Spiral Galaxy Rotation Curve

• Like the Milky Way, external spiral galaxies are supported against collapse by their rotation. (c.f. elliptical galaxies, which aren't).

• By using the Doppler shifts in spectral lines to measure galaxies' line-of-sight velocity as a function of position, we can measure their rotation curves (speed of material following circular orbits around the centre of the galaxy as a function of radius). We can derive this quantity from:
  • 21cm emission from atomic hydrogen
  • Optical emission lines from hotter gas
  • Optical absorption lines from the stellar component

• Most rotation curves look very similar:

  

• Although the enclosed mass, M(r), continues to grow apparently without limit, the enclosed luminosity, L(r), tends to a finite limit as we reach the edge of the luminous material in the galaxy. There must therefore be significant amounts of dark matter which continue to contribute to M(r) out to very large radii.
  • Out to the furthest point measured, typical galaxies have a luminosity of L ~ 10^10 solar luminosities, and a typical enclosed mass of M ~ 10^11 solar masses.

    • The "mass-to-light ratio" M/L is hence ~ 10 solar units.

    • ~ 90% of the material in the galaxy is dark!

• A nice example of the rotation curve of an external galaxy derived from optical data is provided by these data from the disk galaxy NGC 5746.

• Another example is provided by the rotation curve of our own galaxy.