Star-formation modes at different cosmic epochs

The majority of galaxies show a tight correlation between their on-going star-formation and their stellar mass, historically called the main-sequence of star-formation. This sequence seems to imply a balance between stochastic and secular effects. There are however galaxies showing little or no star-formation, called quiescent, and system showing an incredible amount of on-going star-formation, compared to their stellar masses. The latter are called starburst galaxies and different studies have shown that they are a small fraction of the galaxies at low-redshift, at least among relative massive systems.

The analysis of the star-formation-rate versus stellar mass plane (SFR-M*) is therefore a powerful diagnostic to analyse the interplay and the importance of secular effects, like gas depletion and gas in-fall, and more stochastic effects, such as mergers, on the galaxy evolution and star formation.

We consider a sample of 24463 galaxies from the CANDELS/GOODS-S survey to study the SFR-M* plane at redshift 0.5<z<3 over more than three dex in stellar mass, using mainly UV-based SFR. We derive the main-sequence of star-formation using a specific SFR (sSFR) cut to identify all star-forming galaxies and fitting a linear regression to them, as it is usually done in the literature. Moreover, we apply a second method consisting of a multi-Gaussian decomposition of the sSFR distribution to identify the galaxy main sequence, the starburst sequence and the quenched galaxy cloud.

Our main finding are:

the main-sequence derived using a linear regression to all star-forming galaxies highly depends on the stellar mass range analysed
the starburst galaxies show a clear sequence and their fraction increases with redshift (up to 16% at z=2-3) and decreases with stellar mass.
the quenched galaxies may be divided into two modes, one at high stellar masses, where galaxies are separated from the main-sequence and numerous, and a second one at low stellar masses, where galaxies are close to the main sequence and less frequent. Indeed, the fraction of quenched galaxies decreases with decreasing stellar masses and increasing redshift.