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H205 Cosmic Origins APOD Today: Finish Galaxy Evolution Dark Matter EP 5 Two Public Lectures Einstein’s Biggest Blunder: A Cosmic Mystery Story Lawrence Krauss Arizona State Univ. Saturday, April 18 12:30 PM Jordan Hall 124 From the Big Bang to the Nobel Prize and on to the James Webb Space Telescope John Mather Goddard SFC Tuesday, April 21 7:30 PM Whittenberger, IMU Coma Centaurus Exploring Galaxy Evolution in Galaxy Clusters Clusters of galaxies aren’t the Galaxies biggest structures in the Universe gravity holds clusters together Perseus Hercules Distance Number of Spirals Number of Ellipticals Percentage of Ellipticals Nearby Clusters Coma 99 Mpc Perseus 75 Mpc Centaurus 3.7 Mpc 5 7 9 15 13 17 75 65 60 11 2 10 18 18 10 62 90 50 Distant Clusters Abell 851 1700 Mpc Abell 1689 343 Mpc MS 1054 03 355 Mpc DARK MATTER The universe is NOT what it seems… DARK MATTER • “Extraordinary claims require extraordinary evidence.” (Carl Sagan) • “Extraordinary claims require extraordinary proof.” (Marcello Truzzi) • “The weight of evidence for an extraordinary claim must be proportioned to its strangeness” (Laplace) • “A wise man, therefore, proportions his belief to the evidence” (David Hume) Evidence for Dark Matter Rotation of galaxies Velocities of stars in dwarf galaxies Galaxy interactions Velocities of galaxies in clusters Hot gas in galaxy clusters Collisions of galaxy clusters Gravitational lensing Galaxy Rotation Mass within Sun’s orbit: ~1011 MSun Total mass: ~1012 MSun What’s the PROBLEM??? • The orbits of stars suggest that galaxies contain several times more mass that we can find in stars, gas and dust • MISSING MASS! • Dark matter is the material believed to account for the discrepancy between the mass of a galaxy as found from the orbits of stars and the mass observed in the form of gas and dust The visible portion of a galaxy lies deep in the heart of a large halo of dark matter Rot Vel Grav mass lum Lum mass Lum /grav 2 Kpc 100 4.6e9 5e8 1e9 .22 4 Kpc 120 1.3e10 1.7e9 3.4e9 .25 6 Kpc 130 2.4e10 2.8e9 5.6e9 0.23 8 Kpc 130 3.1e10 3.7e9 7.5e9 .23 10 Kpc 165 6.3e10 4.5e9 9e9 0.14 Velocity Dispersions in Dwarf Galaxies Count the stars Add up the light Look for any gas Add up the mass Velocity Dispersions in Dwarf Galaxies • From spectra and the Doppler shift • Measure the velocity dispersion • Determine the total mass astro-ph/0704126 Calculated for a sample of 194 stars with 32-33 stars per bin M/L Ratios for MW Dwarfs Galaxy MV L Radius Total mass M/L (mag) (106 LSun) (pc) (106 MSun) (Sun=1) Gas Fraction Sculptor -11.1 2.15 110 6.4 3.0 0.004 Phoenix -10.1 0.90 310 33 37 0.006 Fornax -13.2 15.5 460 68 4.4 <0.001 Carina -9.3 0.43 210 13 31 <0.001 Leo I -11.9 4.79 215 22 4.6 <0.001 Sextans -9.5 0.50 335 19 39 <0.001 Leo II -9.6 0.58 160 9.7 17 <0.001 Ursa Minor -8.9 0.29 200 23 79 <0.002 Draco -8.8 0.26 180 22 84 <0.001 Galaxy interactions require more mass than we can see Computer simulation Antennae Galaxy (HST) The real thing 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! 85% dark matter 13% hot gas 2% stars A view of the Coma Cluster in optical light (left) and at X-ray (right, from Chandra) wavelengths Clusters contain large amounts of X-ray emitting hot gas Temperature of hot gas (particle motions) tells us cluster mass The mass is much more than gas and galaxies combined 1E 0657-56 – The Bullet Cluster Direct observation of Dark Matter More Evidence for Dark Matter • 1E 0657-56 – A collision of galaxy clusters • A cluster of galaxies consists of three components 1. Galaxies 2. Hot Gas 3. Dark Matter What’s going on with 1E 065756? • TWO clusters of galaxies collide The gas interacts, the dark matter and galaxies don’t • The galaxies and dark matter pass through unimpeded, but the hot gas is separated from the clusters Gravitational Lensing • Light from a distant, bright source is bends around a massive object (such as a massive galaxy or cluster of galaxies) between the source object and the observer • Gravitational lensing is predicted by Einstein's theory of general relativity (Einstein 1936) General Relativity • The lens phenomenon exists because gravity bends the paths of light rays • In general relativity, gravity acts by producing curvature in space-time • The paths of all objects, whether or not they have mass, are curved if they pass