When the combined light of the stars and the hot HII regions pass through these, some of it is absorbed by the colder atoms and re-emitted in random directions, with a probability of being re-emitted in the exact same direction it came in being practically zero. Further out, things are a bit cooler and calmer, and clouds of neutral hydrogen containing variable amounts of heavier elements are more numerous. The very hot, ionized regions describes about are most likely to be found in the central regions of a galaxy. The caption below is mine from my Bachelor's project paper.Ībsorption happens both in the cold, neutral ISM, the hot, ionized ISM and in stellar atmospheres Taking the average will enhance the features that they have in common while suppressing features that are peculiar to individual galaxies. Note the drop in flux on the blue side of the Lyman-$\alpha$ line. The wavelength offsets here can also tell us about the internal kinematics of the galaxy.īelow is a figure from this article that shows an average spectrum from a large number of star forming galaxies. The relative strengths of these lines depend on the chemical composition, temperature and other physical properties of the hot gas, so studying these line intensity ratios can often teach us a lot about the galaxies we observe.īesides, sometimes we can also see flourescent lines, when photons from the hot gas excite atoms in colder, non-ionized clouds which then re-emit them in slightly different wavelengths. The absolute strengths of the emission lines depend on various parameters, of which star formation rate (SFR) is the most important - galaxies with ceased star formation show very few emission lines. The energy radiated in all these lines doesn't come from the clouds themselves, but from the most energy-rich photons, so the gas will leave a drop in intensity on the blue side of these wavelengths in the spectrum (multiple drops of different strength, to be precise). The ions and free electrons will recombine, and the excited states will decay, resulting in a cascade of electrons dropping between energy levels in the different atoms, resulting in a series of emission lines that is added to the spectrum. The galaxy will often have regions of star formation, in which the strong radiation ionize the surrounding interstellar medium, creating a so called HII Region. Emission lines originate in the hot ISM and in stellar atmospheres The shape of this composite spectrum depends heavily on the composition of stellar types - a young, irregular star-burst galaxy and an old, dying elliptical will show widely different shapes of the continuum. There will generally be some stellar absorption features, but not many, and the combined light of the stars will often show a nice, relatively smooth profile. This will often be dominated by the hotter and heavier stars, but the other spectral types also blend in. The spectrum of the galaxy, however, is a mix of all the stars within it. These differences in spectra are actually the basis of the classification of stars in the spectral types O, B, A, F, G, K, M and L. Stellar spectra are approximately a black body spectrum, but this mostly holds true for the most hot and luminous O and B type stars for cooler stars, the image already gets quite a bit more muddled, as they will show more and more absorption happening in the stellar photosphere, as well as other effects that ruin the nice, smooth black body spectrum. The light is emitted by stars in the galaxy. A blend of starlight of different spectral types makes up the continuum. It is still quite lengthy, though, so if you're impatient, I've summarised it at the bottom. A galaxy spectrum is a quite complex and complicated topic, and many entire careers are fully devoted to understanding them, so this can only be a simplified answer.
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