Postperihelion Integral Field Spectroscopy of the Interstellar Comet 3I/ATLAS

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.