MUSE HeIIλ1640 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 10-30 hour deep pointings from MUSE, we obtain 10 z2-4 HeIIλ1640 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 ( 1/20th) metallicity conditions. However, we find that even after considering effects from binary stars, we are unable to reproduce the HeIIλ1640 equivalent widths (EWs). Our analysis suggest that extremely sub-solar metallicities (1/200th) are required to reproduce observed HeIIλ1640 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.
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