High-z galaxies with the JWST and the ELT: Toward Ever-finer Detail

Eros Vanzella

First listed 2026-09-25 | Last updated 2026-09-24

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 (2um). 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 2030s - 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 39m 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 ~ 6 smaller Point-Spread-Function (PSF). An ELT diffraction-limited PSF (with a Full Width Half Maximum, FWHM ~ 8-12 mas) in the near infrared will resolve spatial scales <100 pc at any redshift (z ~< 18), revealing abundant star-forming clumps with sufficient sensitivity. Leveraging gravitational lensing as a cosmic telescope, even with moderate magnification factors (mu ~ 4-8), diffraction-limited 8m and 39m telescopes will probe physical scales ~< 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 2030s - 2040s.

Short digest

Vanzella reviews how JWST’s infrared imaging and spectroscopy have opened the first half-gyr of cosmic history to resolved studies of massive blue and red galaxies, little red dots, metal-poor systems, and lensed star-forming clumps. The central forward-looking result is that diffraction-limited ELT imaging, with near-infrared PSF FWHM of roughly 8–12 mas, should resolve sub-100-pc scales at essentially any redshift up to z≈18, about six times finer than JWST. Combined with moderate gravitational magnification (μ≈4–8), ELT and adaptive-optics facilities could routinely reach ≲25-pc, star-cluster-scale structure in distant galaxies, connecting early star formation, bound clusters, and possible black-hole-seed environments.

Key figures to inspect

• Figure 1. This figure establishes the paper’s core resolution argument by comparing the physical scales reached by HST, JWST, VLT/MAVIS, and ELT/MORFEO–MICADO over redshift. It makes clear why a 10-mas-class ELT PSF changes the science case from galaxy-scale morphology to resolving ∼100-pc structures even in the earliest accessible galaxies.
• Figure 2. This diagnostic combines telescope diffraction limit with lensing magnification for a source at z=10, directly quantifying the route to star-cluster-scale measurements. It shows that modest magnification lets the ELT probe below 25 pc, while stronger lensing opens sub-parsec regimes that the paper identifies as genuinely new observational territory.