High-z galaxies with the JWST and the ELT: toward ever-finer detail

Abstract The exploration of the early Universe is being transformed by the James Webb Space Telescope ( JWST ), which delivers unprecedented angular resolution at infrared wavelengths and opens a unique window redward of the K band (2 µm). Thanks to NIRCam , NIRISS , NIRSpec , and MIRI instruments, which provide both imaging and spectroscopy with exquisite efficiency, new classes of sources have emerged within the first years of operations. The first half-Gyr of cosmic time is now routinely probed, revealing massive blue/red galaxies and a population of Active Galactic Nuclei (AGN) appearing as “little red dots” together with a rest-frame near-infrared/optical view of sources across the reionization and post-reionization epochs. Angular resolution will remain pivotal in the 2030–2040s, when extreme adaptive optics (AO) facilities will be deployed on both (8–10) m (e.g., the VLT Multi-Conjugate Adaptive Optics (MCAO)-Assisted Visible Imager and Spectrograph, MAVIS ) and on extremely large telescopes, like the 39 m ELT (e.g., Multi-conjugate adaptive Optics Relay For ELT Observation, MORFEO ). Operating at the diffraction limit, these facilities will improve JWST s resolution, with ELT achieving a factor of $$\simeq 6$$ ≃ 6 smaller Point-Spread-Function (PSF). An ELT diffraction-limited PSF (with a Full Width Half Maximum, FWHM $$\approx$$ ≈ 8–12 mas) in the near infrared will resolve spatial scales $$<100$$ < 100 pc at any redshift (z $$\lesssim 18$$ ≲ 18 ), revealing abundant star-forming clumps with sufficient sensitivity. Leveraging gravitational lensing as a cosmic telescope, even with moderate magnification factors (µ $$\sim$$ ∼ 4–8), diffraction-limited 8 m and 39 m telescopes will probe physical scales $$\lesssim 25$$ ≲ 25 pc, enabling systematic studies of star formation down to star cluster scale at cosmological distances. Such observations are poised to become routine in the 2030–2040s.