Abstract
Star cluster formation in the early universe and their contribution to
reionization remains to date largely unconstrained. Here we present JWST/NIRCam
imaging of the most highly magnified galaxy known at z ~ 6, the Sunrise arc. We
identify six young massive star clusters (YMCs) with measured radii spanning ~
20 pc down to ~ 1 pc (corrected for lensing magnification), estimated stellar
masses of ~ $10^(6-7)$ Msun, and with ages 1-30 Myr based on SED fitting to
photometry measured in 8 filters extending to rest-frame 7000A. The resulting
stellar mass surface densities are higher than 1000 Msun pc$^-2$ (up to a few
$10^5$ Msun pc$^-2$) and their inferred dynamical ages qualify the majority
of these systems as gravitationally-bound stellar clusters. The star cluster
ages map the progression of star formation along the arc, with to evolved
systems (>~ 10 Myr old) followed by very young clusters. The youngest stellar
clusters (< 5 Myr) show evidence of prominent Hbeta + OIII4959,5007 emission,
based on photometry, with equivalent widths larger than 1000 A rest-frame, and
are hosted in a 200 pc sized star-forming complex. Such a region dominates the
ionizing photon production, with a high efficiency log($\xi_ion$ Hz
erg$^-1$) ~ 25.7. A significant fraction of the recently formed stellar mass
of the galaxy (> 10-30 %) occurred in these YMCs. We speculate that such
sources of ionizing radiation boost the ionizing photon production efficiency
which eventually carve ionized channels that might favor the escape of Lyman
continuum radiation. The survival of some of the clusters would make them the
progenitors of massive and relatively metal-poor globular clusters in the local
Universe.
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