Finlator et al. explores cosmic reionization

What created the light that flowed between galaxies when the Universe was only a billion years old? Measuring it is akin to taking the young Universe’s pulse, and working out where it came from is akin to guessing the Universe’s diet. The light could have come from hot, massive stars within young galaxies.  Alternatively, it could have come from even deeper within the galaxies, where hot, dense gas glowed brightly before disappearing into monstrous black holes (quasars).
As it happens, clouds of diffuse gas that linger between galaxies are irradiated by this light. These clouds contain heavy elements such as carbon and silicon. If the light is redder (as expected from galaxies) then the heavy elements will retain more of their electrons.  Conversely, if the light is bluer (as expected from quasars) then the heavy elements will have had more of their electrons stripped off by the energetic light.
We compared observations of heavy elements in diffuse extragalactic clouds to a numerical simulation using three different models for the extragalactic light.  In conflict with recent suggestions that the light may have come entirely from quasars, we found that models in which the light came mostly from galaxies explained the observations by far the best.  These results add to a growing chorus of arguments that quasars did not generate most of the extragalactic light when the Universe was less than three billion years old.
The paper in MNRAS can be viewed here.

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