Investigating the Relation between CO (3-2) and Far Infrared Luminosities for Nearby Merging Galaxies Using ASTE

TL;DR

This study investigates the relationship between CO (3-2) luminosity and far-infrared luminosity in nearby merging and isolated galaxies, revealing different star formation efficiencies and stages of galaxy interaction.

Contribution

It provides new CO (3-2) emission data for merging galaxies and compares these with isolated galaxies, highlighting differences in star formation efficiency and galaxy evolution stages.

Findings

Merging galaxies have a steeper slope in the L'CO(3-2)-LFIR relation than isolated spirals.

Star formation efficiency increases from isolated to merging to high-z galaxies.

Early stage mergers show inefficient starbursts, while late stage mergers show efficient, centralized starbursts.

Abstract

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.