High star cluster formation efficiency in the strongly lensed Sunburst Lyman-continuum galaxy at z = 2.37
Authors: Vanzella E. , Castellano M. , Bergamini P. , Meneghetti M. , Zanella A. , Calura F. , Caminha G. B. , Rosati P. , Cupani G. , Meštrić U. , Brammer G. , Tozzi P. , Mercurio A. , Grillo C. , Sani E. , Cristiani S. , Nonino M. , Merlin E. , Pignataro G. V. 
Published: December 2021

We investigate the strongly lensed (μ ≃ ×10 − 100) Lyman continuum (LyC) galaxy, dubbed Sunburst, at z = 2.37, taking advantage of a new accurate model of the lens. A characterization of the intrinsic (delensed) properties of the system yields a size of ≃3 sq. kpc, a luminosity of MUV = −20.3, and a stellar mass of M ≃ 109 M⊙; 16% of the ultraviolet light is located in a 3 Myr old gravitationally bound young massive star cluster (YMC), with an effective radius of ∼8 pc (corresponding to 1 milliarcsec without lensing) and a dynamical mass of ∼107 M⊙ (similar to the stellar mass) – from which LyC radiation is detected (λ < 912 Å). The star formation rate and stellar mass surface densities for the YMC are Log10(ΣSFR[M⊙ yr−1 kpc−2]) ≃ 3.7 and Log10(ΣM[M⊙ pc−2]) ≃ 4.1, with sSFR > 330 Gyr−1, consistent with the values observed in local young massive star clusters. The inferred outflowing gas velocity (> 300 km s−1) exceeds the escape velocity of the cluster. The resulting relative escape fraction of the ionizing radiation emerging from the entire galaxy is higher than 6−12%, whilst it is ≳46 − 93% if inferred from the YMC multiple line of sights. At least 12 additional unresolved star-forming knots with radii spanning the interval 3 − 20 pc (the majority of them likely gravitationally bound star clusters) are identified in the galaxy. A significant fraction (40−60%) of the ultraviolet light of the entire galaxy is located in such bound star clusters. In adopting a formation timescale of the star clusters of 20 Myr, a cluster formation efficiency Γ ≳ 30%. The star formation rate surface density of the Sunburst galaxy (Log10(ΣSFR) = 0.5−0.2+0.3) is consistent with the high inferred Γ, as observed in local galaxies experiencing extreme gas physical conditions. Overall, the presence of a bursty event (i.e., the 3 Myr old YMC with large sSFR) significantly influences the morphology (nucleation), photometry (photometric jumps), and spectroscopic output (nebular emission) of the entire galaxy. Without lensing magnification, the YMC would be associated to an unresolved 0.5 kpc–size star-forming clump. The delensed LyC and UV magnitude m1600 (at 1600 Å) of the YMC are ≃30.6 and ≃26.9, whilst the entire galaxy has m1600 ≃ 24.8. The Sunburst galaxy shows a relatively large rest-frame equivalent width of EWrest(Hβ + [O III]λλ4959, 5007) ≃ 450 Å, with the YMC contributing to ∼30% (having a local EWrest ≃ 1100 Å) and ∼1% of the total stellar mass. If O-type (ionizing) stars are mainly forged in star clusters, then such engines were the key ionizing agents during reionization and the increasing occurrence of high equivalent width lines (Hβ + [O III]) observed at z > 6.5 might be an indirect signature of a high frequency of forming massive star clusters (or high Γ) at reionization. Future facilities, which will perform at few tens milliarcsec resolution (e.g., VLT/MAVIS or ELT), will probe bound clusters on moderately magnified (μ < 5 − 10) galaxies across cosmic epochs up to reionization.