Chapter 14: The Milky Way Galaxy The scenario for the formation of galaxies is very reminiscent of the formation process for solar systems. Giant cloud of gas and dust, shrinks under its own gravity, rotation produces a flattened disk. The halo objects probably formed first and retained their spherical distribution and didn’t get pulled into the disk.

Chapter 14: The Milky Way Galaxy Spectroscopic radio astronomy allows astronomers to look through our galaxy and see the Doppler velocities of hydrogen through its 21 cm spin-flip line. With this information astronomers can recreate the 3 dimensional structure of our galaxy including the number of spiral arms, etc. This is what we think our galaxy looks like.

Chapter 14: The Milky Way Galaxy Astronomers still can’t explain well why we have spiral arms in galaxies. The stars, gas, and dust in the galaxy all obey Kepler’s laws of planetary motion. Thus the inner matterial rotates faster than the outer matterial. If spiral arms were tied to the galaxy then after a few 100,000,000 years they would all be wound up and loose their shape.

Chapter 14: The Milky Way Galaxy The density wave theory says that a compression front moves through the disk and compresses the gas and dust to start the star formation process. In this model new matterial is being constantly fed into the density regions as old matterial leaves.

Chapter 14: The Milky Way Galaxy Since matterial orbiting the galaxy must obey Kepler’s laws, if we could measure the distance and orbital period of matterial as a function of distance from the center, we could determine the amount of mass inside that distance. When we reach the “edge” of the galaxy, we expect the rotation speed to diminish, but instead it keeps on increasing as if there is some “invisible” (dark) matter.

Chapter 14: The Milky Way Galaxy One attempt to detect dark matter is to look for gravitational lensing. Even if the matter is dark it should disturb the space-time continuum and produce gravity like any other matter. While we have seen several such events, they could only account for about 1/2 the dark matter.