Diagnostics for PopIII galaxies and Direct Collapse Black Holes in the early universe

TL;DR

This paper develops diagnostic diagrams using photoionisation models to identify and distinguish PopIII galaxies and Direct Collapse Black Holes in the early universe, aiding future observational efforts.

Contribution

It introduces new diagnostic tools involving nebular lines, combining optical and UV diagnostics, to differentiate early universe objects in low metallicity environments.

Findings

Optical diagnostics have high discriminatory power for early universe objects.

UV diagnostics can also effectively distinguish object classes.

Metal lines like [OIII] 5007A and CIV 1549A remain strong even in extremely metal-poor environments.

Abstract

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 photoionisation 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 ISM (Z~10^{-5}-10^{-4}), (metal poor) PopII and AGN in enriched ISM. Diagnostics involving rest-frame optical lines (that will be accessible by JWST) have a better discriminatory power, but also rest-frame UV diagnostics can provide very useful information. Interestingly, we find that metal lines such as [OIII] 5007A and CIV 1549A can remain relatively strong (about a factor of 0.1-1 relative H-beta and HeII 1640A, respectively), even in extremely metal poor environments (Z~10^{-5}-10^{-4}), which could be embedding PopIII galaxies and DCBH.