He II emitters in the VIMOS VLT Deep Survey: Population III star formation or peculiar stellar populations in galaxies at 2< z< 4.6?

Aims. The aim of this work is to identify He II emitters at 2 1200km/s), 3 Active Galactic Nuclei (AGN), and an additional 12 possible He II emitters. The properties of the individual broad emitters are in agreement with expectations from a Wolf-Rayet (W-R) model. Instead, the properties of the narrow emitters are not compatible with this model, nor with predictions of gravitational cooling radiation produced by gas accretion, unless this is severely underestimated by current models by more than two orders of magnitude. Rather, we find that the EW of the na rrow He II line emitters are in agreement with expectations for a Population III (PopIII) star formation, if the episode of sta r formation is continuous, and we calculate that a PopIII Star Formation Rate (SFR) of 0.1 ‐ 10 M⊙yr −1 alone is enough to sustain the observed He II flux. Conclusions. We conclude that narrow He II emitters are powered either by the ionizing flux from a stellar population rare at z∼ 0 but much more common at z∼ 3, or by PopIII star formation. As proposed by Tornatore and collaborators, incomplete Inter Stellar Medium (ISM) mixing may leave some small pockets of pristine gas at the periphery of galaxies from which PopIII may form, even down to z∼ 2 or lower. If this interpretation is correct, we measure at z∼ 3 a Star Formation Rate Density (SFRD) in PopIII stars of 10 −6 M⊙yr −1 M pc −3 , higher than, but qualitatively comparable to the value pre dicted by Tornatore and collaborators.