We present Atacama Large Millimeter/sub-millimeter Array ( ALMA ) observations towards 27 low-redshift ( 0.02 < z < 0.2 ) star-forming galaxies taken from the Valparaíso ALMA/APEX Line Emission Survey ( VALES ) .
We perform stacking analyses of the ^ { 12 } CO ( 1 - 0 ) , ^ { 13 } CO ( 1 - 0 ) and C ^ { 18 } O ( 1 - 0 ) emission lines to explore the L ^ { \prime } ( ^ { 12 } CO ( 1 - 0 ) ) / L ^ { \prime } ( ^ { 13 } CO ( 1 - 0 ) ) ) ( hereafter L ^ { \prime } ( ^ { 12 } CO ) / L ^ { \prime } ( ^ { 13 } CO ) ) and L ^ { \prime } ( ^ { 13 } CO ( 1 - 0 ) ) / L ^ { \prime } ( C ^ { 18 } O ( 1 - 0 ) ) ( hereafter L ^ { \prime } ( ^ { 13 } CO ) / L ^ { \prime } ( C ^ { 18 } O ) line luminosity ratio dependence as a function of different global galaxy parameters related to the star formation activity .
The sample has far-IR luminosities 10 ^ { 10.1 - 11.9 } L _ { \sun } and stellar masses of 10 ^ { 9.8 - 10.9 } M _ { \sun } corresponding to typical star-forming and starburst galaxies at these redshifts .
On average we find a L ^ { \prime } ( ^ { 12 } CO ) / L ^ { \prime } ( ^ { 13 } CO ) line luminosity ratio value of 16.1 \pm 2.5 .
Galaxies with evidences of possible merging activity tend to show higher L ^ { \prime } ( ^ { 12 } CO ) / L ^ { \prime } ( ^ { 13 } CO ) ratios by a factor of two , while variations of this order are also found in galaxy samples with higher star formation rates or star formation efficiencies .
We also find an average L ^ { \prime } ( ^ { 13 } CO ) / L ^ { \prime } ( C ^ { 18 } O ) line luminosity ratio of 2.5 \pm 0.6 , which is in good agreement with those previously reported for starburst galaxies .
We find that galaxy samples with high L _ { \text { IR } } , SFR and SFE show low L ^ { \prime } ( ^ { 13 } CO ) / L ^ { \prime } ( C ^ { 18 } O ) line luminosity ratios with high L ^ { \prime } ( ^ { 12 } CO ) / L ^ { \prime } ( ^ { 13 } CO ) line luminosity ratios , suggesting that these trends are produced by selective enrichment of massive stars in young starbursts .