Galaxy Formation Collapse of an over-dense region of space (containing more gas and dark matter than average) under gravity Disks are produced as the cloud of material spins faster and faster as the gravitational collapse progresses To conserve angular momentum, the spin speed must increase inversely proportional to the decreasing size of the cloud Spiral galaxies — contain much raw material (hydrogen gas) for star formation and a large fraction of newborn stars; ongoing star formation Elliptical galaxies — mostly formed their stars early; little or no current star formation; very little gas or dust

More Galaxy Formation Bulges of spiral galaxies are similar to certain elliptical galaxies; formed as a result of monolithic collapse of a cloud of gas and dark matter with very little angular momentum Other elliptical galaxies and spiral galaxy halos are thought to have formed by merging smaller galaxies; merging process results in disordered stellar orbits Important distinction between stars and hydrogen gas: gas is dissipative (a clump of gas tends to drag surrounding material with it due to viscosity); stars are non-dissipative

Large Scale Structure Galaxy Groups: up to ~10 galaxies; typical speeds of km/s; typical size up to 1 Mpc (e.g., Local Group) Galaxy Clusters: a few hundred galaxies; km/s; 10 Mpc (e.g., Virgo & Coma clusters) Galaxy Superclusters: a few thousand galaxies; not necessarily gravitationally bound together; over 10 Mpc in size (e.g., Perseus-Pisces supercluster) Peculiar velocities — velocity not associated with uniform Universal expansion; “Fingers of God” effect

Disk Galaxies: Structural Components Flattened differentially-rotating disk Dense centrally-concentrated bulge with mostly disordered orbits Extended, not centrally concentrated, mostly dark halo Bulge + Halo = “Spheroid”

Spiral Galaxy Properties Bulge stars are older on average than disk stars Youngest disk stars lie in very thin plane Older disk stars lie in a thicker disk Disk stars, particularly young ones, are organized into spiral arms Spiral density waves in the disk: the most successful explanation of spiral structure

Globular Clusters

Most galaxies, including our own, contain dense clusters of 10 3 – 10 6 stars known as globular clusters The observed distribution of globular clusters tells us that the Sun is NOT at the center of the Milky Way galaxy