Clues on the presence and segregation of very massive stars in the Sunburst Lyman-continuum cluster at z=2.37

Abstract

We report the identification of very massive stars (VMS; mass $>100$\,\msun) that may be segregated in the center of the young massive star cluster at $z$=2.37 hosted in the lensed galaxy called {\tt Sunburst} galaxy. This result is based on two pieces of evidence: (1) VLT/MUSE spectra of several multiple images of the same star cluster show key spectral signatures of VMS, such as the \heii\ broad emission, \nivblue\ emission, and an \niv\ P-Cygni profile. In particular, \heii\ is broad ($\sim 1610\pm 300$ \kms), with an equivalent width of 3Å,\ and asymmetric profile. These features require an extremely young ($\sim 2.5$ Myr) stellar population component in which the masses of the stars exceed 100~\msun. When a Salpeter initial mass function and BPASS models for normal massive stars are assumed, the observed spectral features require $\sim$400 VMS. (2) The same star cluster is detected at a signal-to-noise ratio of~$\sim 100$ in the Lyman continuum domain ($\lambda <900$Å). The Lyman continuum emission emerges from a region with a radius that is at least twice smaller than what is observed at 1700Å~(independently of magnification) and is located in the center of the cluster. After delensing, the effective radii in absolute scales are R${}_{\mathtt{eff}}[\mathtt{LyC}]\sim 4.7\pm 1.5$ pc and R${}_{\mathtt{eff}}[1700]=7.8\pm 1.4$ pc. The Lyman continuum radiation is mainly produced by hot and massive stars, which implies that their spatial distribution (including that of VMS) is preferentially more confined in the central parts of the cluster. Approximately 400 VMS hosted by a cluster of $\sim 10^7$ \msun\ produce $\sim$15\% of the escaping Lyman continuum photons, and the remaining photons are produced by other massive early-type stars.

10 pages, 8 figures, Accepted to publication in A&A

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