Deep Spectroscopic Survey of Lyman Break Galaxies at z~5

Abstract

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 Ly-alpha 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 (EW_rest>20AA) Ly-alpha emission. Combining our results with other spectroscopic observations of galaxies at the similar redshift range, we found a clear luminosity dependence of EW_rest of Ly-alpha emission, i.e., the lack of strong Ly-alpha emission in bright LBGs. If the absence of Ly-alpha 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.

5 pages, 4 postscript figures. To appear in the proceedings of "The Fabulous Destiny of Galaxies: Bridging Past and Present", with minor corrections