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

¹ INAF – OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
e-mail: uros.mestric@inaf.it
² Geneva Observatory, Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
³ LUPM, Université de Montpellier, CNRS, Place Eugène Bataillon, 34095 Montpellier, France
⁴ CNRS, IRAP, 14 avenue E. Belin, 31400 Toulouse, France
⁵ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁶ Dipartimento di Fisica, Università degli Studi di Milano, via Celoria 16, 20133 Milano, Italy
⁷ INAF – IASF Milano, via A. Corti 12, 20133 Milano, Italy
⁸ Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, via Saragat 1, 44122 Ferrara, Italy
⁹ Technical University of Munich, TUM School of Natural Sciences, Department of Physics, James-Franck-Str 1, 85748 Garching, Germany
¹⁰ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
¹¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
¹³ Dipartimento di Fisica “E.R. Caianiello”, Università Degli Studi di Salerno, Via Giovanni Paolo II, 84084 Fisciano (SA), Italy
¹⁴ INAF – INAF – Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
¹⁵ INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy

Received: 11 January 2023
Accepted: 9 March 2023

Abstract

We report the identification of very massive stars (VMS; mass > 100 M_⊙) that may be segregated in the center of the young massive star cluster at z = 2.37 hosted in the lensed galaxy called 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 He IIλ1640 broad emission, N IVλ1486 emission, and an N IVλ1720 P Cygni profile. In particular, He IIλ1640 is broad (∼1610 ± 300 km s⁻¹), with an equivalent width of 3 Å, and asymmetric profile. These features require an extremely young (∼2.5 Myr) stellar population component in which the masses of the stars exceed 100 M_⊙. When a Salpeter initial mass function and BPASS models for normal massive stars are assumed, the observed spectral features require ∼400 VMS. (2) The same star cluster is detected at a signal-to-noise ratio of ∼100 in the Lyman continuum domain (λ < 900 Å). The Lyman continuum emission emerges from a region with a radius that is at least twice smaller than what is observed at 1700 Å (independently of magnification) and is located in the center of the cluster. After delensing, the effective radii in absolute scales are R_eff[LyC] ∼ 4.7 ± 1.5 pc and R_eff[1700] = 7.8 ± 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 ∼10⁷ M_⊙ produce ∼15% of the escaping Lyman continuum photons, and the remaining photons are produced by other massive early-type stars.