Neutral nickel and molecular coma maps of 3I/ATLAS from 2026 January Keck/KCWI spectroscopy

ATel #18130; Bryce T. Bolin (Eureka Scientific), Willem B. Hoogendam (Institute for Astronomy, University of Hawaii), David O. Jones (Institute for Astronomy, University of Hawaii), Benjamin J. Shappee (Institute for Astronomy, University of Hawaii), Ian Wong (Space Telescope Science Institute)
on 30 Oct 2026; 03:27 UT
Credential Certification: Bryce Bolin (bolin.astro@gmail.com)

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We obtained optical integral-field spectra of interstellar comet 3I/ATLAS (Bolin et al. 2025, MNRAS, 542, L139; Seligman et al. 2025, ApJL, 989, L36) with Keck II/KCWI on 2026 January 19 UTC (2025B N068; PI I. Wong). The blue-arm data comprise seven exposures totaling 2000 s. Heliocentric and geocentric distances were 3.25 and 2.27 au, respectively. Spectra were extracted within a 5-arcsec radius (8221 km).

The CN, C3, and Ni maps show emission before and after continuum subtraction. Residuals include spatially extended CN-region emission at 387.55-389.55 nm and a central C3 excess at 397.5-415.0 nm. North is up and east is left; cyan and white arrows indicate the projected antisolar and positive heliocentric-velocity directions, respectively. Scale bars span 5000 km.

Continuum fits give apparent Q(CN) = (0.81-1.26) x 10^24 molecules/s and a C3 rate-equivalent index of (0.41-2.43) x 10^24 molecules/s. The ranges reflect continuum choices, not statistical uncertainties. Ni I lines at rest-air wavelengths 351.505 and 352.454 nm occur in all seven exposures and peak near the comet center; the 361.939-nm identification is tentative. Profile fits give provisional Q(Ni I) = (2.00 +/- 0.10) x 10^23 atoms/s for optically thin, steady, isotropic direct release at 0.5 km/s. We adopt the corrected fluorescence model of Bromley et al. (2021, PSJ, 2, 228; 2026, PSJ, 7, 131) and the neutral lifetime of Guzik & Drahus (2021, Nature, 593, 375). The error includes measurement uncertainty only. Alternative spectral models give (1.27-2.64) x 10^23 atoms/s; absolute-calibration and release-model errors are unquantified.

C2 is undetected over 486-518.5 nm. The largest 3-sigma bounds among the tested models are F(C2) < 3.9 x 10^-14 erg/cm^2/s and Q(C2) < 1.9 x 10^25 molecules/s. The conversion uses a circular Haser model (Haser 1957, Bull. Acad. R. Belg., 43, 740), fluorescence efficiencies and scale lengths from Cochran et al. (2012, Icarus, 218, 144), and a gas speed of 0.5 km/s. These limits exclude absolute-calibration and broad-background errors.

At r_h = 1.51 au on 2025 November 16, Hoogendam et al. (2026, ApJ, 1003, 245; doi:10.3847/1538-4357/ae6243) measured Q(CN) = (1.6 +/- 0.5) x 10^26 and Q(C3) = (5.0 +/- 0.4) x 10^24 molecules/s. Their recommended multilevel Q(Ni I) = (7.47 +/- 0.27) x 10^24 atoms/s gives CN/C3 = 32 and CN/Ni = 21.4. Using the midpoints of our CN and C3 ranges and nominal Ni rate gives illustrative ratios CN/C3 = 0.73 and CN/Ni = 5.2.

These target-of-opportunity observations were triggered during David O. Jones's Keck night, with setup and calibration assistance from Jones and Willem Hoogendam. W. M. Keck Observatory is operated as a scientific partnership among Caltech, the University of California, and NASA, made possible by the W. M. Keck Foundation. We recognize the cultural significance and reverence of Maunakea to the Native Hawaiian community.

2026 January 19 Keck II/KCWI CN, C3, and Ni I maps of 3I/ATLAS