We searched for z = 7.3 Ly \alpha emitters ( LAEs ) behind two gravitationally lensing clusters , Abell 2390 and CL 0024 , using the Subaru Telescope Suprime-Cam and a narrowband filter NB1006 ( \lambda _ { c } \sim 1005 nm , FWHM \sim 21 nm ) .
The combination of the fully depleted CCDs of the Suprime-Cam , sensitive to z \sim 7 Ly \alpha emission at \sim 1 \mu m , and the magnification by the lensing clusters can be potentially a powerful tool to detect faint distant LAEs .
Using the NB1006 and deep optical to mid-infrared images of the clusters taken with the Hubble and Spitzer Space Telescopes , we investigated if there exist objects consistent with the color of z = 7.3 LAEs behind the clusters .
We could not detect any LAEs to the unlensed Ly \alpha line flux limit of F _ { { Ly } \alpha } \simeq 6.9 \times 10 ^ { -18 } erg s ^ { -1 } cm ^ { -2 } .
Using several z = 7 Ly \alpha luminosity functions ( LFs ) from the literatures , we estimated and compared the expected detection numbers of z \sim 7 LAEs in lensing and blank field surveys in the case of using an 8m class ground based telescope .
Given the steep bright-end slope of the LFs , when the detector field-of view ( FOV ) is comparable to the angular extent of a massive lensing cluster , imaging cluster ( s ) is more efficient in detecting z \sim 7 LAEs than imaging a blank field .
However , the gain is expected to be modest , a factor of two at most and likely much less depending on the adopted LFs .
The main advantage of lensing-cluster survey , therefore , remains to be the gain in depth and not necessarily in detection efficiency .
For much larger detectors , the lensing effect becomes negligible and the efficiency of LAE detection is proportional to the instrumental FOV .
We also investigated the NB1006 images of the three z \sim 7 z -dropout galaxy candidates previously detected in Abell 2390 and found that none of them are detected in the NB1006 .
Two of them are consistent with the predictions from the previous studies that they would be at lower redshifts .
The other one has a photometric redshift of z \simeq 7.3 , and if we assume that it is at z = 7.3 , the unlensed Ly \alpha line flux would be very faint : F _ { { Ly } \alpha } < 4.4 \times 10 ^ { -18 } erg s ^ { -1 } cm ^ { -2 } ( 1 \sigma upper limit ) or equivalent width of W _ { { Ly } \alpha } ^ { rest } < 26 Å .
Its Ly \alpha emission might be attenuated by neutral hydrogen , as recent studies show that the fraction of Lyman break galaxies displaying strong Ly \alpha emission is lower at z \sim 7 than at z \la 6 .