Spatial Profiles of 3I/ATLAS CN and Ni Outgassing from Keck/KCWI Integral Field Spectroscopy

Cometary activity from interstellar objects provides a unique window into the environs of other stellar systems. We report blue-sensitive integral field unit spectroscopy of the interstellar object 3I/ATLAS from the Keck-II-mounted Keck Cosmic Web Imager on August 24, 2025 UT. We confirm previously reported CN and Ni outgassing, and present, for the first time, the radial profiles of Ni and CN emission in 3I/ATLAS. We find a characteristic $e$-folding radius of $593.7\pm14.8$ km for Ni and $841.0\pm15.4$ km for CN; this suggests that the Ni emission is more centrally concentrated in the nucleus of the comet and favors hypotheses involving easily dissociated species such as metal carbonyls or metal-polycyclic-aromatic-hydrocarbon molecules. Additional integral field spectroscopy after perihelion will offer a continued opportunity to determine the evolution of the radial distributions of species in interstellar comet 3I/ATLAS.

💡 Research Summary

This paper presents a pioneering analysis of the spatial distribution of gas emissions in the interstellar comet 3I/ATLAS, utilizing integral field unit (IFU) spectroscopy from the Keck Cosmic Web Imager (KCWI) on Keck-II. The observations were taken on August 24, 2025, when the comet was at a heliocentric distance of 2.75 AU. The primary goal was to move beyond simple detection and quantify the radial profiles of two key species: cyanogen (CN) and atomic nickel (Ni), whose outgassing had been previously reported.

The study confirms the presence of CN and Ni emission lines in the 3400-5500 Å spectrum. Using a Haser model, the team derived a CN production rate of (1.7±0.5)×10²⁴ molecules/s. No iron (Fe I) emission was detected at a significant level, allowing the authors to place an upper limit on its flux. The major innovation of this work lies in exploiting the unique spatio-spectral capabilities of the IFU. The researchers created two-dimensional narrow-band images for CN (3883 Å band) and Ni (3610 Å band), visually revealing a more compact emission morphology for Ni compared to the more extended CN coma.

To quantify this difference, the authors performed a detailed radial profile analysis. After accounting for coma asymmetries (jets in the sunward and anti-sunward directions), they azimuthally averaged the flux as a function of physical distance from the nucleus. These profiles were then fitted with an exponential decay model of the form A*exp(-x/τ)+C. The characteristic e-folding scale length (τ) was found to be 593.7±14.8 km for Ni and 841.0±15.4 km for CN. This statistically significant result demonstrates that Ni emission is more centrally concentrated around the nucleus than CN emission.

This finding serves as a critical diagnostic for evaluating potential physical origins of the mysterious nickel in comets. The paper discusses three leading hypotheses: sublimation and photodissociation of (1) metal carbonyls like Ni(CO)₄, (2) metal-polycyclic aromatic hydrocarbon (PAH) complexes, or (3) a hybrid scenario involving “in-situ” formation of Ni(CO)₄ from nickel sulfides eroded from the nucleus. The observed compact Ni distribution strongly favors models involving a parent molecule with a short photodissociation scale length, consistent with all three hypotheses but providing crucial quantitative constraints. The absence of detectable Fe I further supports the metal carbonyl hypothesis, as Ni(CO)₄ is more stable in cometary environments than its iron counterpart.

The research highlights the power of IFU spectroscopy for cometary science, enabling unbiased, wide-field studies of coma chemistry and morphology without the slit-loss and orientation biases of traditional slit spectroscopy. The results suggest that 3I/ATLAS, like 2I/Borisov before it, possesses a unique chemistry involving volatile metal-bearing compounds. The authors conclude that follow-up IFU observations post-perihelion will be vital to track how the radial distributions of these species evolve with changing solar heating, offering an unprecedented opportunity to probe the volatile inventory and thermal physics of an object born around another star.