MUSE HeII$\lambda1640$ analysis at $z=2-4$

TL;DR

This study uses deep MUSE observations to analyze HeII$\lambda1640$ emission at redshifts 2-4, revealing that current stellar models cannot fully explain observed fluxes, suggesting the need for alternative ionizing sources or models.

Contribution

It provides the first extensive observational analysis of HeII$\lambda1640$ emitters at z=2-4, highlighting limitations of existing stellar population models in reproducing observed emission.

Findings

HeII$\lambda1640$ emission driven by star formation at sub-solar metallicities

Current models cannot reproduce observed HeII$\lambda1640$ equivalent widths

Extremely low metallicities or alternative mechanisms are needed to explain observations

Abstract

HeII is the most sought-after emission line to detect and characterize metal free stellar populations. However, current stellar population/photo-ionization models lack sufficient He$^+$ ionising photons to reproduce observed HeII fluxes while being consistent with other emission lines. Using $\sim10-30$ hour deep pointings from MUSE, we obtain $\sim10$ $z\sim2-4$ HeII$\lambda1640$ emitters to study their inter-stellar medium (ISM) and stellar population properties. Emission line ratio diagnostics of our sample suggest that emission lines are driven by star-formation in solar to moderately sub-solar ($\sim 1/20$th) metallicity conditions. However, we find that even after considering effects from binary stars, we are unable to reproduce the HeII$\lambda1640$ equivalent widths (EWs). Our analysis suggest that extremely sub-solar metallicities ($\sim1/200$th) are required to reproduce observed HeII$\lambda1640$ luminosities. Thus, current stellar populations may require alternative mechanisms such as sub-dominant active galactic nuclei (AGN) or top heavy initial-mass-functions (IMFs) to compensate for the missing He$^+$ ionising photons.