article published yes Fuga Komori author Akio K Inoue author Ken Mawatari author Yuma Sugahara author Hideki Umehata author Rhythm Shimakawa author Satoshi Yamanaka author Takuya Hashimoto author Jorryt J Matthee author 7439a258-f3c0-11ec-9501-9df22fe067200000-0003-2871-127X Toru Misawa author JoMa department 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. https://research-explorer.ista.ac.at/download/20545/20549/2025_MonthlyNoticesRAS_Komori.pdf application/pdfno https://creativecommons.org/licenses/by/4.0/ Oxford University Press2025 eng 0035-8711 1365-2966 00159232670000110.1093/mnras/staf1622 54332943-2957 Komori, Fuga, Akio K Inoue, Ken Mawatari, Yuma Sugahara, Hideki Umehata, Rhythm Shimakawa, Satoshi Yamanaka, Takuya Hashimoto, Jorryt J Matthee, and Toru Misawa. “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>. Oxford University Press, 2025. <a href="https://doi.org/10.1093/mnras/staf1622">https://doi.org/10.1093/mnras/staf1622</a>. F. Komori, A.K. Inoue, K. Mawatari, Y. Sugahara, H. Umehata, R. Shimakawa, S. Yamanaka, T. Hashimoto, J.J. Matthee, T. Misawa, Monthly Notices of the Royal Astronomical Society 543 (2025) 2943–2957. Komori, F., Inoue, A. K., Mawatari, K., Sugahara, Y., Umehata, H., Shimakawa, R., … Misawa, T. (2025). 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>. Oxford University Press. <a href="https://doi.org/10.1093/mnras/staf1622">https://doi.org/10.1093/mnras/staf1622</a> Komori F, Inoue AK, Mawatari K, Sugahara Y, Umehata H, Shimakawa R, Yamanaka S, Hashimoto T, Matthee JJ, Misawa T. 2025. The first direct imaging of the silhouette of a damped Lyman α system along the line-of-sight to a background galaxy. Monthly Notices of the Royal Astronomical Society. 543(3), 2943–2957. 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> F. Komori <i>et al.</i>, “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>, vol. 543, no. 3. Oxford University Press, pp. 2943–2957, 2025. Komori, Fuga, 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>, vol. 543, no. 3, Oxford University Press, 2025, pp. 2943–57, doi:<a href="https://doi.org/10.1093/mnras/staf1622">10.1093/mnras/staf1622</a>. 205452025-10-27T08:18:07Z2025-12-01T15:07:43Z