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

TL;DR

This study tests whether nitrogen enrichment in GN-z11 can arise from pollution by supermassive stars (SMSs) using a cosmological zoom-in galaxy simulation with chemical evolution from rotating massive stars, SNe, and AGB stars, followed by a post-processing SMS-pollution analysis. SMSs with masses in the range $M_{ m SMS}=10^{3}$–$10^{5}\,M__ $ are allowed to form in the central 10 pc, and their ejecta are assumed to mix with gas within a Str"omgren sphere; the required pollution fraction to match GN-z11 is $f_{ m SMS}\sim 10$–$30\%$, achievable for central gas densities $n_{ m H}\sim 10^{4}$–$10^{5}\,\mathrm{cm^{-3}}$. The SMS models with $M_{ m SMS}=5\times10^{4}$–$10^{5}\,M__ $ reproduce GN-z11's abundance pattern at $n_e=10^{3}\,\mathrm{cm^{-3}}$, while a $10^{4}\,M__ $ model can also fit at later times with smaller $f_{ m SMS}$. The results suggest SMS pollution is a viable mechanism for early nitrogen enrichment in GN-z11-like galaxies and possibly in other $N/O$-enhanced high-redshift systems, though the exact outcome depends on inner-galaxy density structure and ionized-region geometry.

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 Strömgren 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.

Figures (11)

• Figure 1: The effective temperature evolution of $10^4$$\,\rm{M}_{\odot}$ SMS model performed in Nagele2023.
• Figure 2: Projected surface densities of stars (left), gas (middle), and dark matter (right) at $z=11.20$, 10.77, 10.65, and 10.38 from top to bottom.
• Figure 3: Mass evolution of stars (blue), gas (orange), and dark matter (green) within 1 kpc from the galactic center. Redshifts of each snapshot are noted above the upper $x$ axis. The red vertical lines indicates the moment of $z=10.77$ and $z=10.60$.
• Figure 4: Time evolution of the star formation rate in the central region (within 10 pc) of our simulated galaxy. Red vertical lines indicate the timing of the snapshots. Redshifts of each snapshot are noted above the upper $x$ axis. The shaded region indicates the time range between $z=10.77$ and $10.60$.
• Figure 5: Radial density profiles of dark matter (green), gas (orange), and stars (blue) at $z=10.65$. The shaded regions indicate the possible value ranges of each component during $z=10.77$--$10.60$.