The two regimes of the cosmic sSFR evolution are due to spheroids and discs

TL;DR

This paper explains the two phases of cosmic specific star formation rate evolution by attributing high values at z>2 to spheroids and the gradual decline at lower redshifts to disc galaxy evolution, supported by chemical evolution models.

Contribution

It introduces a model that links the two sSFR phases to different galaxy populations, spheroids and discs, providing a unified explanation for observed trends.

Findings

High sSFR at z>2 is due to spheroid progenitors and bulges.

The decline in sSFR at z<2 is driven by gas fraction evolution in discs.

The model aligns with observed stellar ages and star formation histories.

Abstract

This paper aims at explaining the two phases in the observed specific star formation rate (sSFR), namely the high (>3/Gyr) values at z>2 and the smooth decrease since z=2. In order to do this, we compare to observations the specific star formation rate evolution predicted by well calibrated models of chemical evolution for elliptical and spiral galaxies, using the additional constraints on the mean stellar ages of these galaxies (at a given mass). We can conclude that the two phases of the sSFR evolution across cosmic time are due to different populations of galaxies. At z>2 the contribution comes from spheroids: the progenitors of present-day massive ellipticals (which feature the highest sSFR) as well as halos and bulges in spirals (which contribute with average and lower-than-average sSFR). In each single galaxy the sSFR decreases rapidly and the star formation stops in <1 Gyr. However the combination of different generations of ellipticals in formation might result in an apparent lack of strong evolution of the sSFR (averaged over a population) at high redshift. The z<2 decrease is due to the slow evolution of the gas fraction in discs, modulated by the gas accretion history and regulated by the Schmidt law. The Milky Way makes no exception to this behaviour.