Question

Why is the cosmic star formation history peaking at redshift around 2? Hint: think of which processes are more (or less) active before (or after).

Answer 1

investigation into what the spectra of these galaxies are telling us. For example, astronomers compare emissions lines of elements like hydrogen, nitrogen, sulfur, and oxygen and find that galaxies at cosmic noon (redshift 2)have more ionized nitrogen and oxygen compared to hydrogen than their lower redshift counterparts. We say an atom has been “ionized” when an electron has been knocked out of its orbit around the nucleus of the atom. This process requires an input of energy to pry the electron loose. Therefore, we know that if we have more ionized atoms, the environment is probably filled with more energetic photons capable of ionizing those atoms. In this way, studying the abundance of ionized atoms in galaxies can tell us about what is occurring in the star forming regions.

The takeaway is that it is the change in star formation rate that is changing the electron density with redshift, rather than any other galaxy properties such as mass. Therefore, the increased content of ionized oxygen and nitrogen with respect to hydrogen is likely driven by the increased star formation rate. This could be due to the fact that the presence of star formation means more massive, young stars, which in turn means more energy is being released into the environment via shocks and stellar winds. This would cause the the pressure and density of the regions to increase.