A New Constraint on Reionization by a Census of $z=7.0$ Ly$α$ Emitters with a Deep and Large Sample to 0.3 $L^*$: Evolution of Ly$α$ Luminosity Function and Equivalent Width at $z\sim6-7$ [GA]


We conduct a census of 20 $z=7.0$ Ly$\alpha$ emitters (LAEs) detected in Subaru Deep Field (SDF) and Subaru XMM Deep Survey (SXDS) field to a Ly$\alpha$ luminosity limit $L({\rm Ly}\alpha) = 2.0\times 10^{42}$ erg s$^{-1}$ or $\sim 0.3$ $L^*_{z=7}$ in a volume $V=6.1\times 10^5$ Mpc$^3$ by our $\sim 80$ and 37 hrs of Subaru Telescope Suprime-Cam narrowband NB973 and reddest optical y-band imagings. We compare Ly$\alpha$ luminosity function (LF), rest frame UV LF, Ly$\alpha$ equivalent width (EW) distribution of $z=7.0$ LAEs to those of $z=5.7$, 6.6 and 7.3 LAEs from previous Suprime-Cam surveys to comparable limits. The Ly$\alpha$ LF significantly declines from $z=5.7$ to 7.0 at the bright to faint end and from $z=6.6$ to 7.0 at the faint end. It more significantly declines from $z=7.0$ to 7.3. Meanwhile, though the UV LF of LAEs does not evolve much at $z=5.7$-6.6, it modestly declines from $z=6.6$ to 7.0. Furthermore, in addition to the systematic decrease in Ly$\alpha$ EWs from $z=5.7$ to 6.6 previously found, we find that 2/3 of our $z=7.0$ LAEs detected in the UV continuum have lower EWs than those of $z=6.6$ LAEs. These results indicate that the Ly$\alpha$ LF acceleratingly decreases at $z>5.7$ as suggested by previous studies, and the decline of the UV LF at $z=6.6$-7.0 suggests that galaxy evolution contributes to the declines of the Ly$\alpha$ LF and EWs at this epoch. Comparison of our $z=7.0$ Ly$\alpha$ LF to the one predicted by a recent LAE evolution model further reveals that galaxy evolution alone cannot explain all the decline of the Ly$\alpha$ LF at $z=7.0$. If we attribute the discrepancy to Ly$\alpha$ attenuation by neutral hydrogen, the intergalactic medium transmission of Ly$\alpha$ photons at $z=7.0$ would be $\leq 0.6$-0.7. It is lower (higher) than the one at $z=6.6$ ($z=7.3$) derived by previous studies, suggesting that neutral fraction increases at $z > 6$.

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K. Ota, M. Iye, N. Kashikawa, et. al.
Wed, 8 Mar 17

Comments: 36 Pages, 22 Figures, 10 Tables, Submitted to ApJ