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Galaxies and the Distance Ladder

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Galaxies and the Distance Ladder 1 Lecture 34 Galaxies and Dark Matter January 15c, 2014 2 M 100 -- Spiral Galaxy A grand design spiral galaxy. Click here for more info about this image. 3 NGC 5090 and NGC 5091 The spiral galaxy NGC 5091 may be colliding with elliptical galaxy NGC 5090. Click here for more info on the galaxy pair. 4 Small Magellanic Cloud Our nearest neighbor, an irregular galaxy. Click here for more info on this picture. 5 Distances to Galaxies • Galaxies were first thought to be star forming regions. • It was proposed that the “spiral nebula” were “island universes”. • Spectroscopic parallax is only good for distances up to 10,000 pc. Andromeda Galaxy 6 Using Cepheid Variable Stars to Measure Distances • Variable stars are stars whose brightness varies in a very smooth, predictable way. • Cepheid variables – periods vary from 1-100 days. Figure 23.5, Chaisson and McMillan, 6th ed. Astronomy Today, © 2008 Pearson Prentice Hall • RR Lyrae variables – periods are all less than 1 day. 7 • Cepheids are stars that have moved off of the MS. – Star is expanding and contracting. – Luminosity rises and falls. Figure 23.6, Chaisson and McMillan, 6th ed. Astronomy Today, © 2008 Pearson Prentice Hall 8 Cepheid Variables • Average luminosity is related to pulsation period. • If luminosity and apparent brightness are known, distance can be determined Luminosity Apparent Brightness 4d 2 Figure 23.7, Chaisson and McMillan, 6th ed. Astronomy Today, © 2008 Pearson Prentice Hall 9 Supernovae as Standard Candles • Type Ia supernovae all reach the same maximum luminosity, about 3 × 109 solar luminosities • If supernova is observed in another galaxy and the peak apparent brightness is measured, the distance can be calculated. Luminosity Apparent Brightness 4d 2 10 Tully-Fisher Relation for Determining Distances • Used to determine distances to galaxies where individual stars cannot be seen. • Relates speed of rotation and luminosity of a galaxy. – The faster a galaxy rotates, the higher the luminosity. • If apparent brightness and luminosity are known, distance can be determined 11 • How is rotational speed measured? – Doppler Shift Figure 24.11, Chaisson and McMillan, 6th ed. Astronomy Today, • Tully-Fisher calibrated using © 2008 Pearson Prentice Hall nearby galaxies with variable stars 12 The Distance Ladder 13 Distribution of Galaxies • Most galaxies are clustered – Milky Way has 3 nearby companions – ~40 galaxies in Local Group (Size ~1 Mpc) Map of three-quarters of the members of the Local Group Figure 16-18, Comins and Kaufmann, Artist’s view of the Local Group 7th ed. Discovering the Universe, Figure 16.30, Arny and Schneider, 5th ed. Explorations, © 2005 W.H. Freeman and Company © 2008 The McGraw-Hill Companies, Inc. 14 Which distance measuring method would be most reliable for a nearby galaxy in our Local Group? A. Parallax B. Spectroscopic parallax C. Cepheid variables D. Supernovas E. Tully-Fisher 15 Which distance measuring method would be most reliable for a nearby galaxy in our Local Group? A. Parallax B. Spectroscopic parallax C. Cepheid variables D. Supernovas E. Tully-Fisher 16 Clusters of Galaxies • Galaxies often found in clusters – Rich cluster: many hundreds of galaxies – Poor cluster (or group): only a few dozen galaxies • Held together by gravity • Milky Way is near the Virgo Cluster of ~2500 galaxies (Size ~3Mpc across) 17 Figure 16.36, Arny and Schneider, 5th ed. Explorations, © 2008 The McGraw-Hill Companies 18 Virgo Cluster – SDSS Image 19 Clusters and Superclusters • Large clusters – More Ellipticals found near the center – More Spirals found in outer regions • Superclusters – clusters of clusters of galaxies. – In between clusters -- no gas gas been detected • Most must have been swept up during galaxy formation. 20 Figures 25.23 and 24, Chaisson and McMillan, 6th ed. Astronomy Today, © 2008 Pearson Prentice Hall 21 Abell 2218 22 Role of Interactions • Small interactions may start formation of spiral structure. • Strong interactions (collisions, cannibalism) may alter structure completely – Spirals lose structure, become ellipticals. – Large galaxies “eat” many other galaxies, become very large 23 Galaxy Merger 24 Antennae Galaxy 25 Which of the following is NOT true concerning the Local Group? A. It contains about 40 member galaxies. B. It is roughly spherical in shape with the most massive galaxies near the center. C. It is a poor cluster. D. It is the galaxy cluster to which the Milky Way belongs. 