SCIENTIFIC BACKGROUND

To fully understand the physical mechanisms driving galaxy formation and evolution, we need to retrace the evolutionary path of galaxies across cosmic time. While the local Universe is well anchored by extensive datasets such as SDSS, and the distant Universe is increasingly probed by deep spectroscopic observations such as LEGA-C, the intermediate-redshift window (0.3 < z < 0.7) remains comparatively less explored. Yet this epoch is crucial: over the past 6 billion years - nearly half the age of the Universe - galaxies underwent profound transformations, with the Hubble sequence taking shape, star formation declining dramatically, and environmental processes becoming increasingly important.

The advent of new generation spectroscopic surveys is now opening an unprecedented opportunity to fill this gap. Instruments such as 4MOST, DESI, and WEAVE are providing, or will soon provide, spectra for millions of galaxies across large cosmological volumes, enabling us to study galaxy populations in a statistically powerful and homogeneous way. This wealth of data offers a unique opportunity to investigate galaxies at a key stage of their evolution, linking the well characterised local Universe to the earlier phases probed by deep surveys, and to understand how their stellar populations, structure, and star-formation activity were shaped over time.

The present project is based on proprietary data from three major spectroscopic surveys: WEAVE-StePS, 4MOST-StePS, and 4MOST-WAVES, carried out with two state-of-the-art multi-object spectroscopic facilities, WEAVE on the William Herschel Telescope (Canary islands) and 4MOST on the ESO VISTA telescope (Cerro Paranal - Chile). Furthermore, it will also exploit publicly available spectroscopic data from DESI, whose first public Data Release (DR1) contains spectra for more than 18 million unique targets, including over 13 million galaxies, over a wide area of the sky. Together, these complementary surveys will provide an unprecedented view of galaxy evolution across the intermediate redshift Universe, combining high-quality spectroscopy with large statistical samples.

THE PROJECT

The spectrum of a galaxy is a powerful archaeological record of its formation and evolution. By analysing the absorption features imprinted by its stellar content and the emission lines produced by its ionised gas, we can reconstruct the physical processes that shaped galaxies across cosmic time.

Our analysis focuses on measuring stellar absorption indices and nebular emission lines from individual high-SNR spectra and, crucially, from stacked spectra of galaxies whose individual spectra have too low a SNR for detailed analysis. Stellar absorption indices (see Fig. 1 left panel) provide constraints on the age, metallicity, and star-formation history of the stellar populations, while nebular emission lines (see Fig. 1 right panel) trace the properties of the ionised gas, including star formation, chemical enrichment, and the presence of nuclear activity. By combining these complementary diagnostics, we will characterise the stellar and gaseous components of galaxies at intermediate redshift and investigate how their properties vary with galaxy mass, star-formation activity, and environment. In particular, we will explore galaxies across the cosmic web - from relatively low-density regions such as voids to filaments and high-density environments - to determine how environment influences their evolution.

A key goal is to place these measurements in an evolutionary context. By comparing the stellar populations and gas properties of intermediate-redshift galaxies with those observed at higher and lower redshift, we aim to identify when and how the major transformations in galaxy evolution took place. This will allow us to move beyond a simple description of galaxy populations at different epochs and instead reconstruct the evolutionary pathways connecting the distant and local Universe.

Ultimately, the combination of spectral diagnostics, large statistical samples, and environmental information will provide empirical constraints on galaxy formation and evolution models, helping us understand how galaxies acquired the properties we observe in the present-day Universe.