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 Valpara\'iso 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 $L'$ ($^{ 12}$CO($1-0$))/$L'$($^{ 13}$CO($1-0$))) (hereafter $L'$($^{ 12}$CO)/$L'$($^{ 13}$CO)) and $L'$($^{ 13}$CO($1-0$))/$L'$(C$^{ 18}$O($1-0$)) (hereafter $L'$($^{ 13}$CO)/$L'$(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$_{ \odot}$ and stellar masses of $10^{ 9.8-10.9}$M$_{ \odot}$ corresponding to typical star-forming and starburst galaxies at these redshifts. On average we find a $L'$($^{ 12}$CO)/$L'$($^{ 13}$CO) line luminosity ratio value of 16.1$\pm$2.5. Galaxies with evidences of possible merging activity tend to show higher $L'$($^{ 12}$CO)/$L'$($^{ 13}$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 $L'$($^{ 13}$CO)/$L'$(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 $L'$($^{ 13}$CO)/$L'$(C$^{ 18}$O) line luminosity ratios with high $L'$($^{ 12}$CO)/$L'$($^{ 13}$CO) line luminosity ratios, suggesting that these trends are produced by selective enrichment of massive stars in young starbursts.
Categories: astro-ph.GA astro-ph.IM astro-ph.SR