near a massive body • Prediction confirmed in the 1919 solar eclipse Discovering Gravitational Lenses • Mysterious arcs discovered in 1986 (a) Cluster Abell 370 (left) • cluster redshift z=0.37 • arc redshift z=0.735 (b) Cluster C12244 (right) • cluster redshift z=0.31 • arc redshift of 2.24 • Bright knots on the arcs show the structure of of the galaxies, whose images are strongly distorted • The influence of individual lensing cluster galaxies the arc can also be seen Three Classes of Gravitational Lenses • Strong lensing - easily visible distortions – Einstein rings, arcs, and multiple images The Einstein Cross • Weak lensing - distortions are much smaller – Detected by analyzing large numbers of objects to find distortions of only a few percent. – The lensing shows up statistically as a preferred stretching of the background objects perpendicular to the direction to the center of the lens • Microlensing - no distortion in shape can be seen but the amount of light received from a background object changes with time – Microlensing occurs with stars and extrasolar planets The Double Quasar – the first gravitational lens Unlike optical lenses, gravitational lenses produce multiple images • In an optical lens, maximum bending occurs furthest from the central axis • In a gravitational lens, maximum bending occurs closest to the central axis • A gravitational lens has no single focal point • If the source, the lens, and the observer lie in a straight line, the source will appear as a ring around the lens • If the lens is off-center, multiple images will appear. The lensed image will always be distorted Simulating Gravitational Lenses • HST MDS WFPC2 HST Gravitational Lens Simulation (mds.phys.cmu.edu/ego_cgi.html) source and lens aligned source and lens not aligned • A galaxy having a mass of over 100 billion solar masses will produce multiple images separated by only a few arcseconds • Galaxy clusters can produce separations of several arcminutes Arcs in the Galaxy Cluster Abell 2218 (z=0.175) cluster center • Several arcs surround the cluster center – Arc A0 has a redshift of 2.515; – Near A2 is another image of the same galaxy • More arcs surround a second mass concentration (upper right) • Multiple images of the same distant galaxies allows detailed model of the mass of the lensing cluster Cluster of Galaxies Cl0024+16 • The reddish objects are galaxies in the lensing cluster at z=0.39 • The bluish objects are multiple images of a distant galaxy at z=1.63 lensed by the cluster • Reconstruct the distant galaxy individual pieces of the arc Galaxy Cluster Cl1358+62 • The reddish arc is a lensed image of a background galaxy with z=4.92 – upper right - an enlarged version of the lensed galaxy – lower right - a reconstruction of the unlensed source Abell 2390 • A thick arc with z=0.913 • Two more arc systems are also seen (indicated by the letters A and B) – system A has redshift z=4.04 – system B has redshift z=4.05 The Bottom Line… • The visible matter does not provide enough gravity to produce the gravitational lenses we see from galaxies and galaxy clusters • Dark matter must be present to account for what we observe cluster center Arcs let us map the distribution of dark matter in clusters of galaxies All methods of measuring cluster mass indicate similar amounts of dark matter Dark Matter The universe contains matter we cannot see Dark matter interacts with normal matter through gravity Dark matter does NOT interact with light the way the normal matter does The Universe contains 5 or 6 times MORE dark matter than normal matter All galaxies are embedded in clouds of dark matter Alternative to Dark Matter: MOND Modified Newtonian Dynamics For accelerations a less than a0, reduce gravity acceleration by the factor a/a0 a(a/a0) = GM/r2 This gives flat rotation curves A single value of a0 works for galaxy rotation curves But MOND is untested experimentally MOND can‘t explain DM in clusters and far out in halos MOND can’t explain it all • While MOND can reproduce galaxy rotation curves, it is harder to explain – Galaxy cluster velocity dispersions – Observations of gravitational lenses – The Bullet Cluster and the DM ring • MOND still requires DM to account for all the observations • Which is a simpler explanation, DM or MOND+DM? Summary: Dark Matter Evidence Many dynamical phenomena cannot be explained with the