The first direct imaging of the silhouette of a damped Lyman α system along the line-of-sight to a background galaxy Komori, Fuga Inoue, Akio K Mawatari, Ken Sugahara, Yuma Umehata, Hideki Shimakawa, Rhythm Yamanaka, Satoshi Hashimoto, Takuya Matthee, Jorryt J ; https://orcid.org/0000-0003-2871-127X Misawa, Toru ddc:520 The H i gas distribution in damped Lyman $\alpha$ 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 $10^7$ in the internal structure, implying a sharp edge or patchy structure. A Lyman $\alpha$ emitter (LAE) at $z_{\rm LAE}=3.3433\pm 0.0005$, consistent with the DLA redshift, is detected at an angular separation of $1{{_{.}^{\prime\prime}} }73\pm 0{{_{.}^{\prime\prime}} }28$ ($12.9\pm 2.1$ 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  $\lambda 1302$ absorption at $z_{\rm OI}=3.3288\pm 0.0004$ 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. Oxford University Press 2025 info:eu-repo/semantics/article doc-type:article text http://purl.org/coar/resource_type/c_2df8fbb1 https://research-explorer.ista.ac.at/record/20545 https://research-explorer.ista.ac.at/download/20545/20549 Komori F, Inoue AK, Mawatari K, et al. The first direct imaging of the silhouette of a damped Lyman α system along the line-of-sight to a background galaxy. <i>Monthly Notices of the Royal Astronomical Society</i>. 2025;543(3):2943-2957. doi:<a href="https://doi.org/10.1093/mnras/staf1622">10.1093/mnras/staf1622</a> eng info:eu-repo/semantics/altIdentifier/doi/10.1093/mnras/staf1622 info:eu-repo/semantics/altIdentifier/issn/0035-8711 info:eu-repo/semantics/altIdentifier/e-issn/1365-2966 info:eu-repo/semantics/altIdentifier/wos/001592326700001 https://creativecommons.org/licenses/by/4.0/ info:eu-repo/semantics/openAccess