AST- 105 1st EditionLecture 24Outline of Last LectureI. Historical Understanding of GalaxiesII. Types of GalaxiesOutline of Current LectureI. Galaxy CollisionsII. Dark Matter in ClustersIII. Superclusters Current Lecture I. Galaxy Collisionsa. Galaxy Collisions- low speed collisions that can cause galaxy mergers in small groups. 1. Gravity of each galaxy pulls out stars and gas from other galaxy.2. Spirals can merge to form ellipticals.3. Merges can build up large galaxies near group centers if galaxies have large holes of dark matter.4. High speed collisions in large clusters do not tend to result in merges.5. Collisions can compress gas clouds in the galaxy, causing star formation.b. Merger Evidence: 2 Supermassive Black Holes in the Same Galaxy1. The galaxy NGC 6240 was formed by merger1. Contains 2 nulcei2. Chandra sees type of x-rays from each nucleus expected from a supermassive black hole.3. Black holes are 300 ly apart and will merge in about 100 million years producing strong gravity waves.II. Dark Matter in Clustersa. Dark Matter in clusters determine masses of galaxy clusters from galaxy speeds.1. Gravitational pull of mass accelerates the galaxies2. The higher the speed, the greater the mass of the clusterb. The high galaxy speeds measured imply large cluster mass.c. Less than 20% of mass of a cluster is in visible parts of galaxies and hot gas.1. The rest of the mass is dark matter.1. Thought to be WIMPS.III. Superclustersa. Supercluster- group of galaxy clusters1. Produced by gravity2. About 10^8 ly across3. Features of large scale distribution of galaxies are:1. Filaments- apparent strings of galaxies in space2. Voids- empty regions of space3. Walls- sheets of galaxies in space at edges of
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