A Young Super Star Cluster Powering a Nebula of Retained Massive Star Ejecta

TL;DR

This paper identifies a young, massive super star cluster in a distant galaxy that is actively enriching its surrounding nebula with stellar ejecta, providing insights into cluster evolution and chemical enrichment processes.

Contribution

It presents detailed physical and chemical characterization of a young super star cluster and its nebula, highlighting the retention of stellar ejecta and implications for globular cluster formation.

Findings

Cluster age 4-6 Myr and mass 2 million solar masses

Nebula enriched with nitrogen, helium, and supernova ejecta

High-density, pressurized environment driven by supernova turbulence

Abstract

We suggest that "Godzilla" of the lensed Sunburst galaxy ($z=2.37$) is a young super star cluster powering a nebula of gravitationally trapped stellar ejecta. Employing HST photometry and spectroscopy from VLT/MUSE and VLT/X-Shooter, we infer physical and chemical properties of the cluster and nebula, finding Godzilla is young $4$-$6$Myr, massive $2 \times 10^{6} M_\odot (1000/\mu)$, a stellar metallicity $Z \simeq 0.25 Z_\odot$, and has a compact FUV component $\lesssim 1{\rm pc}(1000/\mu)$, where $\mu$ is the flux magnification factor. The gas is significantly enriched with N and He, indicating stellar wind material, and has highly elevated O relative to the stellar metallicity, indicating entrainment of CCSNe ejecta. The high density $n_{\rm e} \simeq 10^{7-8}{\rm cm}^{-3}$ implies a highly pressurized intracluster environment. We propose the pressure results from CCSN-driven supersonic turbulence in warm, self-shielding gas, which has accumulated in the cluster center after runaway radiative cooling and is dense enough to resist removal by CCSNe. The nebula gas shows sub-solar C/O, Ne/O and Si/O, which may reflect the CCSN element yields for initial stellar masses $>40 M_\odot$. A comparison to element yield synthesis models for young star clusters shows the gas abundances are consistent with complete retention and mixture of stellar winds and CCSNe ejecta until the inferred cluster age. The inferred O and He enhancement may have implications for the formation of multiple stellar populations in globular clusters, as stars formed from this gas would contradict the observed abundances of second-population stars.