ar X iv : a st ro - p h / 05 10 83 0 v 1 3 0 O ct 2 00 5 DEEP SPECTROSCOPIC SURVEY OF LYMAN BREAK GALAXIES AT Z ∼ 5

We report results of deep optical spectroscopy with Subaru/FOCAS for Lyman Break Galaxy (LBG) candidates at z ∼ 5. So far, we made spectroscopic observations for 24 LBG candidates among ∼ 200 bright (z < 25.0) LBG sample and confirmed 9 objects to be LBGs at z ∼ 5. Intriguingly, these bright LBGs show no or a weak Lyα emission and relatively strong low ionization interstellar metal absorption lines. We also identified 2 faint (z > 25.0) objects to be at z ∼ 5 with their strong (EWrest >20Å) Lyα emission. Combining our results with other spectroscopic observations of galaxies at the similar redshift range, we found a clear luminosity dependence of EWrest of Lyα emission, i.e., the lack of strong Lyα emission in bright LBGs. If the absence of Lyα emission is due to dust absorption, these results suggest that bright LBGs at z ∼ 5 are in dusty and more chemically evolved environment than faint ones. This interpretation implies that bright LBGs started star formation earlier than faint ones, suggesting biased star formation.