1. Galaxy Evolution Astronomy 315 Professor Lee Carkner Lecture 22

2. Galaxies  Spiral   Forming stars  Elliptical   Not forming stars  Irregular   Active  Material feeding central black hole

3. Galaxy Questions  How were galaxies originally formed?   Why are there different kinds of galaxies?

4. The Early Universe  About 500 million years after the Big Bang the universe was composed of:   Helium   Material was not uniform, but arranged on cosmic filaments   Dark matter began to clump together on the filaments

5. Protogalaxies   The gas began to form stars  Structures called protogalaxies   By looking for objects 13 billion light years away   Since it takes a long time for the light to get to us

6. Mergers  Protogalaxies were small   About 10% the current size of the Milky Way  The original galaxies start out small and grow via hierarchical mergers   A “bottom-up process”

7. Early Galaxies   Other elements hadn’t had a chance to form yet   Since they were closer together   Due to greater interactivity

8. Interactions  Galaxies evolve by interactions  Galaxies interact via:   Close encounters   Galaxies are larger than they look   What are the signs of interaction?

9. Starburst Galaxies  Starburst galaxies are very bright in infrared   Due to massive star formation shrouded by dust   Forms dozens of stars per year for millions of years  A near-by galaxy is compressing the gas and dust into the center of the galaxy causing waves of star formation

10. Giant Ellipticals  Giant ellipticals can be found at the centers of clusters   Often have multiple nuclei   Many ellipticals have shells   Only large ellipticals seem to be the result of mergers  Dwarf ellipticals show little merger evidence

11. M87   Formed from mergers with near by smaller galaxies  How big is it?   Convert to arcseconds using the plate scale   Plate scale = conversion between linear and angular distances on a photograph

12. M87NGC4478

13. 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

14. Intercluster Gas  The space between galaxies in a cluster is filled with the intercluster medium   Temperatures of millions of degrees  Visible in X-rays   Composed of gas ejected from galaxies during mergers or starbursts

15. Spiral Galaxy Formation  When several protogalaxies merge:   since it can’t interact with anything   Since it is spinning fast   Original early stars in protogalaxies   Is there an interaction mechanism which recreates them?

16. Elliptical Galaxy Formation  Ellipticals form from merging spirals   Remaining stars form a ball  At the cores of clusters ellipticals grow very large 

17. Next Time  Read 26.1-26.4