Spectroscopic properties of luminous Lyman-α emitters at $z\approx 6-7$ and comparison to the Lyman-break population

Abstract

We present spectroscopic follow-up of candidate luminous Ly$\alpha$ emitters (LAEs) at $z=5.7-6.6$ in the SA22 field with VLT/X-SHOOTER. We confirm two new luminous LAEs at $z=5.676$ (SR6) and $z=6.532$ (VR7), and also present {\it HST} follow-up of both sources. These sources have luminosities L${}_{\mathrm{Ly}\alpha }\approx 3\times 10^{43}$ erg s${}^{-1}$, very high rest-frame equivalent widths of EW${}_0\gtrsim 200$ Å and narrow Ly$\alpha$ lines (200-340 km s${}^{-1}$). VR7 is the most UV-luminous LAE at $z>6.5$, with M${}_{1500}=-22.5$, even brighter in the UV than CR7. Besides Ly$\alpha$, we do not detect any other rest-frame UV lines in the spectra of SR6 and VR7, and argue that rest-frame UV lines are easier to observe in bright galaxies with low Ly$\alpha$ equivalent widths. We confirm that Ly$\alpha$ line-widths increase with Ly$\alpha$ luminosity at $z=5.7$, while there are indications that Ly$\alpha$ lines of faint LAEs become broader at $z=6.6$, potentially due to reionisation. We find a large spread of up to 3 dex in UV luminosity for $>L^{\star }$ LAEs, but find that the Ly$\alpha$ luminosity of the brightest LAEs is strongly related to UV luminosity at $z=6.6$. Under basic assumptions, we find that several LAEs at $z\approx 6-7$ have Ly$\alpha$ escape fractions $\gtrsim 100$ \%, indicating bursty star-formation histories, alternative Ly$\alpha$ production mechanisms, or dust attenuating Ly$\alpha$ emission differently than UV emission. Finally, we present a method to compute ${\xi }_{ion}$, the production efficiency of ionising photons, and find that LAEs at $z\approx 6-7$ have high values of log${}_{10}({\xi }_{ion}$/Hz erg${}^{-1})\approx 25.51\pm 0.09$ that may alleviate the need for high Lyman-Continuum escape fractions required for reionisation.

18 pages, 8 figures. Accepted for publication by MNRAS. Added new HST observations of VR7

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