The High-Redshift Universe
Resolving galaxy formation at cosmic noon and beyond
The peak of cosmic star formation occurred between redshifts z ≈ 1–3, when galaxies assembled most of their mass, formed rotating disks, and established the interplay between stars, gas, and dark matter. Understanding how galaxies formed and evolved during this crucial epoch requires spatially resolved spectroscopy on kiloparsec and sub-kiloparsec scales — a regime that is largely inaccessible with current facilities.
HARMONI on the ELT will provide an unprecedented view of galaxies in the distant Universe, resolving their internal structure, dynamics, and physical conditions at a level comparable to what is currently achievable only for nearby galaxies.
Mapping galaxy dynamics at high redshift
A primary science driver for HARMONI is the measurement of two-dimensional kinematics of star-forming galaxies at z ≈ 1–3 using strong rest-frame optical emission lines such as Hα and [O III].
These observations will enable astronomers to:
- Measure rotation curves and velocity dispersion maps
- Identify inflows, outflows, mergers, and tidal features
- Disentangle the contributions of stars, gas, and dark matter to galaxy dynamics
- Test models of disk formation and stability in the early Universe
With spatial resolutions corresponding to a few hundred parsecs at z ≈ 2, HARMONI will resolve the inner regions of galaxies where baryonic processes dominate, while also tracing dynamics to several effective radii where dark matter becomes significant.
The role of dark matter and baryons
One of the key open questions in galaxy evolution concerns the interaction between dark matter and baryons during galaxy assembly. Different theoretical models predict markedly different dark-matter density profiles, especially in lower-mass galaxies.
HARMONI will:
- Measure galaxy dynamics well beyond the effective radius
- Provide direct constraints on dark-matter halo properties
- Reduce systematic uncertainties associated with beam smearing and limited spatial resolution
By extending these measurements to lower-mass galaxies at high redshift, HARMONI will probe regimes where theoretical predictions diverge most strongly, offering powerful tests of galaxy formation models.
Star-forming regions and the interstellar medium
Beyond global galaxy dynamics, HARMONI will resolve individual star-forming clumps and H II regions within distant galaxies.
These observations will allow:
- Measurement of internal motions and turbulence within star-forming regions
- Determination of gas excitation, metallicity, and ionisation conditions
- Studies of feedback processes driven by star formation and active galactic nuclei
This spatially resolved view of the interstellar medium provides critical insight into how star formation is regulated within galaxies during their most active phases.
The first galaxies and cosmic reionisation
At even higher redshifts, HARMONI will contribute to studies of the Epoch of Reionisation, when the first galaxies transformed the intergalactic medium.
By targeting the brightest early systems, HARMONI will:
- Search for spectroscopic signatures of extremely low-metallicity stellar populations
- Constrain the physical conditions in early galaxies
- Complement imaging and photometric studies from facilities such as JWST
These observations will help bridge the gap between the first light in the Universe and the emergence of mature galaxy populations.
Why HARMONI is essential
HARMONI brings together a unique combination of capabilities for high-redshift studies:
- Integral-field spectroscopy, capturing spatial and spectral information simultaneously
- Large field of view, enabling coverage of entire galaxies and their environments
- Adaptive optics correction, delivering sharp images at cosmological distances
- Flexible spectral resolution, optimised for faint emission lines in the near-infrared
Together, these features make HARMONI the ideal instrument to study galaxy evolution during the most active periods of cosmic history.
A transformative view of the distant Universe
By resolving the internal structure of galaxies across cosmic time, HARMONI will:
- Connect local galaxy populations to their high-redshift progenitors
- Reveal how disks, bulges, and halos assemble
- Provide critical benchmarks for numerical simulations of galaxy formation
HARMONI will thus play a central role in building a coherent, physically motivated picture of galaxy evolution from the early Universe to the present day.