Copyright © 2009 Pearson Education, Inc.Chapter 15Galaxies and the Foundationof Modern CosmologyCopyright © 2009 Pearson Education, Inc.15.4 Quasars and Other Active GalacticNucleiOur goals for learning:• What are quasars?• What is the power source for quasars andother active galactic nuclei?• Do supermassive black holes really exist?Copyright © 2009 Pearson Education, Inc.What are quasars?Copyright © 2009 Pearson Education, Inc.If the center of agalaxy isunusually bright,we call it anactive galacticnucleus.Quasars are themost luminousexamples.Active Nucleus in M87Copyright © 2009 Pearson Education, Inc.The highly redshifted spectra of quasars indicate large distances.From brightness and distance, we find that luminosities of somequasars are >1012LSun!Variability shows that all this energy comes from a regionsmaller than the solar system.Copyright © 2009 Pearson Education, Inc.Thought QuestionWhat can you conclude from the fact that quasars usuallyhave very large redshifts?A. They are generally very distant.B. They were more common early in time.C. Galaxy collisions might turn them on.D. Nearby galaxies might hold dead quasars.Copyright © 2009 Pearson Education, Inc.Thought QuestionAll of the above!What can you conclude from the fact that quasars usuallyhave very large redshifts?A. They are generally very distant.B. They were more common early in time.C. Galaxy collisions might turn them on.D. Nearby galaxies might hold dead quasars.Copyright © 2009 Pearson Education, Inc.Galaxiesaroundquasarssometimesappeardisturbed bycollisions.Copyright © 2009 Pearson Education, Inc.Quasars powerfully radiate energy over a very widerange of wavelengths, indicating that they contain matterwith a wide range of temperatures.Copyright © 2009 Pearson Education, Inc.Radio galaxies contain active nuclei shooting out vast jetsof plasma that emit radio waves coming from electronsmoving at near light speed.Copyright © 2009 Pearson Education, Inc.The lobes of radio galaxies can extend over hundreds ofmillions of light-years.Copyright © 2009 Pearson Education, Inc.An active galacticnucleus can shootout blobs ofplasma moving atnearly the speedof light.The speed ofejection suggeststhat a black holeis present.Copyright © 2009 Pearson Education, Inc.Radiogalaxiesdon’t appearas quasarsbecausedusty gasclouds blockour viewof theiraccretiondisks.Copyright © 2009 Pearson Education, Inc.Characteristics of Active Galaxies• Luminosity can be enormous (>1012LSun).• Luminosity can rapidly vary (comes from a spacesmaller than solar system).• They emit energy over a wide range of wavelengths(contain matter with wide temperature range).• Some drive jets of plasma at near light speed.Copyright © 2009 Pearson Education, Inc.What is the power source for quasarsand other active galactic nuclei?Copyright © 2009 Pearson Education, Inc.The accretion of gas onto a supermassive black hole appearsto be the only way to explain all the properties of quasars.Copyright © 2009 Pearson Education, Inc.• The gravitational potential energy of matter fallinginto a black hole turns into kinetic energy.• Friction in the accretion disk turns kinetic energyinto thermal energy (heat).• Heat produces thermal radiation (photons).• This process can convert 10–40% of E = mc2 intoradiation.Energy from a Black HoleCopyright © 2009 Pearson Education, Inc.Jets are thought to come from the twisting of a magneticfield in the inner part of the accretion disk.Copyright © 2009 Pearson Education, Inc.Do supermassive black holesreally exist?Copyright © 2009 Pearson Education, Inc.Orbits of starsat center ofMilky Waystars indicate ablack hole withmass of4 million MSun.Copyright © 2009 Pearson Education, Inc.Orbital speed and distance of gas orbiting center of M87indicate a black hole with mass of 3 billion MSun.Copyright © 2009 Pearson Education, Inc.• Many nearby galaxies—perhaps all ofthem—have supermassive black holes at theircenters.• These black holes seem to be dormant activegalactic nuclei.• All galaxies may have passed through a quasar-like stage earlier in time.Black Holes in GalaxiesCopyright © 2009 Pearson Education, Inc.Galaxies and Black Holes• The mass of agalaxy’scentral blackhole iscloselyrelated to themass of itsbulge.Copyright © 2009 Pearson Education, Inc.Galaxies and Black Holes• Thedevelopmentof a centralblack holemustsomehow berelated togalaxyevolution.Copyright © 2009 Pearson Education, Inc.What have we learned?• What are quasars?— Active galactic nuclei are very bright objectsseen in the centers of some galaxies, andquasars are the most luminous type.• What is the power source for quasars andother active galactic nuclei?— The only model that adequately explains ourobservations holds that supermassive blackholes are the power source.Copyright © 2009 Pearson Education, Inc.What have we learned?• Do supermassive black holes really exist?— Observations of stars and gas clouds orbitingthe centers of galaxies indicate that manygalaxies, perhaps all of them, havesupermassive black holes.Copyright © 2009 Pearson Education, Inc.Chapter 16Dark Matter, Dark Energy, andthe Fate of the UniverseCopyright © 2009 Pearson Education, Inc.16.1 Unseen Influences in the CosmosOur goals for learning:• What do we mean by dark matter and darkenergy?Copyright © 2009 Pearson Education, Inc.What do we mean by dark matterand dark energy?Copyright © 2009 Pearson Education, Inc.Dark matter: An undetected form of mass that emits little orno light but whose existence we infer from its gravitationalinfluenceDark energy: An unknown form of energy that seems to bethe source of a repulsive force causing the expansion of theuniverse to accelerateUnseen InfluencesCopyright © 2009 Pearson Education, Inc.• Normal matter: ~ 4.4%— Normal matter inside stars: ~ 0.6%— Normal matter outside stars: ~ 3.8%• Dark matter: ~ 22%• Dark energy: ~ 74%Contents of UniverseCopyright © 2009 Pearson Education, Inc.What have we learned?• What do we mean by dark matter and darkenergy?— Dark matter is the name given to the unseenmass whose gravity governs the observedmotions of stars and gas clouds.— Dark energy is the name given to whatevermight be causing the expansion of theuniverse to accelerate.Copyright © 2009 Pearson Education, Inc.16.2 Evidence for Dark MatterOur goals for learning:• What is the evidence for dark matter ingalaxies?• What is the evidence for dark matter
View Full Document
Unlocking...