Transcript Disk Galaxies: Structural Components

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
100-500 km/s; typical size up to 1 Mpc (e.g., Local
Group)
• Galaxy Clusters: a few hundred galaxies; 500-2000
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
Globular Clusters
• Most galaxies, including our own, contain dense clusters of
103 – 106 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