2015ApJ...808..139S

Faint Lyα emitters become increasingly rarer toward the reionization epoch (z ∼ 6-7). However, observations from a very large (∼5 deg²) Lyα narrow-band survey at z = 6.6 show that this is not the case for the most luminous emitters, capable of ionizing their own local bubbles. Here we present follow-up observations of the two most luminous Lyα candidates in the COSMOS field: ''MASOSA'' and ''CR7.'' We used X-SHOOTER, SINFONI, and FORS2 on the Very Large Telescope, and DEIMOS on 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 Lyα with a line width of 386±30 km/s (FWHM) and with very high EW₀(>200 Å), but undetected in the continuum, implying very low stellar mass and a likely young, metal-poor stellar population. ''CR7,'' with an observed Lyα luminosity of 10^43.92±0.05 erg/s is the most luminous Lyα emitter ever found at z > 6 and is spatially extended (∼16 kpc). ''CR7'' reveals a narrow Lyα line with 266±15 km/s FWHM, being detected in the near-infrared (NIR) (rest-frame UV; β = -2.3±0.1) and in IRAC/Spitzer. We detect a narrow He ii 1640 Å emission line (6σ, FWHM = 130±30 km/s) in CR7 which can explain the clear 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 (He ii/O iii] 1663 Å > 3 and He ii/C iii] 1908 Å > 2.5). We conclude 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 presumably dominates the mass. Hubble Space Telescope/WFC3 observations show that the light is indeed spatially separated between a very blue component, coincident with Lyα and He ii 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.