Euclid Quick Data Release (Q1): XI. A first look at the fraction of bars in massive galaxies at z < 1

Stellar bars are key structures in disc galaxies, driving angular momentum redistribution and influencing processes such as bulge growth and star formation. Quantifying the bar fraction as a function of redshift and stellar mass is therefore important for constraining the physical processes that drive disc formation and evolution across the history of the Universe. Leveraging the unprecedented resolution and survey area of the Euclid Q1 data release, combined with the Zoobot deep-learning model trained on citizen science labels, we identified 7711 barred galaxies with M* ≳ 1010 M⊙ in a magnitude-selected sample (IE &lt; 20.5) spanning 63.1 deg2. We measured a mean bar fraction of 0.2–0.4, consistent with previous studies. At fixed redshift, massive galaxies exhibit higher bar fractions, while lower-mass systems show a steeper decline with redshift, suggesting earlier disc assembly in massive galaxies. Comparisons with cosmological simulations (e.g. TNG50, Auriga) reveal a broadly consistent bar fraction, but highlight overpredictions for high-mass systems, pointing to potential over-efficiency in central stellar mass build-up in simulations. A semi-empirical model (Decode) in which bar formation is regulated by the gas fraction broadly reproduces the observed bar fractions. These findings demonstrate the transformative potential of Euclid for galaxy morphology studies and underscore the importance of refining theoretical models to better reproduce observed trends. Future work will explore finer mass bins, environmental correlations, and additional morphological indicators.