Lyman 'bump' galaxies - II. A possible signature of massive extremely metal-poor or metal-free stars in z= 3.1 Lyα emitters

Deep narrow-band (NB359) imaging with Subaru telescope by Iwata et al. has detected a surprisingly strong Lyman continuum (LyC; ~900Å in the rest frame) from some Lyman α emitters (LAEs) at z= 3.1. However, the possibility of a redshift misidentification by the previous spectroscopic studies due to a narrow wavelength coverage cannot be rejected. Here we present the results of a new technique, the deep spectroscopy, in which we covered 4000-7000Å with VLT/VIMOS and Subaru/FOCAS for the eight LAEs detected in NB359. All the eight objects have only one detectable emission line around 4970Å, which is most likely to be Lyα at z= 3.1, and thus, the objects are certainly LAEs at the redshift. However, five of them show a ~0.8arcsec spatial offset between the Lyα emission and the source detected in NB359. No indications of the redshifts of the NB359 sources are found although it is statistically difficult that all the five LAEs have a foreground object accounting for the NB359 flux. The rest three LAEs show no significant offset from the NB359 position. Therefore, we conclude that they are truly LyC-emitting LAEs at z= 3.1. We also examine the stellar population which simultaneously accounts for the strength of the LyC and the spectral slope of non-ionizing ultraviolet of the LAEs. We consider the latest statistics of Lyman limit systems to estimate the LyC optical depth in the intergalactic medium (IGM) and an additional contribution of the bound-free LyC from photoionized nebulae to the LyC emissivity. As a result, we find that stellar populations with metallicity Z≥ 1/50Z_⊙ can explain the observed LyC strength only with a very top-heavy initial mass function (IMF; 〈m〉~ 50M_⊙). However, the critical metallicity for such an IMF is expected to be much lower. A very young (~1Myr) and massive (~100M_⊙) extremely metal-poor (Z≤ 5 × 10^-4Z_⊙) or metal-free (so-called Population III) stellar population can reproduce the observed LyC strength. The required mass fraction of such 'primordial' stellar population is ~1-10 per cent in total stellar mass of the LAEs. We also present a possible evolutionary scenario of galaxies emitting strong LyC and implications of the primordial stars at z~ 3 for the metal enrichment in the intergalactic medium and for the ionizing background and reionization.