DISCOVERY of A DAMPED Lyα ABSORBER at z = 3.3 ALONG A GALAXY SIGHT-LINE in the SSA22 FIELD

Using galaxies as background light sources to map the Lyα absorption lines is a novel approach to study Damped Lyα Absorbers (DLAs). We report the discovery of an intervening z = 3.335 ±0.007 DLA along a galaxy sight-line identified among 80 Lyman Break Galaxy (LBG) spectra obtained with our Very Large Telescope/Visible Multi-Object Spectrograph survey in the SSA22 field. The measured DLA neutral hydrogen (H i) column density is log(N_{H I}/cm^-2) = 21.68 ±0.17. The DLA covering fraction over the extended background LBG is >70% (2σ), yielding a conservative constraint on the DLA area of ≳1 kpc². Our search for a counterpart galaxy hosting this DLA concludes that there is no counterpart galaxy with star formation rate larger than a few M_⊙ yr^-1, ruling out an unobscured violent star formation in the DLA gas cloud. We also rule out the possibility that the host galaxy of the DLA is a passive galaxy with M∗ ≳ 5 × 10¹⁰M_⊙ or a heavily dust-obscured galaxy with E(B - V) ≳ 2. The DLA may coincide with a large-scale overdensity of the spectroscopic LBGs. The occurrence rate of the DLA is compatible with that of DLAs found in QSO sight-lines.