Speaker
Description
The discovery of an unexpectedly high N/O ratio in GN-z11 raises questions about star formation in the early universe. Using public NIRSpec data, we have pinpointed additional two galaxies at z=6.23-8.68 with nitrogen-rich gas relative to carbon and oxygen ([N/C] $\gtrsim$ 1 and [N/O] $\gtrsim$ 0.5), suggestive of predominant materials processed by the CNO cycle. Interestingly, using the NIRSpec data and our derived line-spread functions, we have found that CEERS_01019 with [OII] has a high electron density of $n_{\mathrm{e}}\sim1000$ cm$^{-3}$. Such high N/O and high $n_{\mathrm{e}}$ gas is also reported in other z>6 galaxies of GN-z11 and RXCJ2248-ID, but not in a z=6.07 galaxy of the Cosmic Grapes, suggesting a potential correlation between N/O and n_e values. Such high N/O ratios are also observed in the atmospheres of dwarf stars in globular clusters, and high $n_{\mathrm{e}}$ values of $\gtrsim1000$ cm$^{-3}$ are simulated in young massive clusters, promising GC progenitors. This suggests that GCs, which formed from dense gas with high star-formation efficiencies, may preserve nitrogen-rich gas in their dwarf stars. We also find that the 3 nitrogen-rich galaxies, CEERS_01019, GN-z11, and RXCJ2248-ID, have high NIV]/NIII] ratios beyond stellar photoionization models at a given line ratio (e.g., [OIII]/[OII]), suggestive of hard emission from black holes and/or an exotic stellar population such as Wolf-Rayet stars or supermassive stars. Notably, these sources can contribute to nitrogen enrichment by releasing the CNO-cycle materials processed in stellar outer layers via tidal disruption events or stellar winds. This talk will also delve into causal relationships among the rich nitrogen, the high $n_{\mathrm{e}}$, and the hard emission in the early universe.
