Nitrogen-enriched, Highly Pressurized Nebular Clouds Surrounding a Super Star Cluster at Cosmic Noon

Abstract

Abstract Strong lensing offers a precious opportunity for studying the formation and early evolution of super star clusters that are rare in our cosmic backyard. The Sunburst Arc, a lensed Cosmic Noon galaxy, hosts a young super star cluster with escaping Lyman continuum radiation. Analyzing archival Hubble Space Telescope images and emission line data from Very Large Telescope/MUSE and X-shooter, we construct a physical model for the cluster and its surrounding photoionized nebula. We confirm that the cluster is ≲4 Myr old, is extremely massive M ⋆ ∼ 10 7 M ⊙ , and yet has a central component as compact as several parsecs, and we find a gas-phase metallicity Z = (0.22 ± 0.03) Z ⊙ . The cluster is surrounded by ≳10 5 M ⊙ of dense clouds that have been pressurized to P ∼ 10 9 K cm −3 by perhaps stellar radiation at within 10 pc. These should have large neutral columns N HI > 10 22.8 cm −2 to survive rapid ejection by radiation pressure. The clouds are likely dusty as they show gas-phase depletion of silicon, and may be conducive to secondary star formation if N HI > 10 24 cm −2 or if they sink farther toward the cluster center. Detecting strong [N iii ] λ λ 1750,1752, we infer heavy nitrogen enrichment log ( N / O ) = − 0.21 − 0.11 + 0.10 . This requires efficiently retaining ≳500 M ⊙ of nitrogen in the high-pressure clouds from massive stars heavier than 60 M ⊙ up to 4 Myr. We suggest a physical origin of the high-pressure clouds from partial or complete condensation of slow massive star ejecta, which may have an important implication for the puzzle of multiple stellar populations in globular clusters.