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A possible signature of primordial stellar populations in $z=3$ Lyman $α$ emitters

Observations with Subaru telescope have detected surprisingly strong Lyman continuum (LyC; $\sim900$ Å in the rest-frame) from some Lyman $α$ emitters (LAEs) at $z=3.1$. We have examined the stellar population which simultaneously accounts for the strength of the LyC and the spectral slope of non-ionizing ultraviolet of the LAEs. As a result, we have found that stellar populations with metallicity $Z\geq1/50 Z_\odot$ can explain the observed LyC strength only with a very top-heavy initial mass function (IMF; $< m > \sim 50 M_\odot$). However, the critical metallicity for such an IMF is expected to be much lower. A very young ($\sim1$ Myr) and massive ($\sim100$ $M_\odot$) extremely metal-poor ($Z\leq5\times10^{-4}Z_\odot$) or metal-free (so-called Population III) stellar population can also reproduce the observed LyC strength if the mass fraction of such 'primordial' stellar population is $\sim1$% in total stellar mass of the LAEs.