The Galaxy Starburst/Main-sequence Bimodality over Five Decades in Stellar Mass at z ≈ 3–6.5

Abstract

Abstract We study the relation between stellar mass (M *) and star formation rate (SFR) for star-forming galaxies over approximately five decades in stellar mass ( 5.5 ≲ log 10 ( M * / M ⊙ ) ≲ 10.5 ) at z ≈ 3–6.5. This unprecedented coverage has been possible thanks to the joint analysis of blank non-lensed fields (COSMOS/SMUVS) and cluster lensing fields (Hubble Frontier Fields) that allow us to reach very low stellar masses. Previous works have revealed the existence of a clear bimodality in the SFR–M * plane with a star formation Main Sequence and a starburst cloud at z ≈ 4–5. Here we show that this bimodality extends to all star-forming galaxies and is valid in the whole redshift range z ≈ 3–6.5. We find that starbursts constitute at least ≈20% of all star-forming galaxies with M * ≳ 109 M ⊙ at these redshifts and reach a peak of 40% at z = 4–5. More importantly, 60%–90% of the total SFR budget at these redshifts is contained in starburst galaxies, indicating that the starburst mode of star formation is dominant at high redshifts. Almost all the low stellar mass starbursts with log 10 ( M * / M ⊙ ) ≲ 8.5 have ages comparable to the typical timescales of a starburst event, suggesting that these galaxies are being caught in the process of formation. Interestingly, galaxy formation models fail to predict the starburst/main-sequence bimodality and starbursts overall, suggesting that the starburst phenomenon may be driven by physical processes occurring at smaller scales than those probed by these models.