The detectability of bars at high redshift: a case study using Euclid-like mock observations of TNG50 simulated galaxies

ABSTRACT Modern surveys such as Euclid report a decline in the fraction of barred galaxies from the local Universe to $z \sim 1$, whereas the TNG50 simulation predicts higher bar fractions, in tension with observations. This discrepancy may be due to observational biases in bar detectability when comparing simulations with observations. We present a proof-of-concept study quantifying how Euclid-like observational conditions affect bar detectability in TNG50. We analysed the entire galaxy sample at $z=0.5$ and highlight one borderline case with a bar length of 2.1 kpc and strength $A_2=0.4$. Synthetic images were produced with Monte Carlo radiative transfer and realistic post-processing, and analysed with ellipse fitting and Fourier decomposition, as well as the recently constructed Zoobot analysis. Results were compared to idealized noise-free stellar mass maps. In the illustrative case the bar is clearly detected in the mass map and remains visible in the Euclid VIS $I_{\rm E}$ filter, where Zoobot also classifies it as barred, but becomes undetectable in $Y_{\rm E}$ and in the VIS–NISP RGB composite, with all methods failing outside VIS. Extending to the full $z=0.5$ sample, Zoobot recovers only 31/141 galaxies, while $A_2$ and ellipse fitting perform better (80/141 and 67/141) but still miss many short or weak bars. When non-detections are counted as unbarred, the bar fraction of 44 per cent falls to 12–33 per cent depending on method. These results demonstrate the strong impact of observational effects on bar detectability and motivate bar-fraction estimates that incorporate realistic instrumental conditions across redshift in cosmological simulations.