W. B. Hoogendam, David O. Jones, Bin Yang, B. J. Shappee, J. J. Wray, K. J. Meech, Christopher Ashall, D. Desai, Jason T. Hinkle, Andrew Hoffman, K. Medler, Cameron Pfeffer, Ruining Zhao
Abstract The environs of other stellar systems may be directly probed by analyzing the cometary activity of interstellar objects. The recently discovered interstellar object 3I/ATLAS was the subject of an intensive worldwide follow-up campaign in its preperihelion approach. Now, 3I/ATLAS has begun its postperihelion departure from the solar system. In this paper, we report the first postperihelion blue-sensitive integral-field unit spectroscopy of 3I/ATLAS using the Keck Cosmic Web Imager on 2025 November 16. We confirm previously reported CN, Fe, and Ni outgassing along with detections of carbon chain molecules C 2 and C 3 . We calculate production rates for each species. We find Fe and Ni production rates of Q Fe = (9.55 ± 3.96) × 10 25 atoms s −1 and Q Ni = (6.61 ± 2.74) × 10 25 atoms s −1 , resulting in a ratio of log ( Q Ni / Q Fe ) = − 0.16 ± 0.03 , which matches solar system comets well and continues the preperihelion trend of declining log ( Q Ni / Q Fe ) with r h . We investigate the radial distributions of these elemental species and find characteristic e -folding radii of 3880 ± 39 km for Ni, 6053 ± 68 km for CN, 4194 ± 45 km for C 2 , and 3833 ± 45 km for C 3 . Compared to preperihelion measurements, these radii have increased by a factor of ∼6.5–7. Our postperihelion observations reveal that 3I/ATLAS continues to exhibit cometary behavior broadly consistent with solar system comets.