Faint Lyman-$α$ (Ly$α$) emitters become increasingly rarer towards the re-ionisation epoch (z~6-7). However, observations from a very large (~5deg²) Ly$α$ survey at z=6.6 (Matthee et al. 2015) show that this is not the case for the most luminous emitters. Here we present follow-up observations of the two most luminous z~6.6 Ly$α$ candidates in the COSMOS field: `MASOSA' and `CR7'. We used X-SHOOTER, SINFONI and FORS2 (VLT), and DEIMOS (Keck), to confirm both candidates beyond any doubt. We find redshifts of z=6.541 and z=6.604 for MASOSA and CR7, respectively. MASOSA has a strong detection in Ly$α$ with a line width of 386±30 km/s (FWHM) and with high EW₀ (>200 Å), but it is undetected in the continuum. CR7, with an observed Ly$α$ luminosity of 1043.93±0.05erg/s is the most luminous Ly$α$ emitter ever found at z>6. CR7 reveals a narrow Ly$α$ line with 266±15 km/s FWHM, being detected in the NIR (rest-frame UV, with β=-2.3±0.1) with an excess in J, and also strongly detected in IRAC/Spitzer. We detect a narrow HeII1640$Å$ emission line ($6σ$) which explains the excess seen in the J band photometry (EW₀~80 Å). We find no other emission lines from the UV to the NIR in our X-SHOOTER spectra (HeII/OIII]1663A>3 and HeII/CIII]1908A>2.5). We find that CR7 is best explained by a combination of a PopIII-like population which dominates the rest-frame UV and the nebular emission, and a more normal stellar population which dominates the mass. HST/WFC3 observations show that the light is indeed spatially separated between a very blue component, coincident with Ly$α$ and HeII emission, and two red components (~5 kpc away), which dominate the mass. Our findings are consistent with theoretical predictions of a PopIII wave, with PopIII star formation migrating away from the original sites of star formation.
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Sobral et al. (2015) studied this question.