Luminous infrared galaxies ( L _ { 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 ) Â K and gas densities of n _ { H _ { 2 } } = 10 ^ { 3 } cm ^ { -3 } .
We also find weak correlations between r _ { 31 } and both merger progression and star formation efficiency ( L _ { fIR } / L _ { 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 850Â \mu m fluxes measured with SCUBA , which are seen to be significant ( \sim 24 % ) .