The MillenniumTNG project: the galaxy population at z >= 8

The early release science results from JWST have yielded an unexpected abundance of high-redshift luminous galaxies that seems to be in tension with current theories of galaxy formation. However, it is currently difficult to draw definitive conclusions form these results as the sources have not yet been spectroscopically confirmed. It is in any case important to establish baseline predictions from current state-of-the-art galaxy formation models that can be compared and contrasted with these new measurements. In this work, we use the new large-volume (L-box similar to 740 cMpc) hydrodynamic simulation of the MillenniumTNG project, suitably scaled to match results from higher resolution - smaller volume simulations, to make predictions for the high-redshift (z greater than or similar to 8) galaxy population and compare them to recent JWST observations. We show that the simulated galaxy population is broadly consistent with observations until z similar to 10. From z approximate to 10-12, the observations indicate a preference for a galaxy population that is largely dust-free, but is still consistent with the simulations. Beyond z greater than or similar to 12, however, our simulation results underpredict the abundance of luminous galaxies and their star-formation rates by almost an order of magnitude. This indicates either an incomplete understanding of the new JWST data or a need for more sophisticated galaxy formation models that account for additional physical processes such as Population III stars, variable stellar initial mass functions, or even deviations from the standard Lambda CDM model. We emphasize that any new process invoked to explain this tension should only significantly influence the galaxy population beyond z greater than or similar to 10, while leaving the successful galaxy formation predictions of the fiducial model intact below this redshift.

Automated summary

The early release science results from JWST indicate a large abundance of high-redshift luminous galaxies, challenging current theories of galaxy formation. However, the lack of spectroscopic confirmation limits definitive conclusions. This study uses a large-volume hydrodynamic simulation to make predictions for the high-redshift galaxy population and finds that the simulations are consistent with observations until z≈10, but underestimate the abundance of luminous galaxies and star-formation rates beyond z≥12, indicating a need for more sophisticated galaxy formation models.