Stellar Absorption Line Kinematics

• When we take an absorption line spectrum of a galaxy, we observe the summed spectra of many different stars, all with slightly different Doppler shifts in their spectral lines.

• The net result of this is that the lines in the observed galaxy spectrum are broadened and shifted by the characteristic velocities of the stars in the galaxy:

  

• The observed galaxy spectrum is a convolution of the stellar spectrum with the distribution of line-of-sight velocities of all the stars along that particular line of sight through the galaxy.