Die Hard: The on-off cycle of galaxies on the star formation main sequence

¹ Excellence Cluster ORIGINS, Boltzmannstr. 2, 85748 Garching bei München, Germany
² Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 München, Germany
³ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
⁴ Max-Planck-Institute for Extraterrestrial Physics, Giessenbacherstr. 1, 85748 Garching, Germany

^★ Corresponding author: sfortune@usm.lmu.de

Received: 27 March 2025
Accepted: 26 September 2025

Abstract

Context. 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.

Aims. We reveal typical in-situ star formation histories and their relations to large-scale environment as well as gas accretion across cosmic time. We further discuss systematic imprints on stellar masses, ages, and metallicities.

Methods. 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.

Results. 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 evolution, while the density parameter as a tracer of environment shows no significant correlation. A derivation of star formation rates from gas accretion with simple assumptions only works reasonably well in the high-redshift universe, where accreted gas is quickly converted into stars.

Conclusions. We conclude that an evolution scenario consistently on the main sequence is the exception, when regarding galaxies on the main sequence at lower redshifts. Galaxies with rejuvenation cycles can be distinguished well from main-sequence-evolved galaxies, both in their halo accretion modes and in their features at z ≈ 0.