Elliptical Galaxy Dynamics

• Absorption lines in the spectra of ellipticals are broadened and shifted by the stellar motions in the galaxy.
  • Shifts imply net streaming motion of the stars (vbar)
  • Broadening implies random motions of the stars (sigma)

• long slit spectra of ellipticals reveal:
  • They rotate about their minor axes
  • random motions dominate (vbar >> sigma)

• Are ellipticals flattened by rotation, like spiral galaxies?

  • In general:
    • Net streaming contributes to flattening (conservation of angular momentum prevents collapse in one plane).
    • Random motions inhibit flattening

  • So, given the observed ellipticity of an elliptical galaxy, we can predict the balance between ordered and random motion, vbar/sigma.

  • In fact, vbar/sigma is found to be too small for almost all elliptical galaxies.

    • There is not enough net rotation in an elliptical galaxy to explain its observed flattening.

• So, how do we explain the observed flattening of ellipticals?
  • Flattening arises because random motions are not equal in all directions:

    

  • The random motions are now intrinsically a tensor quantity, sigma = (sigma_x, sigma_y, sigma_z)

    • If all three components of the random motions are unequal, the galaxy will have a fully triaxial shape.

• Even though elliptical galaxies have complicated orbital structure and triaxial shapes, we can still obtain a crude estimate of their masses using the same formula as for spiral galaxies:

  

  • You can find an explanation of why this formula remains valid here.

  • Mass-to-light ratios for ellipticals turn out to be ~ 10 (solar units), similar to spirals.

  • But they contain no gas, so there is no 21cm emission, and it is hard to map out the mass distribution at large radii to see if they contain extended dark haloes.

    • The kinematics of polar ring galaxies do suggest that ellipticals are surrounded by extended haloes of dark matter, similar to those around spiral galaxies.