Adaptive optics imaging and optical spectroscopy of a multiple merger in a luminous infrared galaxy

We present near-infrared (NIR) adaptive optics imaging obtained with VLT/NACO and optical spectroscopy from the Southern African Large Telescope of a luminous IR galaxy (LIRG) IRAS 19115-2124. These data are combined with archival Hubble Space Telescope imaging and Spitzer imaging and spectroscopy, allowing us to study this disturbed interacting/merging galaxy, dubbed the Bird, in extraordinary detail. In particular, the data reveal a triple system where the LIRG phenomenon is dominated by the smallest of the components. One nucleus is a regular barred spiral with significant rotation, while another is highly disturbed with a surface brightness distribution intermediate to that of disc and bulge systems, and hints of remaining arm/bar structure. We derive dynamical masses in the range 3-7 x 10(10) M-circle dot for both. The third component appears to be a 1-2 x 10(10) M-circle dot mass irregular galaxy. The total system exhibits H ii galaxy-like optical line ratios and strengths, and no evidence for active galactic nucleus (AGN) activity is found from optical or mid-IR data. The star formation rate is estimated to be similar to 190 M-circle dot yr(-1). We also report a search for supernovae from NIR images separated by five months and search for superstar cluster candidates. We detect outflowing gas from the Bird mostly in the range 100-300 km s(-1) using Na i D absorption features. Overall, the Bird shows kinematic, dynamical and emission-line properties typical for cool ultraluminous IR galaxies (ULIRGs). However, the interesting features setting it apart for future studies are its triple merger nature, and the location of its star formation peak - the strongest star formation, as revealed by Spitzer imaging, does not come from the two major K-band nuclei, but from the third irregular component. This is in contrast to the conventional view that the (U)LIRG phases are powered by infalling gas to the major nuclei of the merging spiral galaxies. Aided by simulations, we discuss scenarios where the irregular component is on its first high-speed encounter with the more massive components.