A pair of leading spiral arms in a luminous infrared galaxy ? 1

Leading spiral arms are a rare phenomenon. We present here one of the very few convincing candidates of spiral arms opening counter-intuitively in the same direction as the galaxy disk is rotating. This detection in a luminous IR galaxy (LIRG) IRAS 18293-3134 is based on near infrared (NIR) adaptive optics imaging with the Very Large Telescope and long-slit NIR spectroscopy with the AngloAustralian Telescope. We discuss the orientation of the galaxy based on imaging and derive rotation curves from both emission and absorption features in the spectrum. The galaxy is strongly star-forming and has a minor companion in a high-velocity encounter. The fact that the arms of IRAS 18293-3134 are not easily traceable from optical images suggests that larger samples of high-quality NIR imaging of interacting systems and LIRGs might uncover further cases of leading arms, placing constraints on spiral arm theories and retrograde encounters, and especially on the relationship between disk masses and dark matter halo masses. Subject headings: galaxies:individual(IRAS 18293-3413) – infrared: galaxies – galaxies: kinematics and dynamics – galaxies: spiral – galaxies: interactions – galaxies: structure Based on observations made with the ESO telescopes at the Paranal Observatory under programme 073.D-0406B and observations made at the Anglo-Australian Telescope. South African Astronomical Observatory, P.O.Box 9, Observatory 7935, Cape Town, South Africa; [email protected] Anglo-Australian Observatory, P.O.Box 296, Epping, NSW 1710, Australia; [email protected] Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, FI21500 Piikkiö, Finland; [email protected]; [email protected]