Public Star Party • Organized by Institute for Astronomy, University of Hawaii. Contact person: Prof. Jim Heasley. • When? Tonight between 8:30 and 10 pm • Where? Kapiolani Park, Honolulu • How to get there? Intersection Paki Avenue with Noela Street • What about the weather? Check 956-6195
Chapter 18: Dark Matter A Dark Mystery Dark Matter in Galaxies Dark Matter in Clusters Ordinary or Extraordinary? Structure Formation
A Dark Mystery • The rotation curve of the Milky Way indicates that there is a large amount of matter in the halo that does not emit light. • The total mass of dark matter in the Milky Way may be 10 times greater than the mass of visible stars. • We assume that we understand gravity correctly.
Dark Matter in Galaxies • From the Doppler shifts of the 21-cm emission line of H, we determine rotation curves for galaxies. • Blueshifted lines on the left side of the disk show how fast that side is rotating toward us. Redshifted lines on the right side show how fast that side is rotating away.
Rotation Curve • A rotation curve contains all the information needed to measure the mass contained within the orbits of the outermost gas clouds. • The tilt of the galaxy has to be taken into account.
Rotation Curves of Spirals • Remarkably flat as far as we can see (up to 150,000 l.y. in some cases). • A flat rotation curve implies that we must keep encircling more and more matter with increasing distance from the center. • In elliptical galaxies only stars can be measured. They also have a great deal of dark matter.
Mass-to-Light Ratio • Within the Sun’s orbit there are 90 billion solar masses and 15 billion solar luminosities. The mass-to-light ratio within the Sun’s orbit is about 6 solar masses per solar luminosity. • Elliptical galaxies have no luminous high-mass stars. They can have mass-to-light ratios of about 10 solar masses per solar luminosity. • The M-L ratio for entire spiral galaxies can be as high as 50 solar masses per solar luminosity.
Dark Matter in Clusters • In the 1930s, Fritz Zwicky used a spectrograph to measure the redshifts of galaxies in clusters. He estimated the speed of galaxies around the cluster. He found that clusters of galaxies have M-L ratios of several hundreds.
Intracluster Medium • Hot gas that fills the space between galaxies and emits primarily as X rays. • Its temperature depends on the mass of the cluster because of gravitational equilibrium. • Xrays=>Tgas=>Ekinetic=>M • M-L rations > 100 MSun/LSun => Dark matter holds the clusters together.
Gravitational Lensing • Masses distort space-time. Massive objects bend light beams passing near them. • A cluster of galaxies produces multiple images of a background blue galaxy. Photons from this distant galaxy are bended by the cluster looking as if they were coming from different directions.
Gravitational Lensing II • A galaxy cluster is a powerful gravitational well which bends light from background distant galaxies. • If light can arrive to Earth from several different directions, we see multiple distorted images of the same galaxy.
Gravitational Lensing III • Arc-shaped galaxies are not cluster members, but normal galaxies lying far beyond the cluster. • Einstein’s theory of general relativity allows to estimate the masses of the lenses. Cluster masses from lensing agree with those derived from X-ray temperatures and galaxy velocities.
Baryonic Dark Matter • Ordinary matter is dominated by protons and neutrons (baryons). It is also called baryonic matter. • MACHOs: massive compact halo objects are dim objects such as brown dwarfs, giant planets and small black holes that are made of baryonic matter. • MACHOs in the line of sight to distant stars can be noticed by their gravitational lensing. Monitoring projects have found them, but their number is not enough to account for dark matter.
Extraordinary Matter • Neutrinos are weakly interacting particles of very low mass. They cannot make the dark matter in galaxies because they escape a galaxy’s gravitational pull due to their high speeds. • WIMPs: Weakly interacting massive particles. They are slower than neutrinos and collect into galaxies. They resist to settling onto the galactic center and disk. They do not emit electromagnetic radiation. They have escaped detection so far. Most wanted particle in astrophysics.
Structure Formation • Clusters of galaxies are gravitationally bound thanks to dark matter. • The gravitational attraction of dark matter is suspected to have pulled galaxies together. • Galaxies have peculiar velocities due to the attraction of superclusters.
Large Scale Structures • 3-D maps of the universe using thousands of redshifts reveal large-scale structures. • Galaxies are not scattered randomly, but lie along sheets and strings interspersed with voids. • On scales larger than a billion light years the distribution is uniform.
Simulations of Structure Development • Supercomputer simulations show how structures progress since when the universe was only 60 million years old (z=50) to the present (z=0). • The high density regions continue to attract more and more matter. • Today’s structures mirror the original distribution of dark matter.
The Fate of the Universe • If gravity is strong enough, expansion will halt, universe will collapse ending in a big crunch. • If gravity is not strong enough, universe will continue expanding forever, growing even colder. • The critical density marks the dividing line between eternal expansion and eventual collapse. It is presently about 10-29 gram per cubic centimeter. • The visible matter contributes less than 1% of the critical density. Is there enough dark matter?
Mysterious Acceleration • Observations of distant WD supernovae indicate that expansion is speeding up. • A mysterious repulsive force is pushing all galaxies apart. • Some speculative ideas for what might generate such a new force are dark energy, quintessence, and cosmological constant.
Expansion Patterns • Recollapsing or closed universe. • Critical or flat universe. • Coasting or open universe. • Accelerating universe.
The Big Picture • Dark matter is real if we understand how gravity operates in large scales. • Measurements of mass and luminosity of galaxies and galaxy clusters indicate that they contain far more dark matter than stars. • Dark matter could be baryonic or non-baryonic. • Superclusters, walls and voids extend over scales larger than clusters of galaxies. • Dark matter holds the key to the fate of the universe.
Homework 5 • Describe the observational evidence for magnetic fields at the center of galaxies. • Draw a model of a quasar and describe the different parts. • Describe the central 25 parsecs of the Milky Way. • Write a 1 page report that summarizes the latest ideas about what dark matter is made of.