Confirmation of double peaked Lyman-alpha emission at z=6.593:
  Witnessing a galaxy directly contributing to the reionisation of the Universe
  Distant luminous Lyman-alpha emitters (LAEs) are excellent targets for
spectroscopic observations of galaxies in the epoch of reionisation (EoR). We
present deep high-resolution (R=5000) VLT/X-SHOOTER observations, along with an
extensive collection of photometric data of `COLA1', a proposed double peaked
LAE at z=6.6 (Hu et al. 2016). We rule out that COLA1's emission line is an
[OII] doublet at z=1.475 on the basis of i) the asymmetric red line-profile and
flux ratio of the peaks (blue/red=$0.31\pm0.03$) and ii) an unphysical [OII]/Ha
ratio ([OII]/Ha > 22). We show that COLA1's observed B-band flux is explained
by a faint extended foreground LAE, for which we detect Lya and [OIII] at
z=2.142. We thus conclude that COLA1 is a real double-peaked LAE at z=6.593,
the first discovered at z>6, confirming the result from Hu et al. (2016). COLA1
is UV luminous (M$_{1500}=-21.6\pm0.3$), has a high equivalent width
(EW$_{0}$~120 \AA) and very compact Lya emission (r$_{50} = 0.3$ kpc).
Relatively weak inferred Hb+[OIII] line-emission from Spitzer/IRAC indicates an
extremely low metallicity of Z<1/20 Z$_{\odot}$ or reduced strength of nebular
lines due to high escape of ionising photons. The small Lya peak separation of
$220\pm20$ km/s implies a low HI column density and an ionising photon escape
fraction of ~15-30 %, providing the first direct evidence that such galaxies
contribute actively to the reionisation of the Universe at z>6. Based on simple
estimates, we find that COLA1 could have provided just enough photons to
reionise its own ~0.3 pMpc bubble, allowing the blue Lya line to be observed.
However, we also discuss alternative scenarios explaining the detected double
peaked nature of COLA1. Our results show that future high-resolution
observations of statistical samples of double peaked LAEs at z>5 are a
promising probe of the occurrence of ionised regions around galaxies in the
EoR.
