Abstract
Young galaxies, observed when the Universe was between 2 and 6 billion years old, have very different morphologies from today's galaxies. Images from the Hubble Space Telescope and James Webb Space Telescope show that most of these galaxies have very bright ultraviolet regions. Observational evidence suggests that these regions are giant star clusters, with stellar masses ranging from a few million to a few hundred million solar masses. Such massive star clusters are rare in today's galaxies. These clusters would have formed in the disks of these young galaxies, which are also highly turbulent and rich in molecular gas, acting as a reservoir for star formation. Numerical simulations of young galaxies suggest that they are gravitationally unstable, so that their disks fragment and form such clusters. These clusters contribute around 50 % of the star formation rate of host galaxies. They therefore play an important role in building the stellar mass of galaxies, and allow us to trace the evolution of galaxies over cosmic time.
We recently observed two young galaxies with the ALMA interferometer, at the same spatial resolution as Hubble images, and with the added benefit of gravitational lensing. These observations revealed very massive molecular clouds, with masses one hundred and times greater than the molecular clouds in today's galaxies. They also exhibit very high densities and internal turbulence. As a result, the gas in these clouds is highly supersonic, so they must be efficient at forming stars. Increased star formation efficiency in young galaxies could be behind the increased star formation rate observed in galaxies over cosmic time.
