Euclid preparation

The evolution of galaxies is profoundly influenced by the environment they reside in. Cosmic voids, the most underdense regions of the Universe, serve as pristine laboratories for studying galaxy evolution in the relative absence or weaker influence of the complex physical processes that dominate denser environments. We investigate the properties and merger histories of galaxies as a function of environment using the GAlaxy Evolution and Assembly (GAEA) mock-observation light cone, which replicates the Euclid Deep Survey as foreseen at the epoch of the first data release. The H α -selected galaxy sample spans the redshift range 0.4 < z < 1.8, corresponding to the interval over which H α is accessible to Euclid slitless spectroscopy. We classified galaxies based on their void-centric distance and local density contrast. We compared their stellar masses, specific star formation rates, bulge-to-total stellar mass ratios, and halo masses across different environments, while controlling for the dependence on stellar mass of the other intrinsic properties. We further analysed the merger histories of these galaxies to study their assembly evolution. We find that galaxies located closer to void centres ( d cc ≲ 0.7 R v ) are generally less massive, more actively star-forming, and more disc-dominated than galaxies in denser regions, with their distributions statistically distinct at more than 4 σ confidence according to Kolmogorov–Smirnov tests. Merger histories indicate that void galaxies do not experience fewer mergers but that these occur later, leading to stellar mass assembly delayed by approximately 0.5–1 Gyr relative to galaxies in high-density regions. This supports a scenario in which the environment regulates the timing and nature of mergers rather than their overall frequency, producing a slower evolutionary path in low-density regions. We conclude by discussing the extent to which these trends are shaped by environmental parametrisation methods and observational selection effects. Our analysis provides a framework for interpreting forthcoming Euclid data and demonstrates that Euclid will enable the identification of cosmic voids and characterisations of their galaxy populations with an unprecedented statistical power and redshift coverage, providing a quantitative forecast of Euclid ’s capability to probe environmental effects on galaxy evolution.

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