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Star-forming galaxies (SFGs) display a continuous distribution of specificstar formation rates (sSFR) which can be approximated by the superposition oftwo log-normal distributions. The 1st of these encompasses the main sequence(MS) of SFGs, the 2nd one a rarer population of starbursts (SB). We show thatthe sSFR-distribution of SBs can be regarded as the result of a physicalprocess (plausibly merging) taking the mathematical form of a log-normalboosting kernel and enhancing star formation activity. We explore the utilityof splitting the star-forming population into MS and SB galaxies - an approachwe term "2-Star Formation Mode" (2-SFM) framework - for understanding theirmolecular gas properties. Variations of star formation efficiency (SFE) and gasfractions among SFGs take a simple, redshift-independent form, once thesequantities are normalized to the value of an average MS galaxy. The change inSFE for galaxies undergoing a starburst event scales supra-linearly with theSFR-increase, as expected for mergers. This implies a continuous distributionof galaxies in the Schmidt-Kennicutt plane that separates more clearly intoloci for SBs and normal galaxies than observed in SFR vs. M* space. SBs withthe largest deviations (>10-fold) from the MS, like many local ULIRGs, are notaverage SBs, but even rarer events with progenitors having larger gas fractionsthan typical MS galaxies. We statistically infer that the gas fractions oftypical SBs decrease by a factor of 2 to 3 with respect to their direct MSprogenitors, as expected to occur in short-term SFR-boosts during whichinternal gas reservoirs are drained more quickly than gas is accreted from thecosmic web. We predict variations of the CO-to-H2 conversion factor in theSFR-M* plane and provide evidence that the higher sSFR of distant galaxies is adirect consequence of larger gas fraction in these systems.

astrophysical journal/the astrophysical journal

Regularity underlying complexity: a redshift-independent description of  the continuous variation of galaxy-scale molecular gas properties in the  mass-star formation rate plane