MUSE HeII$\lambda 1640$ analysis at $z=2-4$

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 $\sim 10-30$ hour deep pointings from MUSE, we obtain $\sim 10$ $z\sim 2-4$ HeII$\lambda 1640$ 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$\lambda 1640$ equivalent widths (EWs). Our analysis suggest that extremely sub-solar metallicities ($\sim 1/200$th) are required to reproduce observed HeII$\lambda 1640$ 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.

To appear in Early Science with ELTs Proceedings IAU Symposium No. 347, 2018, 4 pages, 2 figures

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