Astronomy & Astrophysics manuscript no. aanda c ESO 2021 November 8, 2021 Hi mapping of the Leo Triplet Morphologies and kinematics of tails and bridges Gang Wu 1,2 , David Martínez-Delgado ⋆3 , Christian Henkel 1,4,2 , Pavel Kroupa 5, 6 , Fabian Walter 7 , Nico Krieger 7 , Alberto D. Bolatto 8 , Timothy Robishaw 9 , Joshua D. Simon 10 , Álvaro Ibáñez Pérez 11 , Karl M. Menten 1 , and Jarken Esimbek 2 1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany e-mail: gwu@mpifr-bonn.mpg.de 2 Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 830011 Urumqi, Xinjiang, PR China 3 Instituto de Astrofísica de Andalucía, CSIC, E-18080, Granada, Spain 4 Astronomy Department, King Abdulaziz University, PO Box 80203, Jeddah 21589, Saudi Arabia 5 Helmholtz-Institut für Strahlen und Kernphysik (HISKP), University of Bonn, Nussallee 14-16, D-53115 Bonn, Germany 6 Charles University in Prague, Faculty of Mathematics and Physics, Astronomical Institute, V Holešoviˇ ckách 2 CZ-18000 Praha Czech Republic 7 Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany 8 Department of Astronomy, University of Maryland, College Park, MD 20742, USA 9 National Research Council Canada, Herzberg Programs in Astronomy & Astrophysics, Dominion Radio Astrophysical Observa- tory, P.O. Box 248, Penticton, BC V2A 6J9, Canada 10 Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA 11 Asociación Astronómica AstroHenares, 28823 Coslada, Madrid, Spain Received June 16, 2021; accepted October 22, 2021 ABSTRACT A fully sampled and hitherto highest resolution and sensitivity observation of neutral hydrogen (Hi) in the Leo Triplet (NGC 3628, M 65/NGC 3623, and M 66/NGC 3627) reveals six Hi structures beyond the three galaxies. We present detailed results of the mor- phologies and kinematics of these structures, which can be used for future simulations. In particular, we detect a two-arm structure in the plume of NGC 3628 for the first time, which can be explained by a tidal interaction model. The optical counterpart of the plume is mainly associated with the southern arm. The connecting part (base) of the plume (directed eastward) with NGC 3628 is located at the blueshifted (western) side of NGC3628. Two bases appear to be associated with the two arms of the plume. A clump with a reversed velocity gradient (relative to the velocity gradient of M 66) and a newly detected tail, that is to say M 66SE, is found in the southeast of M 66. We suspect that M 66SE represents gas from NGC 3628, which was captured by M 66 in the recent interaction between the two galaxies. Meanwhile gas is falling toward M 66, resulting in features previously observed in the southeastern part of M 66, such as large line widths and double peaks. An upside-down "Y"-shaped Hi gas component (M 65S) is detected in the south of M 65, which suggests that M 65 may also have been involved in the interaction. We strongly encourage modern hydrodynamical simulations of this interacting group of galaxies to reveal the origin of the gaseous debris surrounding all three galaxies. Key words. galaxies: individual (NGC 3628, M 65/NGC 3623, and M 66/NGC 3627) – galaxies: interactions – galaxies: ISM – galaxies: peculiar – radio lines: galaxies 1. Introduction Interactions between galaxies have significant impacts on their participants, leading to asymmetries, warps, and exchange of gas and momentum. These interactions can finally result in noncircu- lar potentials, leading to an inflow of gas, enhancement of bars, and also triggering starburst activity in the galaxies. An excellent example is the Leo Triplet galaxy system (see the left panel of Fig. 1), also known as Arp 317 (Arp 1966), mainly including the SAB(rs)a galaxy M 65 (NGC 3623), the SAB(s)b galaxy M 66 (NGC 3627), and the SAb pec galaxy NGC 3628. The most famous feature in this system is probably the spec- tacular tail (the plume) from NGC 3628 extending toward the east. The plume was first detected by Zwicky (1956) and Ko- rmendy & Bahcall (1974) via optical observations. Then Rots ⋆ Talentia Senior Fellow (1978) and Haynes et al. (1979) reported neutral hydrogen (Hi) observations in the Leo Triplet and revealed a ≈150 kpc (rescaled to a distance of 11.3±0.5 Mpc) long Hi structure 1 consistent with its optical counterpart. In these Hi observations, an additional bridge-like structure is also detected extruding from NGC 3628 and likely pointing southward to M 66. Rots (1978) applied the methods of Toomre & Toomre (1972) and basically recovered the morphologies and kinematics of Hi plume and bridge by assuming a recent (8×10 8 years ago) tidal encounter between 1 Anand et al. (2021) recently compiled a catalog of the best avail- able distances of 118 galaxies. Here we adopt the average of the red giant branch distance of M 65 (11.3±1.1 Mpc) and the group distances (from the galaxy group and numerical modeling of their orbits) of M 66 (11.32±0.48 Mpc) and NGC 3628 (11.3±1.1 Mpc) in Anand et al. (2021) as the distance to the Leo Triplet. See Jacobs et al. (2009) and Anand et al. (2021) for more details. Article number, page 1 of 20 Article published by EDP Sciences, to be cited as https://doi.org/10.1051/0004-6361/202141565