We have performed deep imaging surveys for Lyman \alpha emitters ( LAEs ) at redshift \sim 7.3 in two blank fields , the Subaru Deep Field ( SDF ) and the Subaru/XMM-Newton Deep survey Field ( SXDF ) , using the Subaru/Suprime-Cam equipped with new red-sensitive CCDs and a new narrow-band filter , NB1006 ( \lambda _ { c } = 10052 Å , FWHM \Delta \lambda = 214 Å ) .
We identified four objects as LAE candidates that exhibit luminosity excess in NB1006 .
By carrying out deep follow-up spectroscopy for three of them using Subaru/FOCAS and Keck/DEIMOS , a definitively asymmetric emission line is detected for one of them , SXDF-NB1006-2 .
Assuming this line is Ly \alpha , this object is a LAE at z = 7.215 which has luminosity of 1.2 ^ { +1.5 } _ { -0.6 } \times 10 ^ { 43 } erg s ^ { -1 } and a weighted skewness S _ { \omega } = 4.90 \pm 0.86 .
Another object , SDF-NB1006-2 , shows variable photometry and is thus probably a quasar ( QSO ) or an active galactic nucleus ( AGN ) .
It shows an asymmetric emission line at 10076 Å which may be due to either Ly \alpha at z = 7.288 or [ O ii ] at z = 1.703 .
The third object , SDF-NB1006-1 , is likely a galaxy with temporal luminosity enhancement associated with a supernova explosion , as the brightness of this object varies between the observed epochs .
Its spectrum does not show any emission lines .
The inferred decrease in the number density of LAEs toward higher redshift is n _ { Ly \alpha } ^ { z = 7.3 } / n _ { Ly \alpha } ^ { z = 5.7 } = 0.05 ^ { +0.11 } _ { -0.05 } from z = 5.7 to 7.3 down to L ^ { Ly \alpha } = 1.0 \times 10 ^ { 43 } erg s ^ { -1 } .
The present result is consistent with the interpretation in previous studies that the neutral hydrogen fraction is rapidly increasing from z = 5.7 to 7.3 .