EMPOWER Science Case
Answering the core questions of galaxy evolution
The EMPOWER campaign is a transformative IFU survey structured to deliver representative snapshots of the galaxy population at three key epochs: the decline (z~0.75), the peak (z~1.6), and the early rise (z~2.3) of star formation (SF), across the full parameter space, tackling three fundamental questions in galaxy evolution:
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When and where do galaxies quench their star formation?
Generally, galaxies exhibit a bimodal distribution of star
formation rates (SFRs): active star-forming galaxies on
the main sequence (SFR vs. stellar mass tight correlation)
and quiescente galaxies below it, separated by the green
valley.
At high stellar masses, the main sequence flattens,
signalling a decline in SF efficiency. This marks a crucial
transition: galaxies stop accreting cold gas from cosmic
filaments and become embedded in hot, virialized halos.
To maintain star formation suppression, the circumgalactic
medium (CGM) must remain hot, with specific processes
(e.g. AGN feedback) providing this necessary heating.
To date, a complete picture of gas inflows, outflows, cooling
and heating is still lacking.
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How do mass, AGN, and environment shape galaxy evolution?
AGN feedback has traditionally been viewed as a mass-dependent quenching mechanism, with limited environmental impact. However, advanced simulations reveal a more complex picture. In IllustrisTNG and Astrid, AGN activity emerges around z~2.5 in dense nodes of the Cosmic Web, whereas Simba and Magneticum show feedback in lower-mass groups along filaments at the same epoch. By z~0.75, Simba and Magneticum predict widespread heating of the intergalactic medium, leading to large-scale quenching, while IllustrisTNG maintains SF in low-mass halos.
The process is further complicated by mergers, morphological transformations, and environmental gas removal processes, such as strangulation, ram-pressure stripping, and tidal interactions, which can both suppress and enhance SF. This highlights that environment is a key factor in galaxy evolution, and understanding when and where these processes occur requires robust observational data across cosmic time.
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How does the Cosmic Web influence these processes over time?
Spatially resolved studies of local galaxies have shown that environmental quenching acts from the outside-in, while AGN-driven outflows act from the inside-out. However, observations at z~0 capture only the final stage of galaxy evolution, where many processes have already occurred, making it difficult to disentangle their contributions. Studying galaxies at earlier epochs is crucial to map the co-evolution of AGN, star formation, and large-scale structure.
EMPOWER leverages the ESO KMOS Science Archive Facility (SAF) to go beyond existing high-redshift datasets, targeting ~900 galaxies in filaments, nodes, groups, clusters, and proto-clusters across cosmic epochs.
At z~0.75 and ~1.6, massive clusters provide mature environments to test directly environmental quenching like ram-pressure stripping and starvation. At z~2.3, proto-clusters capture the earliest phases of massive structure formations, offering a window into the emergence of environmental effects and galaxy transformation during the SF density peak.
The survey will deliver spatially resolved maps of Hβ, [OIII], Hα, and [NII] out to at least 2rₑ, providing robust measurements of ionized gas kinematics, metallicity, dust-corrected SFR, extinction, and spatially resolved BPT diagnostics. Around half of the targets already have archival Hα and [NII] data, which will be deepened, while new observations will be added for the remainder to achieve uniform depth and SNR. Additionally, ~45 quiescent galaxies at z~0.75 will be observed in single-band long exposures to capture absorption features such as CaHK, the 4000 Å break, and Hδ.
By integrating with major surveys (e.g., 4MOST-WAVES-Deep, MOONS/MOONRISE, Euclid, COSMOSWeb) and ancillary X-ray, SZ, and JWST datasets, EMPOWER will connect AGN feedback with halo gas thermodynamics, stellar morphology, and mass assembly.
Its design enables detailed studies of SF regulation and correlation with stellar mass (main sequence bending), quenching pathways, and inside-out versus outside-in suppression, including environmental effects. By stacking galaxy maps and tracing radial gradients in SFR, metallicity, and stellar properties, EMPOWER will track the timing and location of feedback and quenching processes across cosmic time.
Together, these observations will establish a comprehensive high-redshift legacy dataset, enabling both targeted science and serendipitous discovery, and providing a cornerstone for understanding galaxy evolution.
Evolution of the main sequence of star forming galaxies over cosmic time. The turnover stellar mass sets the transition to SFR suppression at the high stellar masses. The lover panel shows the residuals versus the best fit (Daddi et al., 2022, Popesso et al., 2023).
A comparison of four state-of-the-art hydrodynamical simulations (Yongseok et al., 2025) - IllustrisTNG (Pillepich et al., 2019), Simba (Bird et al., 2022), Astrid (Dave et al., 2019), and Magneticum (Dolag et al., 2025) - at the three EMPOWER snapshots reveals significant variation in when and where AGN feedback begins to affect the temperature of the CGM and halo gas across the Cosmic Web. These differences highlights the model-dependent nature of the AGN-driven heating in galaxy environments.
Left panel: IllustrisTNG predictions of the distribution of the EMPOWER targets, Color-coded according to the redshift, in the stellar mass (X-axis)-host halo mass (Y-axis) relation. The symbols indicate centrals (crosses) and satellites (triangles). The parent population distribution is indicated by the blue contours. Right panel: Distribution of the COSMOS-Web and GOODS-S EMPOWER targets in stellar mass-distance from the main sequence plane. The color coding is the same as the left panel.