Lyman break galaxies at z ∼ 5:: Rest-frame ultraviolet spectra

We report initial results for spectroscopic observations of candidates of Lyman break galaxies (LBGs) at z similar to 5 in a region centered on the Hubble Deep Field-North by using the Faint Object Camera and Spectrograph attached to the Subaru Telescope. Eight objects with I-C less than or equal to 25.0 mag, including one active galactic nucleus, are confirmed to be at 4.5 < z < 5.2. The rest-frame UV spectra of seven LBGs commonly show no or weak Lyalpha emission lines (rest-frame equivalent width of 0 - 10 Angstrom) and relatively strong low-ionization interstellar metal absorption lines of Si II lambda1260, O I+ Si II lambda1303, and C II lambda1334 ( mean rest-frame equivalent widths of them are -1.2 to - 5.1 Angstrom). These properties are significantly different from those of the mean rest-frame UV spectrum of LBGs at z similar to 3 but are quite similar to those of subgroups of LBGs at z similar to 3 with no or weak Lyalpha emission. The weakness of Lyalpha emission and strong low-ionization interstellar metal absorption lines may indicate that these LBGs at z similar to 5 are chemically evolved to some degree and have a dusty environment. Since the fraction of such LBGs at z similar to 5 in our sample is larger than that at z similar to 3, we may witness some sign of evolution of LBGs from z similar to 5 to z similar to 3, although the present sample size is very small. It is also possible, however, that the brighter LBGs tend to show no or weak Lyalpha emission, because our spectroscopic sample is bright ( brighter than L*) among LBGs at z similar to 5. More observations are required to establish spectroscopic nature of LBGs at z similar to 5.