Investigating cosmic strings using large-volume hydrodynamical simulations in the context of JWST's massive UV-bright galaxies

Abstract

Recent observations from the James Webb Space Telescope (JWST) have uncovered an unexpectedly large abundance of massive, UV-bright galaxies at high redshifts, presenting a significant challenge to established galaxy formation models within the standard $Λ$CDM cosmological framework. Cosmic strings, predicted by a wide range of particle physics theories beyond the Standard Model, provide a promising potential explanation for these observations. They may act as additional gravitational seeds in the early universe, enhancing the process of high-redshift structure formation, potentially resulting in a more substantial population of massive, efficiently star-forming galaxies. We numerically investigate this prediction in large-volume hydrodynamical simulations using the moving-mesh code AREPO and the well-tested IllustrisTNG galaxy formation model. We evaluate the simulation results in the context of recent JWST data and find that sufficiently energetic cosmic strings produce UV luminosity and stellar mass functions that are in slightly to substantially better agreement with observations at high redshifts. Moreover, we observe that the halos seeded by cosmic strings exhibit a greater efficiency of star formation and enhanced central concentrations. Interestingly, our findings indicate that the simulations incorporating cosmic strings converge with those from a baseline $Λ$CDM model by redshift $z \sim 6$. This convergence suggests that the modified cosmological framework effectively replicates the successful predictions of the standard $Λ$CDM model at lower redshifts, where observational constraints are significantly stronger. Our results provide compelling evidence that cosmic strings may play a crucial role in explaining the galaxy properties observed by JWST at high redshifts while maintaining consistency with well-established models at later epochs.

Figures (9)

• Figure 1: Comparison of halo mass functions from the $\Lambda$CDM run (solid black curves) to the runs modeling cosmic strings with string tension $G\mu=10^{-8}$ (solid red curves) and $G\mu=10^{-10}$ (solid blue curves) with corresponding Poisson errors (shaded error bands) at redshifts $z=6$, 9, and 12. Vertical dash-dotted lines indicate the first halo mass bin with 95% completeness (see text for details). We note that these are color-coded analogously to the solid curves, but overlapping in the plots shown here.
• Figure 2: Simulated stellar mass functions from the $\Lambda$CDM run (solid black curves) and the runs modeling cosmic strings with string tension $G\mu=10^{-8}$ (solid red curves) and $G\mu=10^{-10}$ (solid blue curves) with Poisson errors (shaded error bands) at redshifts $z=6$ to $z=14$. Vertical dash-dotted lines in the corresponding colors show the median stellar mass of halos within the first 95% complete halo mass bin to provide an indication of the typical minimum stellar mass of numerically well-resolved galaxies (see text). Dashed black curves show the Schechter fit of the simulated stellar mass functions from 2023Kannan-MTNG, serving as an additional reference for standard $\Lambda$CDM cosmology from higher-resolution and larger-volume simulations using the same galaxy formation model as our runs. We show results from their closest available output redshift $z_{\rm K23} \sim 15$ in the $z=14$ panel, indicating the slight redshift discrepancy with a reduced opacity of the dashed curve. Observational JWST data from 2024Weibel, 2024Navarro-Carrera, and 2025Harvey are shown as gray symbols (at the closest integer redshift, where applicable). Additionally, the dashed green line in the lower right panel indicates the stellar mass of the spectroscopically confirmed massive $z \sim 14$ galaxy JADES-GS-z14-0 2024Carniani. The predicted number density from our CS-8 runs for such a galaxy at $z=14$ exceeds the $\Lambda$CDM prediction of 2023Kannan-MTNG at their output redshift $z_{\rm K23} \sim 15$ ($z_{\rm K23} \sim 12$) by roughly 3.5 (1.5) orders of magnitude.
• Figure 3: Dust-attenuated UV luminosity functions from our baseline $\Lambda$CDM run (solid black curves) and modified runs modeling cosmic strings with string tension $G\mu=10^{-8}$ (solid red curves) and $G\mu=10^{-10}$ (solid blue curves) with corresponding Poisson errors (shaded error bands) at redshifts $z=8-16$. Dash-dotted vertical lines in the corresponding colors indicate the median UV magnitude of galaxies in the first 95% complete halo mass bin (see Sect. \ref{['subsec:Results_HMF_SMF']} for details). At significantly fainter magnitudes than this, the simulation results are expected to be strongly affected by resolution artifacts. Dashed and dotted black curves show Schechter fits of the dust-attenuated UV luminosity functions from higher-resolution and larger-volume $\Lambda$CDM simulations using the same code base and galaxy formation model as our runs (2023Kannan-MTNG; 2020Vogelsberger-IllTNG_JWST). We note that we show data from the closest available 2023Kannan-MTNG output redshift $z_{\rm K23}$ for $z=13$ ($z_{\rm K23} \sim 12$), as well as for $z=14$ and $z=16$ ($z_{\rm K23} \sim 15$) and indicate these redshift discrepancies with a reduced opacity of the dashed curve. Green and gray symbols show observational estimates from spectroscopically confirmed 2024bHarikane2025Harikane2024Fujimoto2025Napolitano2025Naidu and photometric 2023Bouwens2023Donnan2023aHarikane2023Perez-Gonzalez2024Adams2024Donnan2024Finkelstein2024McLeod2024Robertson2024Willott2025KokorevJWST data, respectively. Where applicable, these estimates are plotted at the closest integer redshift. At high redshifts $z \gtrsim 11$, the observations appear to be in significantly better agreement with the $G\mu=10^{-8}$ results than the $\Lambda$CDM and $G\mu=10^{-10}$ predictions.
• Figure 4: Stellar-to-halo mass ratio of individual simulated galaxies (shaded circles) and median stellar-to-halo mass relation (solid curves) with the 10th to 90th percentile of the distribution (shaded error bands) for our $\Lambda$CDM (black), string tension $G\mu=10^{-8}$ (red), and $G\mu=10^{-10}$ (blue) runs. Shaded circles are only shown for the galaxies outside each mass bin's shaded error bands to improve readability of the figure. We note that the plots include galaxies from all three separate simulations for each of the string tension values and the single baseline $\Lambda$CDM run (cf. Table \ref{['tab:simulations_overview']} and Sect. \ref{['sec:Methods']}). Therefore, the absolute number of $G\mu=10^{-8}$ and $G\mu=10^{-10}$ scatter points is increased by a factor of three relative to the number of $\Lambda$CDM points. We only include halos with masses in or above the first 80% complete halo mass bin (see text for details). Dashed black curves show the stellar-to-halo mass relation from 2023Kannan-MTNG as a further $\Lambda$CDM reference.
• Figure 5: Concentration-mass relation (cMr) of simulated dark matter halos, inferred from a spatially uniform sample of 500 halos in the $\Lambda$CDM (black curves) and CS-8-0 ($G\mu=10^{-8}$, red curves) runs, as well as from CS-8-0 halos seeded by cosmic string loops (dashed green curves), at redshifts $z=6$, 9, and 12. To ensure that individual halos are numerically sufficiently resolved, we only include halos with $M_{\rm halo} \geq 10^{10} \, {\rm M_\odot}$. Filled circles and error bars indicate the mean and standard deviation of the logarithm of the concentrations $\log c$, respectively, in the corresponding mass bins.