Deficiency of large equivalent width Lyα emission in luminous lyman break galaxies at z ∼ 5-6?

We report a deficiency of luminous Lyman break galaxies (LBGs) with a large rest-frame equivalent width (EWrest) of Ly alpha emission at z similar to 5-6. Combining our spectroscopic sample of LBGs at and those from the literature, we found that luminous LBGs at generally show weak Lya emissions, while faint LBGs show a wide range of Ly alpha EWrest and tend to have strong (EWrest greater than or similar to 20 angstrom) Ly alpha emissions; i. e., there is a deficiency of strong Lya emission in luminous LBGs. There seems to be a threshold UV luminosity for the deficiency; it is M-1400 = -21.5 to -21.0 mag, which is close to or somewhat brighter than the M-* of the UV luminosity function at z similar to 5 and 6. Since the large EWrest of Ly alpha emission can be seen among the faint LBGs, the ratio of z similar to 5 Lya emitters to LBGs may change rather abruptly with the UV luminosity. If the weakness of Lya emission is due to dust absorption, the deficiency suggests that luminous LBGs at z = 5 - 6 tend to be in dusty and more chemically evolved environments and that they begin star formation earlier than faint LBGs, although other causes cannot be ruled out.