Lyman break galaxies at z ∼ 5: Rest-frame UV spectra. II

We present spectroscopy results of Lyman break galaxies (LBGs) at z ∼ 5 in the J0053+1234 field. Among five bright candidates with z′ < 25.0mag, two objects are confirmed to be at z ∼ 5. The equivalent widths (EWs) of Lya emission of the 2 LBGs are not so strong as to be detected as Lya emitters, and one of them shows strong low-ionization interstellar (LIS) metal absorption lines. Two faint objects with z′ ≥ 25.0 mag are also confirmed to be at z ∼ 5, and their spectra show strong Lya emission in contrast to bright ones. These results suggest a deficiency of strong Lya emission in bright LBGs at z ∼ 5, as discussed in our previous paper. Combined with our previous spectra of LBGs at z ∼ 5, obtained around the Hubble Deep Field-North (HDF-N), we made a composite spectrum of luminous (M₁₄₀₀ ≤ -21.5 mag) LBGs at z ∼ 5. The resultant spectrum shows weak Lya emission and strong LIS absorptions, which suggest that the bright LBGs at z ∼ 5 have chemically evolved at least to ∼ 0.1 solar metallicity. For a part of our sample in the HDF-N region, we obtained near-to-mid infrared data, which constrain the stellar masses of these objects. With the stellar mass and the metallicity estimated from the LIS absorptions, the metallicities of the LBGs at z ∼ 5 tend to be lower than those of galaxies with the same stellar mass at z ≲ 2, although the uncertainty is very large.