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

TL;DR

This study demonstrates that Euclid-like observational effects substantially reduce the detectability of bars in TNG50 galaxies at $z \sim 0.5$, potentially reconciling simulated bar fractions with observations. By creating realistic SKIRT-based radiative-transfer mock images and applying Zoobot, ellipse fitting, and Fourier $A_{2}$ analyses, the authors quantify how many bars would be missed under high-redshift survey conditions. A representative borderline bar is detectable only in the high-resolution VIS band, while the full sample yields apparent bar fractions of $12$--$33\%$ depending on the method, far below the mass-map baseline of $44\%$. These results highlight the critical impact of instrumental resolution, wavelength, and analysis method on inferred bar demographics and motivate observationally informed bar-fraction estimates in cosmological simulations across redshift.

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 bar 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 idealised, 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 percent falls to $12\!-\!33$ percent depending on the method. These results demonstrate the strong impact of observational effects on bar detectability and motivate bar-fraction estimates which incorporate realistic instrumental conditions across redshift in cosmological simulations.

Figures (8)

• Figure 1: Comparison of the stellar mass map and noise-free mock Euclid images for the barred galaxy ID184179. From left to right: the stellar mass map, the synthetic Euclid VIS $I_{E}$ image and the synthetic Euclid NISP $Y_{E}$ image, each covering a physical size of 60 × 60 kpc and shown in a face-on orientation. No observational noise or PSF convolution was added.
• Figure 2: Mock Euclid images of the barred galaxy ID184179 at $z \sim 0.5$ in the $I_{E}$ and $Y_{E}$ bands, their pixel-wise mean, and an RGB composite image. The first three panels (monochromatic) show the $I_{E}$ band, the $Y_{E}$ band, and their mean image, each displayed using the monochromatic color scale adopted for Zoobot classification, after rebinning, synthetic noise addition, and PSF convolution. The fourth panel shows an RGB composite where the $Y_{E}$ band maps to the red channel, the $I_{E}$ band to blue, and the mean image to green. The Zoobot classification (barred or unbarred) for each image is shown at the top of each panel. All images span 60 $\times$ 60 kpc and are displayed face-on.
• Figure 3: Position of subhalo ID184179 within the bar parameter space at $z \sim 0.5$, based on the distribution of bar radius (in kpc) and bar strengths ($A_2$) for all barred galaxies in snapshot 67 from the catalog of Rosas-Guevara2022. Galaxies with both shorter and weaker bars than our target are shown in red, the remaining barred galaxies appear in blue, and ID184179 is highlighted in black. The hatched shaded regions along the $x$-axis mark the bar-length scales corresponding to $2\times\mathrm{FWHM}$ of Euclid VIS ($\sim 2.1\,\mathrm{kpc}$) and NISP ($\sim 5.0\,\mathrm{kpc}$) at this redshift. This figure illustrates that the selected galaxy occupies an intermediate position in the parameter space, with many galaxies hosting even smaller and weaker bars at this redshift.
• Figure 4: Stellar mass map and mock Euclid images of the barred galaxy ID184179, with overplotted ellipse fits for each panel. From left to right: the stellar mass map, the mock Euclid $I_{E}$ image, the mock Euclid $Y_{E}$ image, and the pixel-wise mean of $I_{E}$ and $Y_{E}$, all shown face-on and covering 36 $\times$ 36 kpc. Ellipse fits derived from isophotal analysis are overplotted in blue (mass map), orange ($I_{E}$), green ($Y_{E}$), and purple (pixel-wise mean), respectively.
• Figure 5: Ellipse fitting results for the barred galaxy ID184179 in the stellar mass map and mock Euclid images. From top to bottom: position angle (PA), PA variation ($\Delta$PA), and ellipticity as functions of semi-major axis (SMA) in kpc. Continuous lines show measurements for each image: blue (mass map), orange ($I_{E}$), green ($Y_{E}$), and purple (pixel-wise mean of $I_{E}$ and $Y_{E}$). In both lower panels, points and dashed vertical lines mark the SMA of maximum ellipticity and the SMA where PA deviates from its inner constant plateau, both used as bar length estimates.