1How we know what we do:Radar ranging - distances among planets + AUMeasured parallaxes of starsComparison of main sequences of star clustersUse of standard candles– Determining distance to Andromeda galaxy using variable stars led to ...– ...Milky Way was just one of many galaxies– Use of variable stars to find distance to Virgo Supercluster– Use of Hubble’s Law out to billions of light years awayAdvances in telescope technology and space telescopes!Friday, December 2, 11Where the Galaxies Are.Friday, December 2, 113Hint…..Astronomers create three-dimensional maps of the Universe by…A. using the position of galaxies on the sky and their apparent brightness as a measure of distance along the line of sightB. using the position on the sky and the redshift to determine a distance along the line of sight C. interpreting the peculiar velocities of each galaxy within its cluster through comparison of computer models of the structure formation with observationsHint:Friday, December 2, 113Hint…..Astronomers create three-dimensional maps of the Universe by…A. using the position of galaxies on the sky and their apparent brightness as a measure of distance along the line of sightB. using the position on the sky and the redshift to determine a distance along the line of sight C. interpreting the peculiar velocities of each galaxy within its cluster through comparison of computer models of the structure formation with observationsHint:Friday, December 2, 114• Stars, galaxies, clusters: all gravitationally bound systems.• Gravity has overwhelmed the expansion at these levels.• Expansion is among the huge filaments containing clusters of galaxiesReminder:Friday, December 2, 115Learning 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 gasState the role dark matter played in the formation of large-scale structureFriday, December 2, 115Learning 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 gasState the role dark matter played in the formation of large-scale structureFIRST: YOU BE THE JUDGE!Friday, December 2, 11Harvard Center for Astrophysics + Sloan Digital Sky Survey6Friday, December 2, 11Sloan Digital Sky Survey7Friday, December 2, 112MASS survey (Two Micron All Sky Survey)http://www.ipac.caltech.edu/2mass/releases/allsky/doc/sec2_3d2.movies.html8Friday, December 2, 119http://en.wikipedia.org/wiki/Great_AttractorFriday, December 2, 1110The Local Group• Our neighborhood of galaxies is rather sparse compared to others. • Roughly 40 galaxies; all but 3 (maybe 4) being small irregular or dwarf galaxies.• No giant ellipticals!Friday, December 2, 1111Virgo Supercluster ~60,000,000 light years awayRedshift measures 400 km/sec LESS than Hubble expansion would suggest. Huge cluster is gradually slowing the Local Group down.Friday, December 2, 1112HerculesFornaxHydraOther “nearby” clusters of galaxiesAll at same scale?Friday, December 2, 11Friday, December 2, 1114Discovering dark matterHow do we know it exists if we can’t “SEE” it?Summarize the evidence for dark matterFriday, December 2, 1115What observational steps are needed to get velocities?How do we know the distance to the stars and gas in our galaxy? How do we know the distances from center-to-edge of distant galaxies? “Weighing” galaxiesSummarize the evidence for dark matterFriday, December 2, 1116Summarize the evidence for dark matter - galaxiesSummarize the evidence for dark matterFriday, December 2, 1117Measure the orbital speeds of individual galaxies around center of mass of the cluster. [Called “peculiar motion” although it is not. We are recording the Doppler shift of galaxies.]What do we find?Summarize the evidence for dark matter[Think about Bo - the middle-school student on the play field merry-go-round.]Summarize the evidence for dark matterFriday, December 2, 1117Measure the orbital speeds of individual galaxies around center of mass of the cluster. [Called “peculiar motion” although it is not. We are recording the Doppler shift of galaxies.]What do we find?Galaxies are moving way too fast be held in the cluster by the mass traced by the stars.Summarize the evidence for dark matter[Think about Bo - the middle-school student on the play field merry-go-round.]Summarize the evidence for dark matterFriday, December 2, 1118NOVA Dark MatterClickSummarize the evidence for dark matterFriday, December 2, 1119Intracluster medium•Hot gas (X rays) -- 10 - 100 million degrees•Something keeps it within the cluster!Summarize the evidence for dark matterFriday, December 2, 1120Stronger lensing than luminous matter can account for!Weighing galaxy clustersSummarize the evidence for dark matterGravitational lensingFriday, December 2, 1121Summarize the evidence for dark matterFriday, December 2, 1121Summarize the evidence for dark matterFriday, December 2, 1122Gravitational lensing of lightClickSummarize the evidence for dark matterFriday, December 2, 1123Dark matter “seeded” formation of galaxies, clusters, superclusters, and “great walls.”Structures observed today mirror distribution of dark matter in very early universe.Called COLD DARK MATTER.State role of dark matter formation of large-scale structureSummarize the evidence for dark matterFriday, December 2, 11Distribution of dark matter24http://hubblesite.org/newscenter/archive/releases/2007/01/image/a/grav/Friday, December 2, 11Distribution of dark matter24http://hubblesite.org/newscenter/archive/releases/2007/01/image/a/grav/Friday, December 2,
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