VALES: IV. Exploring the transition of star formation efficiencies between normal and starburst galaxies using APEX/SEPIA Band-5 and ALMA at low redshift

In this work we present new APEX/SEPIA Band-5 observations targeting the CO ($J=2\text{-}1$) emission line of 24 Herschel-detected galaxies at $z=0.1-0.2$. Combining this sample {with} our recent new Valpara'iso ALMA Line Emission Survey (VALES), we investigate the star formation efficiencies (SFEs = SFR/$M_{{\mathrm{H}}_2}$) of galaxies at low redshift. We find the SFE of our sample bridges the gap between normal star-forming galaxies and Ultra-Luminous Infrared Galaxies (ULIRGs), which are thought to be triggered by different star formation modes. Considering the $\mathrm{S}\mathrm{F}{\mathrm{E}}^{\mathrm{\prime }}$ as the SFR and the $L_{\mathrm{C}\mathrm{O}}^{\mathrm{\prime }}$ ratio, our data show a continuous and smooth increment as a function of infrared luminosity (or star formation rate) with a scatter about 0.5 dex, instead of a steep jump with a bimodal behaviour. This result is due to the use of a sample with a much larger range of sSFR/sSFR${}_{\mathrm{m}\mathrm{s}}$ using LIRGs, with luminosities covering the range between normal and ULIRGs. We conclude that the main parameters controlling the scatter of the SFE in star-forming galaxies are the systematic uncertainty of the ${\alpha }_{\mathrm{C}\mathrm{O}}$ conversion factor, the gas fraction and physical size.