26 Which of the following is NOT true concerning the Local Group? A. It contains about 40 member galaxies. B. It is roughly spherical in shape with the most massive galaxies near the center. C. It is a poor cluster. D. It is the galaxy cluster to which the Milky Way belongs. 27 Measuring the Mass of Galaxies – Kepler’s Third Law • Mass inside of radius of rotation can be measured using Kepler’s Third Law 2 3 M TOT P a • Period can be measured from the velocity of gas • Observe H to measure mass outside of stellar part of the disk. Figure 23.21, Chaisson and McMillan, 6th ed. Astronomy Today, © 2008 Pearson Prentice Hall 28 Measuring the Observable Mass • Add up mass of all of the stars, gas, dust, etc. that can be observed. • Observable Mass and Mass measured by rotation ARE NOT THE SAME!! Expected rotation curve from observable mass 29 Dark Matter • There is more mass than we can account for with known stars and gas. • The missing mass cannot be observed so it is called dark matter • In the Milky Way, about half of the matter is dark matter. • Other galaxies: 0-90% is dark matter. 30 Andromeda Galaxy Rotation Curve The M31 major axis mean optical radial velocities and the rotation curve, r <120 arcmin, superposed on the M31 image from the Palomar Sky Survey. Velocities from radio observations are indicated by triangles, 90< r <150 arcmin. Rotation velocities remain flat well beyond the optical galaxy, implying that the M31 cumulative mass rises linearly with radius. (Image by Vera Rubin and Janice Dunlap.) Physics Today, December 2006, p. 9 31 Using the Doppler effect to measure velocities, we find that the actual mass of the galaxy cluster is 10 to 100 times more than what is suggested by the luminous matter. Figure 25.2, Chaisson and McMillan, 6th ed. Astronomy Today, © 2008 Pearson Prentice Hall 32 Dark Matter revealed by Galaxy Cluster Collision This image was made by superimposing a picture made a visible wavelength, an image made at X-ray wavelengths (revealing hot gas as red blobs), and a map of dark matter (blue blobs) deduced from gravitational lensing. Figure 16.35, Arny and Schneider, 5th ed. Explorations, © 2008 The McGraw-Hill Companies, Inc. 33 Dark Matter in Galaxies and Clusters • Dark matter has been detected – in galaxies (by rotation curves) – in clusters (by galactic motions and by gravitational lensing) • Mass of cluster ~ 10-100 times observable mass • If true, universe is Figure 25.1b, Chaisson and McMillan, 6th ed. Astronomy Today, © 2008 Pearson Prentice Hall 90% Dark Matter. 34 • Where is the dark matter? – Believed to be distributed in large halo surrounding galaxy. – Also in clusters? • What is dark matter? -- not sure – Brown dwarfs = protostars that never started fusion. – Black dwarfs = cooled white dwarfs. – Black holes – Sub-atomic particles 35 Which of the following is NOT a reason why astronomers believe that dark matter exists? A. They can detect it with radio telescopes. B. The outer parts of galaxies rotate faster than expected on the basis of the luminous matter. C. The galaxies in clusters move faster than expected on the basis of the luminous matter. D. It explains some of the gravitational lensing that is observed at long distances. 36 Which of the following is NOT a reason why astronomers believe that dark matter exists? A. They can detect it with radio telescopes. B. The outer parts of galaxies rotate faster than expected on the basis of the luminous matter. C. The galaxies in clusters move faster than expected on the basis of the luminous matter. D. It explains some of the gravitational lensing that is observed at long distances. 37 Galaxy Formation • No set theory for galaxy formation yet. Galaxies form from one large cloud of gas Galaxies form from the merger of a few medium size gas clouds Galaxies form from many small gas clouds 38 Questions on Galaxy Formation • Why are ellipticals and spirals so different? – Ellipticals have mainly older stars = stars formed early on and little new star formation has occurred. – Spirals have new and old stars = more continuous star formation • What role do interactions play in creating galaxies? • Spiral galaxies are more common at large distances (in the past) -- where are they now? 39 Formation of Milky Way One model for the formation of the Milky Way. There still remains debate among scientists about the role that collisions and mergers play in the formation of large galaxies like the Milky way. Figure 15.27, Arny and Schneider, 5th ed. Explorations, © 2008 The McGraw-Hill Companies .
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