1. Galactic Evolution PHYS390 Astrophysics Professor Lee Carkner Lecture 21

2. Galaxy Questions   Why do galaxies have multiple components?   Why are some spirals and some ellipticals?

3. Interaction   Galaxies are close together relative to their size   Interaction scenarios   Merger  Tidal interaction

4. Collision  When galaxies collide, the stars are too small to impact each other   Results in dynamical friction   As the satellite attracts gas and dust as it passes through   Satellite slows down, losing kinetic energy to the material it attracts

5. Friction Force  We can write the force in terms of the mass and velocity of the satellite galaxy (M and v M ) and the density of the surrounding material (  ) f d ~ (CG 2 M 2  )/v M 2   Range: C ~

6. Dynamical Lifetime   Depends on r, radius from center of main galaxy  t = (2  v M r i 2 )/(CGM)  Can also solve for most distant satellite that could have been captured in galaxy lifetime r max = (t max CGM/2  v M ) 1/2

7. Mergers   Large galaxies tend to “eat” smaller satellite galaxies  Accounts for   Counter-rotating cores  Irregular galaxies

8. Impulse  Some collisions occur so fast, the stars don’t have time to respond   Can produce ring galaxies and dust lane ellipticals

9. Tidal Interactions   Is a differential force and tries to pull galaxies apart   Effect strongest in close, slow encounters and where one galaxy orbits another  Also produces warped disks

10. Starburst Galaxies   98% of total light in IR  Due to massive star formation shrouded by dust   Forms dozens of stars per year for millions of years  Tidal interactions slow down gas orbits, causing it to fall into and be concentrated in the center

11. Elliptical Mergers   Found at the bottom of the cluster’s gravity well   Left by merging galaxies settling into new orbits  Only large ellipticals seem to be the result of mergers  Dwarf ellipticals show little merger evidence

12. Double Nucleus   Central black hole should fall into the core   Observed as double X-ray source in core   Supermassive black holes may be built up through mergers

13. Intercluster Gas   Mostly in the form of hot gas  Mass ~ 5X10 13 M sun  T ~ 5X10 7 K   Mass may exceed that of stars in the cluster  Shows iron lines   Comes from  mergers   ram-pressure stripping

14. Spiral Galaxy Formation   Many are low mass (10 6 -10 8 M sun )   Some stay separate to form globular clusters   Distant galaxies tend to be bluer than near-by, indicating that star formation is concentrated in the early history of spirals

15. Elliptical Galaxy Formation   Disk is destroyed and material settles into spheroid  Ellipticals may also form naturally at the center of cluster gravity wells 

16. Next Time  Read 28.1-28.2  Homework: 26.4, 26.7, 26.3, 28.5, 28.7a  Extra-galactic presentations on Monday