11How we know what we do:How we know what we do: Radar ranging - distances among planets + AU Measured parallaxes of stars Comparison of main sequences of star clusters Use of standard candles– Determining distance to Andromeda galaxy using variablestars– Milky Way was just one of many galaxies– Use of variable stars to find distance to Virgo Supercluster– Use of Hubble’s Law Advances in telescope technology and space telescopes!2Edwin HubbleEdwin Hubble’’s Distance to Andromeda Galaxys Distance to Andromeda Galaxy In early 1900’s it was thought thatwhole Universe — planets, stars,and fuzzy objects called nebulae— was contained within Galaxy. Our galaxy was the Universe. In 1923 using Hooker telescopeon hazy patch of sky calledAndromeda Nebula, Hubble foundstars just like those in our galaxy,only dimmer -- Cepheid variables. More observations turned up moreof these stars. The rest is history.3Hint…..Astronomers create three-dimensional maps of the Universeby…A. using the position of galaxies on the sky and their apparentbrightness as a measure of distance along the line of sightB. using the position on the sky and the redshift to determine adistance along the line of sightC. interpreting the peculiar velocities of each galaxy within itscluster through comparison of computer models of thestructure formation with observationsHint:4Learning ObjectivesLearning ObjectivesBe able to : Describe what the Universe looks like on the largest scales. Explain how we know this. Summarize the evidence for dark matter– Galaxy rotation curves– Peculiar motion– Presence of hot gas State the role dark matter played in the formation of large-scalestructure25Structure of Universeon grander scaleComputerModel ofLarge-ScaleStructureGalaxies are not distributedrandomlyHumongous filamentssurrounding voidsThe Universe on the largest scales.The Universe on the largest scales.6• Stars, galaxies, clusters: all gravitationally bound systems.• Gravity has overwhelmed the expansion at these levels.• Expansion is among the huge filaments containing clusters ofgalaxiesReminder:Reminder:7The Local GroupThe Local Group• Our neighborhood ofgalaxies is rather sparsecompared to others.• Roughly 40 galaxies; allbut 3 (maybe 4) beingsmall irregular or dwarfgalaxies.• No giant ellipticals!8Perspective?Scale?You are hereRotatearound y axisWhat the UniverseWhat the Universelooks like on largerlooks like on largerscale.scale.~ 200,000,000 ly39Virgo Supercluster ~60,000,000 light years awayVirgo Supercluster ~60,000,000 light years awayRedshift measures 400 km/sec LESS than Hubble expansion wouldsuggest.Huge cluster is gradually slowing the Local Group down.10HerculesFornaxHydraOther Other ““nearbynearby”” clusters of galaxies clusters of galaxiesAll at same scale?11http://en.wikipedia.org/wiki/Great_Attractor12Discovering dark matterDiscovering dark matterHow do we know it exists ifwe can’t “SEE” it?413What observational steps needed to getvelocities? (write list)How do we know the distance to thestars and gas in our galaxy? (write answer)How do we know the distances fromcenter-to-edge of distant galaxies?Weighing galaxiesWeighing galaxies14What would velocity curve look like if ALL of themass was in or near the center? How do we know?Summarize the evidence for dark matter - galaxiesSummarize the evidence for dark matter - galaxies15Measure the orbitalMeasure the orbital speeds of individual galaxies around center of speeds of individual galaxies around center ofmass of the clustermass of the cluster..Called “peculiarmotion” although it isnot. We see the Dopplershift of galaxies.Galaxies are movingway too fast be held inthe cluster by the masstraced by the stars.Summarize the evidence for dark matterSummarize the evidence for dark matter16Stronger lensing thanluminous matter canaccount for!Weighing galaxy clustersWeighing galaxy clustersSummarize the evidence for dark matter51718Gravitational lensing of lightGravitational lensing of light19NOVA Dark MatterNOVA Dark Matter20Intracluster Intracluster mediummedium•Hot gas (X rays) -- 10 - 100 million degrees•Something keeps it within the cluster!Summarize the evidence for dark matterSummarize the evidence for dark matter621Dark matter “seeded” formation ofgalaxies, clusters, superclusters, and“great walls.”Structures observed today mirrordistribution of dark matter in veryearly universe.Called COLD DARK MATTER.State role of dark matter formation of large-scale structureState role of dark matter formation of large-scale structure22•High resolution dark matter simulation of formationof small group of galaxies similar to Local Group.Currently studying dark matter environment of thesegroups with some low gas simulations.http://hpcc.astro.washington.edu/nbody/cosmo/rotbox1.mpegComputer simulations - CDM modelComputer simulations - CDM modelhttp://www-hpcc.astro.washington.edu/picture/movies/grpo1.2.mpegColor density animation of the dark matter densitydistribution in a Cold Dark Matter Universedominated by dark energy. The box is 300 millionlight years across. The mass contained in the box is4 x 1016 solar masses. The biggest halos have asize comparable to large galaxy clusters.23Structure of Universeon grander scaleComputerModel ofLarge-ScaleStructureGalaxies are not distributedrandomlyHumongous filamentssurrounding voidsSUMMARY: The Universe on the largest scales.SUMMARY: The Universe on the largest
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