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 (EW_rest) of Lyα emission at z ∼ 5-6. Combining our spectroscopic sample of LBGs at z ∼ 5 and those from the literature, we found that luminous LBGs at z ∼ 5-6 generally show weak Lyα emissions, while faint LBGs show a wide range of Lyα EW_rest and tend to have strong (EW_rest ≳ 20 Å) Lyα emissions; i.e., there is a deficiency of strong Lyα emission in luminous LBGs. There seems to be a threshold UV luminosity for the deficiency; it is M₁₄₀₀ = -21.5 to -21.0 mag, which is close to or somewhat brighter than the M_* of the UV luminosity function at z ∼ 5 and 6. Since the large EW_rest of Lyα emission can be seen among the faint LBGs, the ratio of Lyα emitters to LBGs may change rather abruptly with the UV luminosity. If the weakness of Lyα 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.