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A CO(3-2) survey of a merging sequence of luminous infrared galaxies

Luminous infrared galaxies ($L_{\rm{IR}}>10^{11} L_{\odot}$) are often associated with interacting galactic systems and are thought to be powered by merger--induced starbursts and/or dust--enshrouded AGN. In such systems, the evolution of the dense, star forming molecular gas as a function of merger separation is of particular interest. Here, we present observations of the CO(3-2) emission from a sample of luminous infrared galaxy mergers that span a range of galaxy-galaxy separations. The excitation of the molecular gas is studied by examining the CO(3-2)/CO(1-0) line ratio, $r_{31}$, as a function of merger extent. We find these line ratios, $r_{31}$, to be consistent with kinetic temperatures of $T_k$=(30--50) K and gas densities of $n_{\rm{H}_2}=10^3 \rm{cm}^{-3}$. We also find weak correlations between $r_{31}$ and both merger progression and star formation efficiency ($L_{\rm{fIR}} / L_{\rm{CO(1-0)}}$). These correlations show a tendency for gas excitation to increase as the merger progresses and the star formation efficiency rises. To conclude, we calculate the contributions of the CO(3-2) line to the 850 $μ$m fluxes measured with SCUBA, which are seen to be significant ($\sim$24%).