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

¹ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
e-mail: paolo.cassata@oamp.fr
² UPMC–CNRS, UMR 7095, Institut d’Astrophysique de Paris, 75014 Paris, France
³ Institut de Recherche en Astrophysique et Planétologie, CNRS, Université de Toulouse, 14 avenue E. Belin, 31400 Toulouse, France
⁴ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ IASF–INAF, via Bassini 15, 20133 Milano, Italy
⁶ Astronomical Observatory of the Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
⁷ National Centre for Nuclear Research, ul. Hoża 69, 00-681 Warsaw, Poland
⁸ INAF IASFBO, via P. Gobetti 101, 40129 Bologna, Italy

Received: 20 December 2012
Accepted: 22 April 2013

Abstract

Aims. The aim of this work is to identify He II emitters at 2 < z < 4.6 and to constrain the source of the hard ionizing continuum that powers the He II emission.

Methods. We assembled a sample of 277 galaxies with a highly reliable spectroscopic redshift at 2 < z < 4.6 from the VIMOS-VLT Deep Survey (VVDS) Deep and Ultra-Deep data, and we identified 39 He II λ1640 emitters. We studied their spectral properties, measuring the fluxes, equivalent widths (EW), and full width at half maximum (FWHM) for most relevant lines, including He II λ1640, Lyα line, Si II λ1527, and C IV λ1549.

Results. About 10% of galaxies at z ~ 3 and i_AB ≤ 24.75 show He II in emission, with rest frame equivalent widths EW₀ ~ 1–7 Å, equally distributed between galaxies with Lyα in emission or in absorption. We find 11 (3.9% of the global population) reliable He II emitters with unresolved He II lines (FWHM₀ < 1200 km s^-1), 13 (4.6% of the global population) reliable emitters with broad He II emission (FWHM₀ > 1200 km s^-1), 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 narrow He II line emitters are in agreement with expectations for a Population III (PopIII) star formation, if the episode of star 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 interstellar medium 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 in PopIII stars of 10^-6  M_⊙   yr^-1   Mpc^-3, higher than, but qualitatively comparable to the value predicted by Tornatore and collaborators.