Investigating the Relation between CO (3-2) and Far Infrared
  Luminosities for Nearby Merging Galaxies Using ASTE
  We present the new single dish CO (3-2) emission data obtained toward 19
early stage and 7 late stage nearby merging galaxies using the Atacama
Submillimeter Telescope Experiment (ASTE). Combining with the single dish and
interferometric data of galaxies observed in previous studies, we investigate
the relation between the CO (3-2) luminosity (L'CO(3-2)) and the far Infrared
luminosity (LFIR) in a sample of 29 early stage and 31 late stage merging
galaxies, and 28 nearby isolated spiral galaxies. We find that normal isolated
spiral galaxies and merging galaxies have different slopes (alpha) in the log
L'CO(3-2) - log LFIR plane (alpha ~ 0.79 for spirals and ~ 1.12 for mergers).
The large slope (alpha > 1) for merging galaxies can be interpreted as an
evidence for increasing Star Formation Efficiency (SFE=LFIR/L'CO(3-2)) as a
function of LFIR. Comparing our results with sub-kpc scale local star formation
and global star-burst activity in the high-z Universe, we find deviations from
the linear relationship in the log L'CO(3-2) - log LFIR plane for the late
stage mergers and high-z star forming galaxies. Finally, we find that the
average SFE gradually increases from isolated galaxies, merging galaxies, and
to high-z submillimeter galaxies / quasi-stellar objects (SMGs/QSOs). By
comparing our findings with the results from numerical simulations, we suggest;
(1) inefficient star-bursts triggered by disk-wide dense clumps occur in the
early stage of interaction and (2) efficient star-bursts triggered by central
concentration of gas occur in the final stage. A systematic high spatial
resolution survey of diffuse and dense gas tracers is a key to confirm this
scenario.
