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The aim of this work is to identify HeII emitters at 2<z<4.6 and to constrainthe source of the hard ionizing continuum that powers the HeII emission. Wehave assembled a sample of 277 galaxies with a high quality spectroscopicredshift at 2<z<4.6 from the VVDS survey, and we have identified 39 HeII1640Aemitters. We study their spectral properties, measuring the fluxes, equivalentwidths (EW) and FWHM for most relevant lines. About 10% of galaxies at z~3 showHeII in emission, with rest frame equivalent widths EW0~1-7A, equallydistributed between galaxies with Lya in emission or in absorption. We find 11high-quality HeII emitters with unresolved HeII line (FWHM_0<1200km/s), 13high-quality emitters with broad He II emission (FWHM_0>1200km/s), 3 AGN, andan additional 12 possible HeII emitters. The properties of the individual broademitters are in agreement with expectations from a W-R model. On the contrary,the properties of the narrow emitters are not compatible with such model,neither with predictions of gravitational cooling radiation produced by gasaccretion. Rather, we find that the EW of the narrow HeII line emitters are inagreement with expectations for a PopIII star formation, if the episode of starformation is continuous, and we calculate that a PopIII SFR of 0.1-10 Mo yr-1only is enough to sustain the observed HeII flux. We conclude that narrow HeIIemitters are either powered by the ionizing flux from a stellar population rareat z~0 but much more common at z~3, or by PopIII star formation. As proposed byTornatore et al. (2007), incomplete ISM mixing may leave some small pockets ofpristine gas at the periphery of galaxies from which PopIII may form, even downto z~2 or lower. If this interpretation is correct, we measure at z~3 a SFRD inPopIII stars of 10^6Mo yr^-1 Mpc^-3 qualitatively comparable to the valuepredicted by Tornatore et al. (2007).

HeII emitters in the VIMOS VLT Deep Survey: PopIII star formation or peculiar stellar populations in galaxies at 2