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 ∼ 5 in a region centered on the Bubble Deep Field-North by using the Faint Object Camera and Spectrograph attached to the Subaru Telescope. Eight objects with Ic ≤ 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 Lyα emission lines (rest-frame equivalent width of 0-10 Å) and relatively strong low-ionization interstellar metal absorption lines of Si II λ1260, O I+Si II λ1303, and C II λ1334 (mean rest-frame equivalent widths of them are -1.2 to -5.1 Å). These properties are significantly different from those of the mean rest-frame UV spectrum of LBGs at z ∼ 3 but are quite similar to those of subgroups of LBGs at z ∼ 3 with no or weak Lyα emission. The weakness of Lyα emission and strong low-ionization interstellar metal absorption lines may indicate that these LBGs at z ∼ 5 are chemically evolved to some degree and have a dusty environment. Since the fraction of such LBGs at z ∼ 5 in our sample is larger than that at z ∼ 3, we may witness some sign of evolution of LBGs from z ∼ 5 to z ∼ 3, although the present sample size is very small. It is also possible, however, that the brighter LBGs tend to show no or weak Lyα emission, because our spectroscopic sample is bright (brighter than L*) among LBGs at z ∼ 5. More observations are required to establish spectroscopic nature of LBGs at z ∼ 5.