostly empty • The clusters occupy only about 5% of the space Lecture 22

16. Slices of the Universe • Enormous volumes of space lie beyond the Local Supercluster • This space has not been completely mapped • One striking structure that has been found is the “Great Wall” • There are obvious sheets and filaments separated by huge voids Lecture 22

17. When Did Galaxies Form? • We can study old, distant galaxies and get information about times near the beginning of the universe • Most galaxies we can see are least a few billion years old • We can learn about a galaxy by measuring its color • Blue means young, hot stars • Yellow or red means old stars • Another way to learn about a galaxy is to study its shape • Spiral galaxies are young • Elliptical galaxies are old Lecture 22

18. The Ages and Compositions of Galaxies • Nearly all galaxies are old • The Milky Way contains stars that about the age of the universe • Distant galaxies show evidence for heavier elements that were not present at the beginning of the universe • A least one generation of stars has passed • Star formation has stopped in elliptical galaxies while it continues in spiral galaxies • Elliptical are poor in interstellar gas but galaxy clusters have a large amount • Galaxies in clusters collide! Lecture 22

19. Colliding Galaxies • Galaxies can collide which stimulates star formation • Indivdual stars are not affected much because of the large distance between stars Lecture 22

20. The Life History of Galaxies • Elliptical galaxies formed early and turned all the gas and dust to stars in the first 3 billion years • Spirals converted gas and dust at a much slower rate and are still producing stars today • The peak of star formation occurred when the universe was between 3 and billion years old • When the universe was 3 - 6 billion years old, the galaxies were small • Galaxies have merged to form larger galaxies since that time Lecture 22

21. Cosmology • The study of the universe as a whole is called cosmology • How did the universe come into being? • What will its ultimate fate be? • What have we observed about the universe? • All galaxies show a redshift proportional to distance, implying that the universe is expanding • The distribution of galaxies on the largest scale is isotropic and homogeneous • The contents of the universe evolve with time: hydrogen and helium are changed into heavier elements inside stars • Gravity warps the fabric of space-time Lecture 22

22. The Age of the Universe • The universe cannot be static • The universe must either be contracting or expanding • If we had a movie of the expanding universe and ran it backwards, we would see the galaxies moving together until they were all in one place • The big bang • We can estimate how long the galaxies would take to be back in the same place • v = Hd, Hubble’s Law • From physics we know d = vt • t = d/v = d/(Hd) = 1/H • Hubble time • H = 20  2 km/s per million LY • t = 15  1.5 billion years Lecture 22

23. Estimates of the Age of the Universe • The estimates of the age of the universe depend on our knowledge of the distance of galaxies • An error of a factor of 2 in the distance would mean a factor of 2 in the age of the universe • Over the past 20 years debate has raged among astronomers about the value of H • It has varied from 15 to 35 km/s per million LY • Recent data from the Hubble Space Telescope have yielded • 20  2km/s per million LY • 70  7 km/s per million parsecs Lecture 22

24. Deceleration/Acceleration • The Hubble time is the correct age of the universe only if this expansion has been constant throughout the age of the universe • Constant H • Gravity creates attraction and should slow the expansion of the universe • Deceleration • The universe would actually be younger than the Hubble time • New measurements of type Ia supernovae can interpreted to mean that the universe is accelerating • The universe is expanding more slowly now than in the past • The universe would be older than the Hubble time • Based on the observation that distant type Ia supernovae are 20% dimmer than they should be if expansion were constant • Other explanations? Lecture 22

25. The Age of the Universe • Astronomers generally agree that the modifications to the Hubble time for acceleration and deceleration make the age of the universe 15  5 billion years • Another way to estimate the age of the universe is to find the oldest objects whose age we can measure • Computer models show that the age of globular clusters is about 13 billion years and assuming that it took a billion years for stars to form, the oldest stars are younger than the age of the universe • This agreement has only occurred in recent years • Previously the Hubble time was shorter • Previously the age of globular clusters was longer Lecture 22

26. The Geometry of Spacetime • The gravity from the matter of the entire universe warps spacetime • We must consider a fourth dimension of space • Thinking in 4 dimensions is difficult so we will think in 3 dimensions (2 + 1) • In the world of 2 dimensions the third dimension if curvature • A 2-dimensional observer would observe odd things in his 2-dimensional curved world • Let’s use a balloon as an example Lecture 22

27. A Balloon Universe • If you go in one direction, you get back to where you started • There is no center, all points on the balloon are the same • If the balloon grows, all points move away from each other • The points move away from other other because the balloon is growing, not because any point is doing something special • This example represents a closed universe • An open universe is also possibility but harder to visualize Lecture 22

28. The Expanding Universe • If the universe is dense enough, it will stop expanding and collapse • If the universe not dense enough, it continue to expand forever • At a critical density, the universe will just stop expanding at infinite time • Closed universe • Open universe • Universe with critical density • Universe with less than critical density and positive cosmological constant Lecture 22

