Title: Shutting down or powering up a (U)LIRG? Merger components in distinctly different evolutionary states in IRAS 19115-2124 (The Bird),
Abstract: We present new SINFONI near-infrared integral field unit (IFU) spectroscopy
and SALT optical long-slit spectroscopy characterising the history of a nearby
merging luminous infrared galaxy, dubbed the Bird (IRAS19115-2114). The NIR
line-ratio maps of the IFU data-cubes and stellar population fitting of the
SALT spectra now allow dating of the star formation (SF) over the triple system
uncovered from our previous adaptive optics data. The distinct components
separate very clearly in a line-ratio diagnostic diagram. An off-nuclear pure
starburst dominates the current SF of the Bird with 60-70% of the total, with a
4-7 Myr age, and signs of a fairly constant long-term star formation of the
underlying stellar population. The most massive nucleus, in contrast, is
quenched with a starburst age of >40 Myr and shows hints of budding AGN
activity. The secondary massive nucleus is at an intermediate stage. The two
major components have a population of older stars, consistent with a starburst
triggered 1 Gyr ago in a first encounter. The simplest explanation of the
history is that of a triple merger, where the strongly star forming component
has joined later. We detect multiple gas flows in different phases. The Bird
offers an opportunity to witness multiple stages of galaxy evolution in the
same system; triggering as well as quenching of SF, and the early appearance of
AGN activity. It also serves as a cautionary note on interpretations of
observations with lower spatial resolution and/or without infrared data. At
high-redshift the system would look like a clumpy starburst with crucial pieces
of its puzzle hidden, in danger of misinterpretations.