High star cluster formation efficiency in the strongly lensed Sunburst Lyman-continuum galaxy at z=2.37

We investigate the strongly lensed (mu similar or equal to x10-100) 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 similar or equal to 3 sq. kpc, a luminosity of M-UV = -20.3, and a stellar mass of M similar or equal to 10(9) M-circle dot; 16% of the ultraviolet light is located in a 3 Myr old gravitationally bound young massive star cluster (YMC), with an effective radius of similar to 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 (lambda < 912 angstrom). The star formation rate and stellar mass surface densities for the YMC are Log(10)(Sigma(SFR)[M(circle dot)yr(-1) kpc(-2)]) similar or equal to 3.7 and Log(10)(Sigma(M)[M-circle dot pc(-2)]) similar or equal to 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 greater than or similar to 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 Gamma greater than or similar to 30%. The star formation rate surface density of the Sunburst galaxy (Log(10)(Sigma(SFR)) = 0.5(-0.2)(+0.3)) is consistent with the high inferred Gamma, 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 starforming clump. The delensed LyC and UV magnitude m(1600) (at 1600 angstrom) of the YMC are similar or equal to 30.6 and similar or equal to 26.9, whilst the entire galaxy has M1600 similar or equal to 24.8. The Sunburst galaxy shows a relatively large rest-frame equivalent width of EWrest (H beta [O III]lambda lambda 4959, 5007) similar or equal to 450 angstrom, with the YMC contributing to similar to 30% (having a local EWrest 1100 angstrom) and similar to 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 (H beta + [O III]) observed at z > 6.5 might be an indirect signature of a high frequency of forming massive star clusters (or high Gamma) 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 (mu < 5-10) galaxies across cosmic epochs up to reionization.

Automated summary

This article investigates the strongly lensed Lyman continuum (LyC) galaxy Sunburst at a redshift of z = 2.37. The intrinsic properties of the system, such as its size, luminosity, and stellar mass, are characterized. The presence of a young massive star cluster (YMC) and its impact on the star formation rate, stellar mass surface density, and ionizing radiation are discussed. The article also highlights the importance of future facilities in studying magnified galaxies and bound star clusters across cosmic epochs.