Bright star-forming galaxies naturally forming at z>10 in the Shark semi-analytic model

Abstract: The James Webb Space Telescope (JWST) has unveiled the existence of numerous z>10 bright ultraviolet (UV) galaxies, potentially challenging galaxy formation models in a Lambda Cold Dark Matter (LCDM) universe. Modifications to star formation and stellar feedback models have been suggested to alleviate the tension. However, the fundamental challenge is to design a galaxy formation model that simultaneously reproduces observations from z=0 to those at the highest redshifts. Here, we present predictions from the Shark semi-analytic model of galaxy formation, which is tuned to reproduce the z=0 universe. We show that the same model is capable of reproducing the current UV luminosity function constraints even up to z=17 without the need to invoke variations in the baryon physics model. This model is also capable of reproducing reasonably well the stellar mass function evolution from z=0 to z=10 and the cosmic star formation rate (SFR) density at 0<z\<15, again with the same baryon physics and parameters. The key to the success of Shark is the triggering of starbursts by violent disk instabilities included in the model, which leads to an increase in the variance of the UV luminosity with increasing halo mass at z\>10. We demonstrate that without this mechanism, galaxies are not bursty enough in the model to reproduce the observations at z>10.