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We present the ultra-deep Subaru narrowband imaging survey for Lya emitters(LAEs) at $z=7.3$ in SXDS and COSMOS fields with a total integration time of106 hours. Exploiting our new sharp bandwidth filter, NB101, installed onSuprime-Cam, we have reached $L(Lya)=2.4\times10^{42} \ erg \ s^{-1}$($5\sigma$) for $z=7.3$ LAEs, about 4 times deeper than previous Subaru $z\gtrsim 7$ studies, which allows us to reliably investigate evolution of Lyaluminosity function (LF), for the first time, down to the luminosity limit sameas those of Subaru $z=3.1-6.6$ LAE samples. Surprisingly, we only find threeand four LAEs in SXDS and COSMOS fields, respectively, while one expects atotal of $\sim 65$ LAEs by our survey in the case of no Lya LF evolution from$z=6.6$ to $7.3$.We identify a decrease of Lya LF from $z=6.6$ to $7.3$ at the$>90\%$ confidence level from our $z=7.3$ Lya LF.Moreover, the evolution of LyaLF is clearly accelerated at $z>6.6$ beyond the measurement uncertaintiesincluding cosmic variance. Because no such accelerated evolution ofUV-continuum LF or cosmic star-formation rate (SFR) is found at $z\sim 7$, butsuggested only at $z>8$ (Oesch et al. 2013, Bouwens et al. 2014), thisaccelerated Lya LF evolution is explained by physical mechanisms different froma pure SFR decrease but related to Lya production and escape in the process ofcosmic reionization. Because a simple accelerating increase of IGM neutralhydrogen absorbing Lya would not reconcile with Thomson scattering opticaldepth measurements from WMAP and Planck, our findings may support new physicalpictures suggested by recent theoretical studies, such as the existence of HIclumpy clouds within cosmic ionized bubbles selectively absorbing Lya and thelarge ionizing photon escape fraction of galaxies making weak Lya emission.

astrophysical journal/the astrophysical journal

Accelerated Evolution of Ly$\alpha$ Luminosity Function at $\textit{z}  \gtrsim 7$ Revealed by the Subaru Ultra-Deep Survey for Ly$\alpha$ Emitters  at $\textit{z}=7.3$