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

Sven P. K. Koehler, Hao Jiao+1 more|arXiv (Cornell University)|Nov 29, 2024

Astronomy and Astrophysical Research4 citations

TL;DR

This study uses large-volume hydrodynamical simulations with the arepo code and the IllustrisTNG galaxy formation model to test whether cosmic strings—topological defects from early-universe phase transitions—can explain the unexpectedly high abundance of massive, UV-bright galaxies at z ≳ 10 observed by JWST. It finds that cosmic strings with tension Gμ = 10⁻⁸ significantly enhance high-redshift galaxy formation, improving agreement with observed UV luminosity and stellar mass functions, while simulations converge with ΛCDM by z ≈ 6–8.

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.

Motivation & Objective

• To investigate whether cosmic strings can resolve the tension between observed high-redshift massive, UV-bright galaxies and predictions from standard ΛCDM galaxy formation models.
• To assess the impact of cosmic strings on the formation of high-redshift galaxies using large-volume hydrodynamical simulations with realistic galaxy physics.
• To evaluate whether cosmic strings can naturally produce enhanced star formation efficiency and higher halo concentrations without altering the underlying galaxy formation model.
• To determine the redshift at which simulations including cosmic strings converge with baseline ΛCDM results, assessing consistency with later-time observations.
• To constrain viable cosmic string tensions by comparing simulated UV luminosity and stellar mass functions with recent JWST data.

Proposed method

• Employed large-volume cosmological hydrodynamical simulations using the moving-mesh code arepo with the well-tested IllustrisTNG galaxy formation model.
• Incorporated cosmic strings with varying tensions (Gμ = 10⁻¹⁰ and Gμ = 10⁻⁸) as additional gravitational seeds in the early universe to seed structure formation.
• Tracked the evolution of halo and galaxy properties, including UV luminosity functions, stellar mass functions, and NFW halo concentration profiles.
• Compared simulation outputs with recent JWST observations of high-redshift (z ≳ 10) massive, UV-bright galaxies.
• Used rescaling techniques to correct for limited numerical resolution, based on higher-resolution simulations with the same model and code.
• Analyzed convergence of cosmic string simulations with baseline ΛCDM by z ≈ 6–8 to assess long-term consistency with observational constraints.

Experimental results

Research questions

• RQ1 Can cosmic strings with Gμ = 10⁻⁸ produce UV luminosity and stellar mass functions at high redshift that better match observed JWST data compared to standard ΛCDM?
• RQ2 Do halos seeded by cosmic strings exhibit enhanced star formation efficiency and higher concentrations compared to ΛCDM halos of similar mass?
• RQ3 What is the impact of lower string tension (Gμ = 10⁻¹⁰) on galaxy and halo properties, and does it remain consistent with ΛCDM predictions?
• RQ4 At what redshift do simulations including cosmic strings converge with baseline ΛCDM results, and what does this imply for model consistency at lower redshifts?
• RQ5 Can the cosmic string model explain the observed overabundance of massive, efficiently star-forming galaxies at z ≳ 10 without modifying the underlying galaxy formation physics?

Key findings

• Cosmic strings with Gμ = 10⁻⁸ produce UV luminosity and stellar mass functions that are in substantially better agreement with high-redshift JWST observations than the baseline ΛCDM model.
• Halo populations seeded by Gμ = 10⁻⁸ cosmic strings exhibit higher star formation efficiency and enhanced central concentrations compared to ΛCDM halos of similar mass.
• The median concentration of loop-seeded halos is higher than in ΛCDM, indicating earlier formation times and more compact structures.
• Simulations with Gμ = 10⁻¹⁰ show no significant deviation from ΛCDM results across all redshifts, suggesting such low tensions are inconsistent with observed high-redshift galaxy abundance.
• All simulation runs, including those with cosmic strings, converge with the baseline ΛCDM model by redshift z ≈ 6–8, indicating consistency with later-time observational constraints.
• The results suggest that cosmic strings with Gμ = 10⁻⁸ may provide a viable explanation for the high-redshift galaxy overabundance, while future constraints from wider-area surveys and 21 cm/CMB observations could further test this model.