JWST Identification of Extremely Low C/N Galaxies with [N/O] ≳ 0.5 at z ∼ 6–10 Evidencing the Early CNO-cycle Enrichment and a Connection with Globular Cluster Formation
Yuki Isobe, Masami Ouchi, Nozomu Tominaga, Kuria Watanabe, Kimihiko Nakajima, Hiroya Umeda, Hidenobu Yajima, Yuichi Harikane, Hajime Fukushima, Yi Xu, Yoshiaki Ono, Yechi Zhang
Abstract
Abstract We present chemical abundance ratios of 70 star-forming galaxies at z ∼ 4–10 observed by the JWST/NIRSpec Early Release Observations, GLASS, and CEERS programs. Among the 70 galaxies, we have pinpointed two galaxies, CEERS_01019 at z = 8.68 and GLASS_150008 at z = 6.23, with extremely low C/N ([C/N] ≲ −1), evidenced with C iii ] λλ 1907,1909, N iii ] λ 1750, and N iv ] λλ 1483,1486, which show high N/O ratios ([N/O] ≳ 0.5) comparable with the one of GN-z11, regardless of whether stellar or active galactic nucleus radiation is assumed. Such low C/N and high N/O ratios found in CEERS_01019 and GLASS_150008 (additionally identified in GN-z11) are largely biased toward the equilibrium of the CNO cycle, suggesting that these three galaxies are enriched by metals processed by the CNO cycle. On the C/N versus O/H plane, these three galaxies do not coincide with Galactic H ii regions, normal star-forming galaxies, and nitrogen-loud quasars with asymptotic giant branch stars, but with globular-cluster (GC) stars, indicating a connection with GC formation. We compare the C/O and N/O of these three galaxies with those of theoretical models and find that these three galaxies are explained by scenarios with dominant CNO-cycle materials, i.e., Wolf–Rayet stars, supermassive (10 3 –10 5 M ⊙ ) stars, and tidal disruption events, interestingly with a requirement of frequent direct collapses. For all the 70 galaxies, we present measurements of Ne/O, S/O, and Ar/O, together with C/O and N/O. We identify four galaxies with very low Ne/O, log(Ne/O) < −1.0, indicating abundant massive (≳30 M ⊙ ) stars.