The Anti-Solar Tail of Comet 3I/ATLAS Seen in Deep Multi-band Imaging Taken at Gemini South

ATel #17363; Bryce Bolin (Eureka Scientific), Laura-May Abron (Griffith Observatory), Matthew Belyakov (CIT), Juan Pablo Carvajal (PUCC), Christoffer Fremling (CIT), Josef Hanus (Charles University), Carl Ingebretsen (JHU), Piera King (Gemini Observatory), Baltasar Luco (PUCC), Keith Noll (NASA GSFC), Thomas H. Puzia (PUCC), Rohan Rahatgaonkar (PUCC), Karleyne Silva (Gemini Observatory), Ian Wong (STScI)
on 29 Aug 2025; 04:10 UT
Credential Certification: Bryce Bolin (bolin.astro@gmail.com)

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We report deep imaging and u, g, r, i color measurements of interstellar comet 3I/ATLAS taken with the Gemini South 8.2-m telescope at Cerro Pachon (observatory code I11). We used the Gemini Multi-Object Spectrograph (GMOS) to observe 3I/ATLAS on 2025 August 27 23:53 UTC (see attached u, g, r, and i composite images). The comet has an extended appearance with a tail ~30 arcsec long, pointing towards the anti-solar direction in the South East. Additionally, the comet had a coma ~10 arcsec wide, significantly more extended than its compact appearance seen in initial imaging (Bolin et al. 2025, MNRAS:L, 542, 1, pp. L139-L143). The seeing measured using nearby stars was ~0.5 arcsec. Using our multi-band images, we measured the following color indices using an aperture with a 5.3 arcsec, equivalent to 10,000 km at the 2.59 au geocentric distance of the comet: u-g = 1.45 +/- 0.04, g-r = 0.68 +/- 0.01, r-i = 0.32 +/- 0.01, and g-i = 1.00 +/- 0.01, equivalent to a spectral slope of 16.1 +/- 0.4 % / 100 nm. The g-r and r-i color and overall color slope measured with the GMOS appear to be in line with the spectral slope measurements for the comet reported in other recent results (e.g., Puzia et al. 2025, arXiv: 2508.02777, Santana-Ros et al. 2025, arXiv: 2508.00808, Beniyama 2025, arXiv:2508.08829, Rahatgaonkar et al. 2025, arXiv: 2508.18382). This study is 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 National Science Foundation on behalf of the Gemini Observatory partnership.

Images of 3I/ATLAS taken in u, g, r, and i bands on 2025 August 27 UTC