Galaxy types: Elliptical, Normal Spiral, Barred spiral, Irregular

Properties of a galaxy

• Mass
• size
• amount of gas
• age of stellar population
• presence of star formation
• Central black hole seen in larger galaxies

Rotation curves

$$ M(R)=\frac{v^2R}{G} $$

• $M(R)$ mass at a radius $R$
• For point masses: $v\propto r^{-\frac 12}$
• For constant density galaxy $\rho(r)=\rho_0$ then $v\propto r$
• When $R$ is large $v$ becomes constant

Dark matter halo

• Flat rotation curves require $p \propto r^{-2}$, this shows that galaxies have much more mass far out than is seen in stars and gas