A Search for z=7.3 Ly{\alpha} Emitters behind Gravitationally Lensing
  Clusters
  We searched for z=7.3 Lya emitters (LAEs) behind two lensing clusters, Abell
2390 and CL 0024, with the Subaru Telescope Suprime-Cam and a narrowband NB1006
(FWHM ~ 21 nm centered at 1005 nm). We investigated if there exist objects
consistent with the color of z=7.3 LAEs behind the clusters but could not
detect any LAEs to the unlensed line limit F(Lya) ~ 6.9 x 10^{-18} erg/s/cm^2.
Using several z=7 Lya luminosity functions (LFs) from the literature, we
estimated and compared the expected detection numbers of z ~ 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 ~ 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 inspected NB1006 images of three
z ~ 7 z-dropouts previously detected in Abell 2390 and found that none of them
are detected in NB1006. Two of them are consistent with predictions from the
previous studies that they would be at lower redshifts. The other one has a
photometric redshift of z ~ 7.3, and if it is at z=7.3, its unlensed Lya line
flux would be very faint: F(Lya) < 4.4 x 10^{-18} erg/s/cm^2 (1 sigma upper
limit) or rest frame equivalent width of W(Lya) < 26A. Its Lya emission might
be attenuated by neutral hydrogen, as recent studies show that the fraction of
Lyman break galaxies displaying strong Lya emission is lower at z ~ 7 than at z
<~ 6.
