A PAIR OF LEADING SPIRAL ARMS IN A LUMINOUS INFRARED GALAXY?

Leading spiral arms are a rare phenomenon. We present here one of the very few convincing candidates of spiral arms opening counterintuitively in the same direction as the galaxy disk is rotating. This detection in the luminous IR galaxy (LIRG) IRAS 18293 - 3413 is based on near-infrared (NIR) adaptive optics imaging with the Very Large Telescope and long-slit NIR spectroscopy with the Anglo-Australian 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 - 3413 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 retrogade encounters, and especially on the relationship between disk masses and dark matter halo masses.