29. Facing the Future • If the mass density of the universe is high enough, the expansion of the universe will reverse and the universe will collapse • The Big Crunch • If the mass density of the universe is low enough, the universe will expand forever and slowly die out • At critical density, the universe can just barely expand forever • Flat universe • Zero curvature Lecture 22

30. Who’s Winning? • The observed matter density is too low to close the universe • Dark matter may play a role • The ages of stars suggest that we live in an open universe • Type Ia supernovae suggest that the universe if accelerating • The lookback time is how long ago the light from an object was emitted • Depends on our model of the universe Lecture 22

31. The Big Bang • The big bang theory states that the universe began as a gigantic explosion • This idea has entered popular culture Lecture 22

32. History of the Idea of the Big Bang • Georges Lemaitre proposed a big bang-like theory in the early 1920s involving fission • In the 1940s, George Gamov proposed the a big bang model incorporating fusion • Since that time, many astronomers and physicists have added their work to what is now known as the standard model of the big bang • Three main ideas underlie the big bang model • The universe cools as it expands • In very early times, the universe was mostly radiation • The more hotter the universe, the more energetic photons are available to make matter and anti-matter Lecture 22

33. The Evolution of the Early Universe • With the three previous ideas in mind, we can trace the evolution of the universe back to when it was 0.01 s old and had a temperature of 100 billion K • We can go back farther but not all the way to zero time • At 10-43 s most of our physical laws become impractical • At times before 0.01 s, the universe was filled with quarks and gluons Lecture 22

34. After 0.01 s • Our picture after 0.01 s is that the universe was filled with radiation and with types of matter that exist today • Protons and neutrons • Photons and neutrinos • The temperature was no longer hot enough to create neutrons and protons in collisions of photons • At about 3 minutes, nuclei begin to form • 75% hydrogen, 25% helium, some lithium Lecture 22

35. Learning from Deuterium • All the deuterium in the universe was formed in the first 3 minutes • If the universe was very hot and dense when the deuterium formed, it would have been broken up • If the universe expanded and then out thinned out rapidly, deuterium would survive • The density extracted from the surviving deuterium is 5 x 10-31 g/cm3 • Suggests a low enough mass that the universe is open • Dark matter may still play a role Lecture 22

36. The Universe Becomes Transparent • For several hundred thousand years the universe resembled the interior of a star • After that time, atoms began to form • The universe became transparent • Radiation and matter decoupled • 1 billion years after the big bang, stars and galaxies began to form • The radiation from the big bang faded but it left an indelible fingerprint, the cosmic radiation background Lecture 22

37. The Cosmic Radiation Background • In the 1940s Adler and Herman realized that just before matter and radiation decoupled, the universe must have been radiating like a blackbody at a temperature of 3000 K • That was 15 billion years ago, and the universe has expanded, leaving an afterglow of the big bang with a temperature of 3 K • In the 1960s, Penzias and Wilson were using a microwave antenna to study the sky • They could not make their receiver work without background noise that seemed to be coming from everywhere in the sky • They thought is was their detector but soon realized that it was real and was coming from space • Penzias and Wilson got in touch with some cosmologists from Princeton and who realized that this radiation was the cosmic background radiation (CBR) Lecture 22

38. Properties of the CBR • The first accurate measurements of the CBR were made by the COBE satellite • They observed that the CBR matched perfectly with a blackbody with a temperature of 2.73 K • Astronomers concluded that the universe we see today evolved from a hot, uniform state • We live in an evolving universe • The universe looks uniform in all directions but not completely uniform Lecture 22

39. Problems with the Big Bang Model • The standard big bang model explains many things but there are remaining issues • It does not explain why there is more matter than antimatter in the universe • It does not explain the observed uniformity of the universe • Parts of the universe could never have been in contact yet they show the same background temperature • It does not explain why the density of the universe is close to the critical density Lecture 22

40. Grand Unified Theories • There are 4 forces • Gravity, weak, electromagnetic, nuclear • At high temperatures, these forces become one force • Theories exist that unify weak, electromagnetic and nuclear • Grand unified theories (GUTs) • No theory yet exists incorporating gravity Lecture 22

41. The Inflationary Hypothesis • GUTs predict that at 10-35 s, a rapid, early expansion took place • Prior to this inflation, the universe was small enough to communicate with itself • After inflation, parts of the universe were beyond each other’s horizon • The inflationary model also predicts that the universe is exactly at critical density Lecture 22

42. Lucky Accidents • The temperature of the radiation emitted when the universe became transparent varies by about 16 millionth of a K • Smaller variations would have led to no galaxies • Larger variations would have led to black holes • The fine balance between expansion and contraction • The existence of only matter and not anti-matter • The production rate of nuclei in the big bang produced only hydrogen and helium, and did not go all the way to iron • Neutrinos have to have just the right interaction properties with matter to allow supernovae • Anthropic principle Lecture 22