IFU Observations of Comet 3I/ATLAS = C/2025 N1 (ATLAS) Using the Hobby-Eberly Telescope with the VIRUS Instrument

We observed interstellar object 3I/ATLAS = C/2025 N1 (ATLAS) on 2025 December 9 UT with the VIRUS spectrograph on the Hobby--Eberly Telescope. VIRUS is a low spectral resolving power, multi-integral field unit (IFU), optical spectrograph. We used a single IFU for the observations and were able to obtain spectra spaced every 1″ (1327 km) from the optocenter out to about 85,000 km in the sunward direction. The spectra include molecular features normally seen in the optical for spectra of solar system comets, with the addition of many very strong lines of Fe I and Ni I. We modeled these observations using the Haser (1957) model framework and find that the data can only be fit if we assume that the outflow velocity of the gas is half that of comets in our solar system observed at the same heliocentric distance. We derived Haser model scale lengths for three Ni I and four Fe I lines. We discuss the implication of the similarities and differences between this object and normal solar system comets. The excellent spatial resolution of VIRUS, coupled with a field of view (FOV) of 50″ × 50″ and a geocentric distance of 1.83 au, allowed us to probe the coma of 3I in great detail. An accidental offset of the optocenter of the comet to the edge of the FOV meant that we probed to a large distance (>85,000 km) in the sunward direction from the optocenter, adding new information to the extensive literature on 3I, even with a single epoch of observations. *Based on observations obtained with the Hobby--Eberly Telescope (HET), which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximillians-Universitaet Muenchen, and Georg-August Universitaet Goettingen. The HET is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly.