1. Large Scale Structure PHYS390 Astrophysics Professor Lee Carkner Lecture 23

2. 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

3. 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%

4. 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)

5. 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)

6. 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)

7. 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

8. 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

9. 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

10. Clusters  Galaxies are grouped in clusters and superclusters   Groups   Size ~2 Mpc  Clusters   Size ~10 Mpc  Superclusters   Size ~50 Mpc

11. 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

12. Virgo Cluster   Nearest large cluster   Bright galaxies dominated by spirals, faint by dwarf ellipticals

13. Intercluster Medium   Most visible in X-rays  T ~ 10 8 K   Free electrons fly past free protons and emit braking radiation  Dominates visible mass

14. Cluster Dynamics  Galaxies interact as they move through the cluster   May form giant ellipticals at cluster core

15. Superclusters   We are near the edge of the Local Supercluster   Gravity of supercluster slows Local Group expansion

16. 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

17. 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 

18. 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