Speaker
Description
The observations conducted with JWST have disclosed galaxies at high redshifts characterized by extremely high N/O and low C/N ratios, exemplified by GN-z11. The high N/O and low C/N ratios of these galaxies are largely biased toward the equilibrium of the CNO cycle, suggesting that these three galaxies are enriched by metals processed by the CNO cycle. The low C/N and high N/O ratios of these three galaxies cannot be explained by typical chemical evolution models including asymptotic giant branch stars in their early chemical enrichment stages. We thus investigate three scenarios associated with dominant CNO-cycle materials, i.e., Wolf-Rayet stars, supermassive stars (with $10^3 - 10^5~M_\odot$ ), and tidal disruption events. We develop the chemical evolution models of these scenarios, assuming star formation based on various initial mass functions with core-collapse supernovae (CCSN) based on the theoretical yields. We find that the C/O and N/O ratios of these three galaxies are explained by any of these three scenarios. However, the N/O values of the three models decrease quickly after the CCSNe take place due to a large amount of oxygen ejected from CCSNe. Because CCSNe appear in a very short time scale (<1 Myr) for massive star progenitors, the massive stars need to directly collapse into black holes without CCSNe to prevent oxygen enrichment. This mechanism may be related to the seed black hole formation of the supermassive black holes found at high redshift.
