The galaxy starburst/main-sequence bimodality over five decades in
  stellar mass at z ~ 3-6.5
  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
<~ log10(M*/Msun) <~ 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) which 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* >~ 10^9 Msun at these redshifts and reach a peak of 40% at z=4-5. More
importantly, 60% to 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
log10(M*/Msun) <~ 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.
