“Star clusters shaping the morphology and tracing the ISM of galaxies out to the reionisation epoch”
The HST observations have revealed that more than 60% of star-forming galaxies at cosmic noon have peculiar morphologies characterised by UV-bright clumps. Such morphologies have now been found to be ubiquitous in many galaxies out to the cosmic reionisation epoch thanks to the new JWST observations. When combined with the strong gravitational lensing by foreground massive galaxy clusters, we are able to resolve these clumps down to tens of parsecs and derive their physical properties. We find that they are young, massive (10^5-10^8 Msun), and dense star clusters complexes, significantly contributing to the recent star formation of the host galaxy and the build-up of its stellar mass. We explore the physical properties of these stellar clumps throughout the redshifts, and we find their star formation rate densities, together with their stellar mass densities (but at a more moderate level), increase with redshift. We associate this increase to the overall evolution of the host galaxy ISM conditions, which are more turbulent, rich in molecular cold gas, denser, with a higher stellar radiation field and higher disk hydrostatic pressure in the early times. Moreover, we link these star cluster complexes with molecular gas clouds detected with ALMA, and estimate the star formation efficiency to be ~30% at z=1. This is an important finding showing the possible increase of the star-formation efficiency toward higher redshifts, as such an increase could be one possible explanation to the problem of the numerous UV-luminous galaxies detected with JWST toward the epoch of reionisation. Globally, we demonstrate that multi-wavelength observations of high-redshift lensed galaxies start probing the star-formation cycle from the collapse of molecular clouds to the formation of stars in clustered star-forming regions at <100 pc scales.