Galaxy evolution in the last 7 Gyr through observed star formation histories in cosmological simulations

Large extragalactic campaigns, like the Sloan Digital Sky Survey, enriched our knowledge about multiple galaxy populations in the local Universe. However, several open questions regarding galaxy evolution still need to be answered. When do galaxies quench their star formation? Do quenched galaxies rejuvenate during their lifetime? What is the main channel for the gas supply for the rejuvenated galaxies? Anewopportunitytoaddressthesequestionsrecentlyarrived. TheStellarPopulation at intermediate redshift Survey (StePS) is one of the eight surveys that will be carried out with WEAVE, the new wide-field spectroscopic facility for the 4.2m William Herschel Telescope in the Canary Islands. WEAVE-StePS aims to obtain high-resolution (R ∼5000) spectra of galaxies in the redshift range 0.3 < z < 0.7, thus providing reliable measurements of the absorption features in the stellar continuum for a large sample of galaxies. The excellent dataset that WEAVE-StePS promises to deliver will allow us to characterize with unprecedented accuracy the last 7 Gyr of galaxy evolution. However, models and observations still need to converge to a coherent description of the star formation history of massive galaxies. Providing for the need for a more realistic comparison between theory and observa- tions, in this manuscript I describe the creation of a synthetic spectroscopic dataset of ∼750 massive galaxies from the TNG50 cosmological simulation tailored for WEAVE- StePS observations. I compare the star formation history of these galaxies, retrieved using a full spectral fitting analysis. This strategy will allow a proper comparison be- tween observed star formation histories and those inferred from cosmological simulations, looking for possible systematics and observational biases. Applying this framework, I find that the fractions of quiescent and green–valley systems increase toward lower redshift, consistent with ongoing quenching and the build-up of the red sequence; the green–valley fraction peaks at z=0.5 (11%), indicating an elevated incidence of transitioning galaxies. I introduce the simple diagnostic ∆AGE ≡AGEMW−AGELW to flag rejuvenation episodes and show that it robustly identifies recent star formation in otherwise quiescent systems. Rejuvenation signatures are stronger in integrated than in fiber-like spectra, suggesting that young populations preferentially reside outside galaxy centers. I further extend the analysis to the local Universe to assess environmental effects, combining archival IFU data for Virgo and Fornax cluster galaxies with forward-modeled TNG100 analogs selected to match the host clusters’ halo mass and virial radius. The main advantage of this approach is that, unlike observations that can only provide i a snapshot of the present-day universe, cosmological simulations offer comprehensive insights into the past of each galaxy, enabling a direct connection between present-day SFHs and assembly pathways. Overall, I find that infall time (e.g., when the galaxy enters the cluster) regulates the SFH timescale at low stellar mass, while mergers increasingly govern mass assembly at high stellar mass. Together, these results deliver validated tools and benchmarks for interpreting forthcoming WEAVE–StePS spectra, and they sharpen constraints on the quenching and rejuvenation channels that shape the mass-assembly histories of massive galaxies.