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

TL;DR

This study investigates how star formation efficiencies transition from normal galaxies to starburst galaxies at low redshift, revealing a smooth continuum rather than a bimodal distribution, influenced by physical and systematic factors.

Contribution

It provides new observational evidence that star formation efficiencies form a continuous spectrum, challenging the traditional bimodal view, using extensive CO emission data at low redshift.

Findings

SFE bridges the gap between normal and starburst galaxies.

SFE shows a smooth increase with infrared luminosity.

Scatter in SFE is mainly due to systematic uncertainties and physical parameters.

Abstract

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.