Figure: Galactic and stellar spectra

A comparison between a galactic spectrum (black line) and a K0 stellar template (red line) shifted to the radial velocity of the galaxy (otherwise, because of the radial velocity of this galaxy, there would be a lateral shift in the vertical match of these two spectra).

Why is this comparison meaningful? This galaxy is an elliptical galaxy, which light is dominated by a single population of stars, K0 giants. Therefore, on first order, one can assume that the entire galaxy is made up of a single type of stars, in this case a K0 giant star which serve as a "template" i.e., a model for the whole galaxy.

The galactic absorption lines are broadened with respect to the same lines in the stellar spectrum. This is caused by the distribution of stellar velocities (also called Velocity Profile) along the line-of-sight. These stars have different velocities and therefore have slightly Doppler-shifted spectra from one another. The final observed galactic spectrum (= the sum of all these stellar spectra) has therefore broader absorption lines. The width of the lines is a measure of the width of the velocity distribution of stars: the wider the range of velocities (e.g., up to very large velocities like in giant elliptical galaxies), the broader the absorption lines.

Studying galactic absorption lines enables the study of the stellar motions inside the galaxy. These motions may, in some cases, betray the presence of some unseen (dark) matter, such as black holes. Indeed, if the velocities predicted by the luminous stellar mass underestimate some observed velocities, some extra non-luminous mass may be present and exert the extra gravitational forces needed to accelerate the stars up to such observed velocities.

Credit: J. Gerssen, "Stellar kinematics in disk galaxies", PhD thesis, Rijksuniversiteit Groningen, Figure 1.1

"Stellar kinematics in disk galaxies",
Dissertation by Joris Gerssen (University of Groningen) available at: http://dissertations.ub.rug.nl/faculties/science/2000/j.gerssen/

http://irs.ub.rug.nl/ppn/240061896