Exploring He II λ 1640 emission line properties at z ~2-4

Deep optical spectroscopic surveys of galaxies provide a unique opportunity to investigate rest-frame ultra-violet (UV) emission line properties of galaxies at z~ 2-4.5. Here we combine VLT/MUSE Guaranteed Time Observations of the Hubble Deep Field South, Ultra Deep Field, COSMOS, and several quasar fields with other publicly available data from VLT/VIMOS and VLT/FORS2 to construct a catalogue of He IIλ1640 emitters at z2. The deepest areas of our MUSE pointings reach a 3σ line flux limit of 3.1 × 10-19 erg s-1 cm-2. After discarding broad-line active galactic nuclei, we find 13 He IIλ1640 detections from MUSE with a median MUV=-20.1 and 21 tentative He IIλ1640 detections from other public surveys. Excluding Lyα, all except two galaxies in our sample show at least one other rest-UV emission line, with C III] λ1907, λ1909 being the most prominent. We use multi-wavelength data available in the Hubble legacy fields to derive basic galaxy properties of our sample through spectral energy distribution fitting techniques. Taking advantage of the high-quality spectra obtained by MUSE (~10-30 h of exposure time per pointing), we use photo-ionisation models to study the rest-UV emission line diagnostics of the He IIλ1640 emitters. Line ratios of our sample can be reproduced by moderately sub-solar photo-ionisation models, however, we find that including effects of binary stars lead to degeneracies in most free parameters. Even after considering extra ionising photons produced by extreme sub-solar metallicity binary stellar models, photo-ionisation models are unable to reproduce rest-frame He IIλ1640 equivalent widths (~0.2-10 Å), thus additional mechanisms are necessary in models to match the observed He IIλ1640 properties.