Spatially resolved spectroscopy of 3I/ATLAS with Gemini North/GMOS IFU on 2025 November 26

ATel #18134; Bryce T. Bolin (Eureka Scientific), Brian Lemaux (Gemini Observatory), Laura-May Abron (Griffith Observatory), Pablo Candia (Gemini Observatory), Rodrigo Carrasco (Gemini Observatory), Kristin Chiboucas (Gemini Observatory), Veronica Firpo (Gemini Observatory), Christoffer Fremling (Caltech), Josef Hanus (Charles University), Jeong-Eun Heo (Gemini Observatory), Piera King (Gemini Observatory), Jameeka Marshall (UC Santa Cruz), Keith Noll (Planetary Science Institute), Karleyne Silva (Gemini Observatory), Ian Wong (STScI)
on 31 Oct 2026; 11:13 UT
Credential Certification: Bryce Bolin (bolin.astro@gmail.com)

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We obtained integral-field spectra of interstellar comet 3I/ATLAS (Bolin et al. 2025, MNRAS, 542, L139) on 2025 November 26 UTC with Gemini North/GMOS (GN-2025B-DD-102; PI B. Bolin). These observations were part of the Shadow the Scientists program. Four 300-s exposures used the one-slit IFU and B480+ grating, split equally between central wavelengths of 540 and 550 nm. The heliocentric and geocentric distances were 1.701 and 1.962 au.

Maps and extracted spectra with captions show the CN, C3 and C2 regions before and after continuum subtraction. A narrow 515.94-516.81-nm excess repeats in all four exposures; its C2 identification depends on the overlapping solar spectrum. North is up and east left; cyan Anti-Sun and white +v_hel arrows show projected directions, and bars span 2000 km. Overview spectra use complete 1-nm bins; rejecting partial bins and omitting isolated points widens apparent gaps relative to the native-sampling zooms. This display choice does not affect the production estimates.

Within a 1.25-arcsec (1779-km) radius, CN band integration with model completion gives a conditional Q(CN) = 4.12 x 10^25 molecules/s. C2 sequence fits give 1.32 x 10^25 and 1.91 x 10^25 molecules/s at 474 and 517 nm; the latter becomes 1.55 x 10^25 with sky-fiber subtraction. A red-side continuum gives a C3 rate-equivalent index of 6.86 x 10^24 molecules/s, but local continua give residuals consistent with zero. This is not a total C3 rate or upper limit.

Same-day long-slit estimates are Q(CN) = (4.85 +/- 0.92) x 10^25 and Q(C2) = (2.94 +/- 0.43) and (2.77 +/- 0.23) x 10^25 molecules/s at 474 and 517 nm. The IFU C2 values are 45% and 69% of these central values. Different apertures, flux scales, sky treatment and CN conversions prevent interpreting these as calibrated spatial differences.

Conversions use a circular Haser model (Haser 1957, Bull. Acad. R. Sci. Belg., 43, 740), 0.5 km/s outflow, Cochran et al. (2012, Icarus, 218, 144) scale lengths and C2/C3 fluorescence efficiencies, and Schleicher (2010, AJ, 140, 973) CN efficiencies. Spectroscopic methods follow Bolin et al. (2024, MNRAS, 527, L42; 2021, AJ, 161, 116).

The IFU flux reconstruction carries an 18.7% shared normalization uncertainty plus unquantified wavelength-response and background errors. These model-dependent estimates do not establish abundance differences; the maps retain their separate display scales.

Based on observations obtained at the international Gemini Observatory, a program of NSF's NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the U.S. National Science Foundation on behalf of the Gemini Observatory partnership: the U.S. National Science Foundation (United States), National Research Council (Canada), Agencia Nacional de Investigacion y Desarrollo (Chile), Ministerio de Ciencia, Tecnologia e Innovacion (Argentina), Ministerio da Ciencia, Tecnologia e Inovacoes (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea). We thank Shadow the Scientists for facilitating public engagement.

2025 November 26 GMOS IFU spectrum of interstellar comet 3I/ATLAS