Lyman Break Galaxies at z$\sim$5: Rest-Frame UV Spectra. II

Abstract

Abstract We present spectroscopy results of Lyman break galaxies (LBGs) at z$\sim$5 in the J0053$+$1234 field. Among five bright candidates with z$'$$\;<\;$25.0 mag, two objects are confirmed to be at z$\;\sim\;$5. The equivalent widths (EWs) of Ly$\alpha$ emission of the 2 LBGs are not so strong as to be detected as Ly$\alpha$ emitters, and one of them shows strong low-ionization interstellar (LIS) metal absorption lines. Two faint objects with z$'$$\geq$25.0 mag are also confirmed to be at z$\;\sim\;$5, and their spectra show strong Ly$\alpha$ emission in contrast to bright ones. These results suggest a deficiency of strong Ly$\alpha$ emission in bright LBGs at z$\;\sim\;$5, as discussed in our previous paper. Combined with our previous spectra of LBGs at z$\;\sim\;$5, obtained around the Hubble Deep Field-North (HDF-N), we made a composite spectrum of luminous ($M_{1400} \leq -$21.5 mag) LBGs at z$\;\sim\;$5. The resultant spectrum shows weak Ly$\alpha$ emission and strong LIS absorptions, which suggest that the bright LBGs at z$\;\sim\;$5 have chemically evolved at least to $\sim$0.1 solar metallicity. For a part of our sample in the HDF-N region, we obtained near-to-mid infrared data, which constrain the stellar masses of these objects. With the stellar mass and the metallicity estimated from the LIS absorptions, the metallicities of the LBGs at z$\;\sim\;$5 tend to be lower than those of galaxies with the same stellar mass at z$\lesssim 2$, although the uncertainty is very large.