Die Hard: The On-Off-Cycle of Galaxies on the Star Formation Main Sequence

Abstract

Our picture of galaxy evolution currently assumes that galaxies spend their life on the star formation main sequence (SFMS) until they are eventually quenched. However, recent observations show indications that the full picture might be more complicated. We reveal typical in-situ star formation histories and their relations to large-scale environment as well as gas accretion across cosmic time. We follow the evolution of central galaxies in the highest-resolution box of the Magneticum Pathfinder cosmological hydrodynamical simulations and classify their evolution scenarios with respect to the SFMS. We find that a major fraction of the galaxies undergoes long-term cycles of quenching and rejuvenation on gigayear timescales. This expands the framework of galaxy evolution from a secular evolution to a sequence of multiple active and passive phases. Only 14% of field galaxies on the SFMS at z=0 actually evolved along the scaling relation, while the bulk of star-forming galaxies in the local Universe have undergone cycles of quenching and rejuvenation. In this work we describe the statistics of these galaxy evolution modes and how this impacts their mean stellar masses, ages, and metallicities today. We further explore possible explanations and find that the geometry of gas accretion at the halo outskirts shows a strong correlation with the star formation rate (SFR) evolution, while the density parameter as a tracer of environment shows no significant correlation. A derivation of SFRs from gas accretion with simple assumptions only works in the high-z universe, where accreted gas is quickly converted into stars. We conclude that an evolution scenario consistently on the SFMS is the exception, when regarding galaxies on the SFMS at z=0. Galaxies with rejuvenation cycles can be distinguished well from SFMS-evolved galaxies, both in their halo accretion modes and in their features at z=0.