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

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

Massimo Pascale, Liang Dai, Christopher F. McKee, Benny T.-H. Tsang.
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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 HST images and emission line data from VLT/MUSE and X-shooter, we construct a physical model for the cluster and its surrounding photoionized nebula. We confirm that the cluster is \lesssim4\,Myr old, is extremely massive M_\star \sim 10^7\,M_\odot and yet has a central component as compact as several parsecs, and we find a gas-phase metallicity Z=(0.22\pm0.03)\,Z_\odot. The cluster is surrounded by \gtrsim 10^5\,M_\odot of dense clouds that have been pressurized to P\sim 10^9\,{\rm K}\,{\rm cm}^{-3} by perhaps stellar radiation at within ten parsecs. These should have large neutral columns N_{\rm HI} > 10^{22.5}\,{\rm 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_{\rm HI} > 10^{24}\,{\rm cm}^{-2} or if they sink further toward the cluster center. Detecting strong {\rm N III]}\lambda\lambda1750,1752, we infer heavy nitrogen enrichment \log({\rm N/O})=-0.21^{+0.10}_{-0.11}. This requires efficiently retaining \gtrsim 500\,M_\odot of nitrogen in the high-pressure clouds from massive stars heavier than 60\,M_\odot 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 important implication for the puzzle of multiple stellar populations in globular clusters.

Associated Faculty