- 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
- There is not enough net rotation in an elliptical galaxy to explain its
- So, how do we explain the observed flattening of ellipticals?
- Flattening arises because random motions are not equal in all
- 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
- The kinematics of polar ring galaxies do suggest that ellipticals are
surrounded by extended haloes of dark matter, similar to those around spiral