DISCOVERY OF A DAMPED Lyα ABSORBER AT z = 3.3 ALONG A GALAXY SIGHT-LINE IN THE SSA22 FIELD

Abstract

Using galaxies as background light sources to map the Ly α absorption lines is a novel approach to study Damped Ly α 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% (2 σ ), yielding a conservative constraint on the DLA area of ≳1 kpc 2 . 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 × 10 10 M ⊙ 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.