Increased burstiness at high redshift in multi-physics models combining supernova feedback, radiative transfer and cosmic rays

We study star formation variability, or burstiness, as a method to constrain and compare different galaxy formation models at high redshift using the Azahar simulation suite. The models range from magneto-hydrodynamics with turbulence-driven star formation to more sophisticated setups incorporating radiative transfer and cosmic ray physics. Analysing a sample of galaxies at redshifts z = 4 − 10, we find that including both radiative transfer and cosmic rays results in more regular star formation periodicity, as revealed by the Lomb-Scargle periodogram. While both radiative transfer and cosmic rays amplify star formation stochasticity, their combination leads to the largest scatter in burst intensity and the most pronounced deviations from the star-forming main sequence. To compare this comprehensive model against observations, we generate a mock spectrum of a low-mass galaxy during a mini-quenching event at z = 7.5. The resulting spectrum aligns well with the low-mass quiescent galaxy JADES-GS-z7-01-QU observed at z = 7.3, though discrepancies attributed to stellar metallicity suggest it may have a composite nature. Our findings highlight the importance of including complex physical processes like cosmic rays and radiative transfer in simulations to accurately capture the bursty nature of star formation in early galaxy formation. Future JWST observations, particularly of the scatter around the star-forming main sequence, might provide critical constraints for numerical models of galaxy formation at high redshift.