Galaxy Spectra: Emission and Absorption Lines?

Question

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?

Answer 1

Hi there,

Atoms make up everything.

And atoms can do BOTH emit and absorb photons.

Photons are absorbed when the wavelength of photon is equal to one of the energy gaps of the atom (since wavelength/frequency is proportional to energy of a photon). These appears as dark lines or no peak (depending what program you're using to analyze these lines) because they were absorbed by the electron and that electron gets excited to the next energy level. Then, the excited electron will want to go to a lower energy state (with appropriate conditions) and when it drops to the lower levels, it releases (or emits) that energy in the form of photons.