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

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 $\mathrm{log}({\rm{N}}/{\rm{O}})=-{0.21}_{-0.11}^{+0.10}$ . 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.

Keywords

• Stellar populations
• Star clusters
• Galaxies