2022MNRAS.513.5134N

Forthcoming observational facilities will make the exploration of the early universe routine, likely probing large populations of galaxies at very low metallicities. It will therefore be important to have diagnostics that can solidly identify and distinguish different classes of objects in such low metallicity regimes. We use new photoionization models to develop diagnostic diagrams involving various nebular lines. We show that combinations of these diagrams allow the identification and discrimination of the following classes of objects in the early universe: PopIII and direct collapse black holes (DCBH) in pristine environments, PopIII and DCBH embedded in slightly enriched interstellar medium (ISM; Z ∼10^–5 - 10^–4), and (metal poor) PopII and active galactic nucleus in enriched ISM. Diagnostics involving rest-frame optical lines (that will be accessible by James Webb Space Telescope) have a better discriminatory power but also rest-frame ultraviolet diagnostics can provide very useful information. Interestingly, we find that metal lines such as [O III]λ5007 and C IVλ1549 can remain relatively strong (about a factor of 0.1-1 relative H β and He IIλ1640, respectively), even in extremely metal poor environments (Z ∼10^–5-10^-4), which could be embedding PopIII galaxies and DCBH.