Galaxy Spectra: Emission and Absorption Lines?
Spectra from galaxies include both absorption and emission lines. I do understand how both types of spectral lines are produced but I am not quite sure where each type is coming from when we observe a galaxy. Few month ago I was told that emission lines are coming from the gas of the galaxy and absorption lines are coming from the stars of the galaxy. Is that correct? If it is, why the opposite doesn't happen? Why can't we have absorption lines coming from the gas of the galaxy? I am mostly (but not exclusively) interested in the Hydrogen lines.I have been looking at some spiral galaxy integral filed spectroscopy datacubes. I always see a very strong Ha emission line, never an Ha absorption line. Why?