Lyα emitters at z= 6.5 in the SSA22 field: an area more neutral or void at the end of the re-ionization epoch


  • This work is based on data collected at Subaru Telescope and obtained from the Subaru–Mitaka–Okayama–Kiso Archive System (SMOKA), which is operated by the Astronomy Data Center, National Astronomical Observatory of Japan.



We present results of a survey of Lyα emitters (LAEs) at z= 6.5 which is thought to be the final epoch of the cosmic re-ionization. In a ≈530 arcmin2 deep image of the SSA22 field taken through a narrow-band filter NB912 installed in the Subaru/Suprime-Cam, we have found only 14 LAE candidates with LLyα≳ 3 × 1042 erg s−1. Even applying the same colour selection criteria, the number density of the LAE candidates is a factor of 3 smaller than that found at the same redshift in the Subaru Deep Field (SDF). Assuming the number density in the SDF is a cosmic average, the probability to have a number density equal to or smaller than that found in the SSA22 field is only 7 per cent if we consider fluctuation by the large-scale structure (i.e. cosmic variance) and Poisson error. Therefore, the SSA22 field may be a rare void at z= 6.5. On the other hand, we have found that the number density of i′-drop galaxies with 25.5 < z′ < 26.0 in the SSA22 field agrees well with that in the SDF. If we consider a scenario that a larger neutral fraction of intergalactic hydrogen, xH I, in the SSA22 field obscures a part of Lyα emission, xH I in the SSA22 field should be about two times larger than that in the SDF. This can be translated into xH I < 0.9 at z= 6.5 in the SSA22 field. A much larger survey area than previous ones is required to overcome a large fluctuation reported here and to obtain a robust constraint on xH I at the end of the re-ionization from LAEs.