Lecture 6: Weighing Galaxies --Dark Matter Astronomy 5: The Formation and Evolution of the Universe Sandra M. Faber Spring Quarter 2007 UC Santa Cruz The Doppler effect The Doppler effect only exists along the line of motion. Motion crosswise to line of sight has no effect. No effect this way Fritz Zwicky: cluster mass measurements, 1932 Fritz Zwicky, Caltech astronomer, started making mass estimates of clusters of galaxies in He was the first to notice that galaxies inside the clusters were moving too fast. How could the clusters hold together? They should fly apart! This was the beginning of the discovery of dark matter. How a spiral galaxy rotates: differential rotation Interstellar gasStars Movie by TJ Cox, UCSC Physics Dept. Stars form from dense clouds of gas Messier 33 galaxy, a nearby member of the Local Group Giant H II region in Messier 33 Galaxy spectrum showing spectral features: H II regions plus starlight H O H O S H Stars 400 nm450 nm500 nm 550 nm600 nm650 nm700 nm Wavelength (nm) O N How rotation translates into a rotation curve This plot of orbital speed versus radius is called a rotation curve. Typical observed galaxy rotation curve Raw spectrumSky Subtracted OH emission from Earths atmosphere Red end Blue end Bright central streak is starlight from middle, where galaxy is brightest Emission lines from galaxy Which side is going away? Unexpected: rotation curves are flat in the outer parts of galaxies! from stars alone A Milky Way-like external galaxy seen edge on NGC 891 disk bulge Schematic model of a visible galaxy surrounded by its dark matter halo 100,000 lyr 1,000,000 lyr A binary galaxy Another binary galaxy M51, the Whirlpool galaxy, and its companion. A small group: Hickson 87 Another small group: Seyferts Sextet A giant cluster: Abell 1689 Another giant cluster: Cl Gravitationally lensed background galaxies Galaxy cluster Abell 2218: note grav lenses! The simplest nuclear reaction that makes stars shine