The first direct imaging of the silhouette of a damped Lyman α system along the line-of-sight to a background galaxy

The H i gas distribution in damped Lyman α absorbers (DLAs) has remained elusive due to the point-source nature of background quasar emission. Observing DLAs against spatially extended background galaxies provides a new method for constraining their size and structure. Using the Keck Cosmic Web Imager, we present the first “silhouette” image of a DLA at z = 3.34, identified in the spectrum of a background galaxy at z = 3.61. Although the silhouette remains unresolved due to limited spatial resolution, this represents a successful proof-of-concept for studying DLA morphology using extended background sources. Possible residual emission in the DLA trough suggests an optical depth contrast exceeding 107 in the internal structure, implying a sharp edge or patchy structure. A Lyman α emitter (LAE) at zLAE = 3.3433 ± 0.0005, consistent with the DLA redshift, is detected at an angular separation of 1.″73 ± 0.″28 (12.9 ± 2.1 kpc). The DLA is surrounded by three galaxies within 140 kpc in projected distance and 500 km s−1 in line-of-sight velocity, indicating that it resides in the circumgalactic medium of the LAE or within a galaxy group/protocluster environment. An O i λ1302 absorption at zOI = 3.3288 ± 0.0004 is also detected along the line of sight. This absorber may trace metal-enriched outflow from the LAE or a gas-rich galaxy exhibiting the highest star formation activity among the surrounding galaxies. Future large spectroscopic surveys of galaxies will expand such a DLA sample, and three-dimensional spectroscopy for it will shed new light on the role of intergalactic dense gas in galaxy formation and evolution.