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/$M_{\rm H_{2}}$) 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 $\rm SFE'$ as the SFR and the $L'_{\rm CO}$
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$_{\rm ms}$ 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 $\alpha_{\rm CO}$ conversion factor, the gas
fraction and physical size.
