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 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 Reff[LyC] ∼ 4.7 ± 1.5 pc and Reff[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.

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

“…Properties of the star cluster are shown in Figure 5. Assuming a single burst, we derive a cluster age = -+ t 2.4 age 1.0 1.6 Myr from the central component, which agrees with the independent UV spectroscopy analysis of Chisholm et al (2019) when the BPASS SED model is used, and is broadly consistent with the age requirement for very massive stars powering broad He IIλ1640 wind emission as suggested by Mestric et al (2023).…”

“…The data disfavor the extended component being older than 7 Myr, and it is possible that the extended component is coeval with the central component. While our model suggests that the extended component may contribute as much as ∼25% of the ionizing flux, no extended source of escaping LyC radiation is seen in image (10) in F275W (Mainali et al 2022;Mestric et al 2023; also see Figure 9). The low-density channels that allow LyC photons to pass freely may be connecting only to the central part of the cluster.…”

“…High signal-to-noise ratio spectroscopy (Mainali et al 2022) revealed evidences that the system drives outflowing nebular gas at ∼300 km s −1 , more dramatically so than in other Sunburst systems, which do not allow LyC photons to escape. Mestric et al (2023) suggest that a few dozen >100 M e very massive stars aggregate at the cluster center from which escaping LyC photons appear to originate, and can account for the observed broad He IIλ1640 emission.…”

“…However, we measure a magnification ratio μ 10 /μ 9 = 3.4 ± 0.1 in the F275W filter, which probes escaping LyC radiation. Interestingly, the extended component is undetected in image (10) for the F275W filter, which is sensitive only to LyC photons (Mestric et al 2023;Mainali et al 2022). The discrepancy in the magnification ratio suggests that the extended component has a higher magnification than the central component.…”

Small Region, Big Impact: Highly Anisotropic Lyman-continuum Escape from a Compact Starburst Region with Extreme Physical Properties

“…Due to lensing magnification, the galaxy's stretched rest-frame LyC image reveals that only one particular star-forming region shows escaping ionizing radiation, while other regions within the galaxy do not (Rivera-Thorsen et al 2019). Indeed, due to its uniqueness as a bright lensing-magnified LyC emitter, the Sunburst Arc has been of great interest in numerous studies concerning the physics of ionizing radiation production and escape (Rivera-Thorsen et al 2017Chisholm et al 2019;Mainali et al 2022;Sharon et al 2022;Vanzella et al 2022;Meštrić et al 2023;Pascale et al 2023) since its discovery (Dahle et al 2016).…”