1204.3234 (Kazuaki Ota et al.)

A Search for z=7.3 Lyα Emitters behind Gravitationally Lensing Clusters    [PDF]

Kazuaki Ota, Johan Richard, Masanori Iye, Takatoshi Shibuya, Eiichi Egami, Nobunari Kashikawa

We searched for z=7.3 Ly{\alpha} emitters (LAEs) behind two lensing clusters, Abell 2390 and CL 0024, using the Subaru Telescope Suprime-Cam and a narrowband NB1006 ({\lambda}c ~ 1005 nm, FWHM ~ 21 nm). Combination of the fully depleted CCDs of the Suprime-Cam, sensitive to z ~ 7 Ly{\alpha} at ~ 1 {\mu}m, and magnification by the lensing clusters can be potentially a powerful tool to detect faint distant LAEs. Using NB1006 and deep optical-infrared images 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} flux limit F(Ly{\alpha}) ~ 6.9 x 10^{-18} erg s^{-1} cm^{-2}. Comparison with z ~ 7 LAE field searches suggests that a blank field survey covering an area sufficiently larger than lensing clusters is more efficient in finding a large number of z ~ 7 LAEs than both a lensing survey observing many clusters with shallow imaging and a lensing survey imaging one cluster to a deeper luminosity limit, expected from the bright end slope of several z ~ 7 Ly{\alpha} luminosity functions. We also investigated the NB1006 images of the three z ~ 7 z-dropout galaxy candidates 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 we assume that it is at z=7.3, the unlensed Ly{\alpha} line flux would be very faint: F(Ly{\alpha}) < 4.4 x 10^{-18} erg s^{-1} cm^{-2} (1 {\sigma} upper limit) or rest frame equivalent width of W(Ly{\alpha}) < 26A. 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 ~ 7 than at z <~ 6.

View original: http://arxiv.org/abs/1204.3234