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Large Scale Structure PHYS390 Astrophysics Professor Lee Carkner Lecture 23
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Distance We can see patterns of stars, galaxies and nebula but we don’t know how far away they are First extragalactic measurement of distance by Hubble in 1923 Need accurate distances to near-by things to calibrate method used for more distant things
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Distance Methods But most distance measurements depend on a standard candle We measure flux and get distance from F = L/4 d 2 Can try to determine it independently Usually about 0.3 mag or about 10%
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Distance Ladder Parallax and spectroscopic parallax For stars in our galaxy (1 Mpc) Pulsating stars For near-by galaxies (~30 Mpc) Globular cluster or planetary nebula luminosity function For intermediate galaxies (~50 Mpc) Tully-Fisher, D- relation For distant galaxies (~100 Mpc) Type Ia supernova For largest distances (~1000 Mpc)
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Parallax Shift in position of star due to viewing from opposite ends of earth’s orbit d = 1/p Does not depend on extinction or finding luminosity d = 10 (m-M+5-A)/5 Very inaccurate (M +/- 1 mag)
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Cepheids Brighter stars have longer periods Used by Shapley to find center of Milky Way (1917) and Hubble to find distance to M31 (1923) Can use to find absolute magnitude from P (days) and color M V = -3.53 log P d - 2.13 + 2.13(B-V)
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Luminosity Function Rather than using a small sample of objects, we can create a luminosity function Can use with globular clusters or planetary nebula Calibrate so you know M for some point on the graph and compare to the observed m
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Galaxy Rotation Can find M from V max using Tully-Fisher relation Have to take inclination into account For elliptical galaxies we can relate the diameter (D) to the velocity dispersion ( ) Called D- relation
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Type Ia Supernova From the lightcurve of a supernova we can find the peak luminosity Since M ~ -19.3, can be used to very large distances Best distance and accuracy
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Clusters Galaxies are grouped in clusters and superclusters Groups Size ~2 Mpc Clusters Size ~10 Mpc Superclusters Size ~50 Mpc
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Local Group Dominated by Milky Way, M31 and M33 About 20 other small groups are within 10 Mpc About 80% of all galaxies are in such groups
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Virgo Cluster Nearest large cluster Bright galaxies dominated by spirals, faint by dwarf ellipticals
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Intercluster Medium Most visible in X-rays T ~ 10 8 K Free electrons fly past free protons and emit braking radiation Dominates visible mass
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Cluster Dynamics Galaxies interact as they move through the cluster May form giant ellipticals at cluster core
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Superclusters We are near the edge of the Local Supercluster Gravity of supercluster slows Local Group expansion
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Great Attractor All near-by galaxies are moving in the direction of the constellation Centaurus May be due to an unobserved supercluster called the Great Attractor Mass ~ 10000 mass of Milky Way
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Largest Scales Reveal a “sponge-like” or “cluster of bubbles” structure Universe consists of large voids about 100Mpc across with galaxies clustered on the boundaries of the voids Voids too large for galaxies to have formed in them and then be pulled out
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Next Time Final Exam, Thursday Feb 17, 9-11am 2/3 covers everything since test 3 1/3 covers rest of course 3 old + 1 new equation sheet given 20% of grade
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