Post-Perihelion 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 pre-perihelion approach. Now, 3I/ATLAS has begun its post-perihelion departure from the Solar System. In this letter, we report the first post-perihelion blue-sensitive integral-field unit spectroscopy of 3I/ATLAS using the Keck Cosmic Web Imager on November 16, 2025. We confirm previously reported CN, Fe, and Ni outgassing along with detections of carbon chain molecules ${\mathrm{C}}_2$ and ${\mathrm{C}}_3$. We calculate production rates for each species. We find Fe and Ni production rates of ${\mathrm{Q}}_{\mathrm{Fe}}=(9.55\pm 3.96)\times 10^{25}$ atoms s${}^{-1}$, and ${\mathrm{Q}}_{\mathrm{Ni}}=(6.61\pm 2.74)\times 10^{25}$ atoms s${}^{-1}$, resulting in a ratio of $\log ({\mathrm{Q}}_{\mathrm{Ni}}/{\mathrm{Q}}_{\mathrm{Fe}})=-0.16\pm 0.03$, which matches Solar System comets well and continues the pre-perihelion trend of declining $\log ({\mathrm{Q}}_{\mathrm{Ni}}/{\mathrm{Q}}_{\mathrm{Fe}})$ with $r_h$. We investigate the radial distributions of these elemental species and find characteristic $e$-folding radii of 3880$\pm$39 km for Ni, 6053$\pm$68 km for CN, 4194$\pm$45 km for ${\mathrm{C}}_2$, and 3833$\pm$45 km for ${\mathrm{C}}_3$. Compared to pre-perihelion measurements, these radii have increased by a factor of $\sim$6.5--7. Our post-perihelion observations reveal that 3I/ATLAS continues to exhibit cometary behavior broadly consistent with Solar System comets.

11 pages, 8 figures, to be submitted AAS Journals. Comet Comments Welcome!