Very Massive Stars and High N/O: A Tale of the Nitrogen-enriched Super Star Cluster in the Sunburst Arc

Abstract

The lensed Sunburst Arc ($z = 2.369$) hosts a young ($\sim2$--$4\,\rm Myr$), massive ($M_\star \sim 10^7\,M_\odot$), compact ($R_{\rm eff} \sim 8\,\rm pc$) Lyman-continuum (LyC) leaking super star cluster, which powers a compact ($< 10\,\rm pc$), high-pressure nebula at sub-solar metallicity $\sim0.2\,Z_\odot$ and with an anomalously elevated nitrogen-to-oxygen ratio $\log({\rm N/O}) \sim -0.2$. We present semi-analytic models and 3D magnetohydrodynamic simulations with radiative feedback in an attempt to reproduce this system. The results indicate that the progenitor giant molecular cloud (GMC) may have $M_{\rm cloud} \gtrsim 3 \times 10^7\,M_\odot$ and $R_{\rm cloud} \sim 70\,\rm pc$, corresponding to a surface density $\sim10^3$--$10^4\,M_\odot\,{\rm pc}^{-2}$. Incorporating feedback from individual Very Massive Stars (VMSs; $\ge 100\,M_\odot$) sampled from the Kroupa initial mass function, we find that their winds rapidly enrich $\sim 10^4\,M_\odot$ of nearby gas with nitrogen ($\sim 1\,$dex) and helium ($\sim 0.1$--$0.2\,$dex). In the first $1$--$3\,$Myr, some cold gas falls to the system center where a central cluster builds up from sub-cluster mergers. There, the gas is photoionized, pressurized, and chemically enriched by the newly formed VMSs, before being radiatively expelled in the next $\sim1\,\rm Myr$. We find that both VMS feedback and a high-surface-density progenitor GMC are necessary to reproduce the observed nebular properties, such as high N/O, high pressure, and stellar proximity. Low metallicity ($Z \le 0.004$) may be essential to avoid overproduction of carbon from WC stars. Such enrichment processes localized to compact starburst events may have caused strong nitrogen emission from dense ionized gas as observed in high-redshift galaxies such as GN-z11 and GS_3073.