Nitrogen enhancement of GN-z11 by metal pollution from supermassive stars

TL;DR

This study uses galaxy formation simulations to explore how supermassive star pollution can explain high nitrogen-to-oxygen ratios observed in early high-redshift galaxies like GN-z11.

Contribution

It demonstrates that SMS ejecta can account for observed abundance patterns in high-redshift galaxies through a detailed chemical evolution model.

Findings

SMS pollution enhances N/O ratio matching observations.

Pollution mass fraction of 10-30% reproduces galaxy abundance patterns.

High-density gas ionized by SMS can facilitate pollution within realistic conditions.

Abstract

Spectroscopic observations by the James Webb Space Telescope (JWST) have revealed young, compact, high-redshift ($z$) galaxies with high nitrogen-to-oxygen (N/O) ratios. GN-z11 at z=10.6 is one of these galaxies. One possible scenario for such a high N/O ratio is pollution from supermassive stars (SMSs), from which stellar winds are expected to be nitrogen-rich. The abundance pattern is determined by both galaxy evolution and SMS pollution, but so far, simple one-zone models have been used. Using a galaxy formation simulation, we tested the SMS scenario. We used a cosmological zoom-in simulation that includes chemical evolution driven by rotating massive stars (Wolf-Rayet stars), supernovae, and asymptotic giant branch stars. As a post-process, we assumed the formation of an SMS with a mass between $10^3$ and $10^5$ $M_\odot$ and investigated the contribution of its ejecta to the abundance pattern. The N/O ratio was enhanced by the SMS ejecta, and the abundance pattern of GN-z11, including carbon-to-oxygen and oxygen-to-hydrogen ratios, was reproduced by our SMS pollution model if the pollution mass fraction ranges within 10-30 per cent. Such a pollution fraction can be realized when the gas ionized by the SMS is polluted, and the gas density is $10^4$-$10^5$ cm$^{-3}$ assuming a Str\"omgren sphere. We also compared the abundance pattern with those of other N/O-enhanced high-$z$ galaxies. Some of these galaxies can also be explained by SMS pollution.