observed mass content of the universe Problem can be solved with one radical assumption 85% of all matter is dark matter initially distributed as ordinary matter interacts with normal matter only through gravity Stars, gas are now more concentrated than dark matter Why is DARK MATTER important? The formation of structure and of galaxies requires the extra mass provided by dark matter. Without dark matter, the Universe would not exist as we know it Dark Matter Dominates the Structure of the Universe Center for Cosmological Physics, University of Chicago http://cosmicweb.uchicago.edu/index.html • The formation of clusters and filaments in a universe filled with cold dark matter • The box is 43 million parsecs (or 140 million light years) • Simulation begins at z=30 - the Universe is less than 1% of its current age and matter is uniformly distributed • Small fluctuations grow to large structures • Structures formed by z=0.5 The Evolution of Dark Matter Observed with Hubble • Dark matter filaments form under the pull of gravity, and clump • Dark matter filaments provide the structure for the formation of stars and galaxies from ordinary matter • Gravity from dark matter needed to form structures and galaxies Forming Galaxy Groups (like ours!) The formation and evolution of these groups, which are very common in the Universe, are dominated by the gravitational pull of dark matter 4.3 Mpc or 14 million LY • Formation proceeds hierarchically • Small-mass objects form at z>5, grow and merge, to form larger and larger systems • Galactic "cannibalism" ongoing • The two objects approaching at z~0 will merge in about a billion years • Many of the small systems become satellites orbiting larger systems Galaxy Formation • A disk galaxy forming when the virtual universe was "only" one and a half billion years old • The galaxy forms where several large-scale filaments of dark matter intersect • These filaments provide gas and dark matter to the galaxy • The gas fuels star formation, while the galaxy grows by accreting dark matter and smaller galaxies Dark matter provides the gravitational mass necessary for galaxy formation to proceed 36 kpc 72 kpc 144 kpc 288 kpc Galaxies Grow through Mergers Intergalactic gas Galaxy building blocks observed with Hubble Clumps concentrated by dark matter lead to galaxies Simulation The cosmic web of dark matter, gas, and galaxies in a young universe The real thing What is DARK MATTER? Can’t see it, taste it, touch it, smell it… We can only detect it by gravity We don’t know! Detecting Dark Matter is one of the most active areas of high energy physics, and a reason to build large accelerators. So, What Is Dark Matter? • Non-baryonic, to reconcile with primordial nucleosynthesis and large-scale structure growth • Slow Moving: must not escape from potential wells (slow moving = cold) • Dark Matter Candidates: – Black holes – Low-mass objects (“MACHO”s, free-floating planets) (but this stuff is baryonic) – Elementary particles Can Dark Matter Be Black Holes?? Plausible mass range: 6 ~10 Msun Such massive black holes cannot be the dark matter in dwarf galaxies That many BH’s in Draco would disrupt the galaxy! What about Big, Dark Rocks? Or Loose Planets? MACHO’s: Massive Compact Halo Objects Mass range: 0.08 MSun (stellar limit) to MEarth Observational test: gravitational microlensing if all the dark matter in the Milky Way’s halo was MACHOS one in 106 chance that a star has a MACHO exactly along the line of sight focussing brightening of the star’s image as star moves brightness changes Searching for Microlenses Large Magellanic Cloud Micro-Lensing Cartoon Lensing Lightcurve Are MACHOs the Dark Matter? •NO – Not enough lensing events are detected •MACHO’s make up (at most) 15% of the Milky Ways halo mass •Inferred mass range for MACHOs: 0.4MSun (Faint MW or LMC stars) MACHOs are not the solution to the dark matter problem What about WIMPS?? • “Weakly Interacting Massive Particles” – As yet undiscovered elementary particles • High energy particle theories suggest such elementary particles exist WIMPS are a plausible, but not firm, consequence of several theories in particle physics Dark Matter • Cold, collisionless, dark matter explains a wide range of phenomena (not only rotation curves) • Nature of dark matter unknown •We only know what it is NOT! For Wednesday Chapter 22 – Dark Energy Complete EP5