What does this research mean for the field?

Euclid Deep Fields can effectively identify high-redshift (z>6) Lyman break galaxies using combined signal-to-noise cuts and SED fitting, with initial data already revealing ultra-luminous z>9 candidates that support a double power-law ultraviolet luminosity function. Novelty: ClaimNovelty.NOVEL_FINDING. Consensus alignment: ConsensusAlignment.NEUTRAL.

What question did this study set out to answer?

The aim is to forecast the number of luminous galaxies at redshifts greater than 6 using the Euclid Deep Fields dataset.

March 26, 2026

Euclid Quick Data Release (Q1). Hunting for luminous z > 6 galaxies in the Euclid Deep Fields -- Forecasts and the first bright detections

Key Points

The aim is to forecast the number of luminous galaxies at redshifts greater than 6 using the Euclid Deep Fields dataset.
Utilized synthetic photometry for spectral energy distributions of high-z and low-z galaxies.
Investigated selection methodologies combining signal-to-noise ratio cuts with SED fitting.
Analyzed optical coverage from auxiliary data sources for high-z source recovery.
Expected to find over 100,000 Lyman break galaxies at z=6-12.
Forecasts suggest detection of more than 100 galaxies at z>12.
Achieved recovery of over 76% of input synthetic z>6 LBGs, with low-z contamination below 10%.

Abstract

The evolution of the rest-frame ultraviolet luminosity function (UV LF) is a powerful probe of early star formation and galaxy stellar mass build-up. At z>6, its bright end (M_ ̊m UV 6 Lyman break galaxies (LBGs) and definitively constrain the bright end of the UV LF. With its NIR coverage extending to sim2, , Euclid has the power to detect galaxies out to z Observatory will distinguish genuine Lyman breaks from contaminant features, while 9. This highlights the power of Euclid in constraining the bright end of the UV LF in the early Universe and in identifying the most luminous sources that are valuable for further follow-up observations. Here, we present a forecast for the number densities of z>6 galaxies that Euclid is expected to observe in the final EDF dataset. Using synthetic photometry from spectral energy distribution (SED) templates of z=5--15 galaxies, z=1--4 interlopers, and Milky Way M-, L-, and T-type dwarfs, we investigate optimal selection methodologies for high-z LBGs in the EDF datasets. We find that a combination of signal-to-noise ratio (S/N) cuts with SED fitting (over optical to MIR bands) yields the highest fidelity sample, recovering more than 76% of the input synthetic z>6 LBGs, while limiting low-z contamination to less than 10%. This contamination does not include effects from instrumental artefacts, which will impact the first Euclid data releases. Auxiliary data prove critical: optical coverage from the Hyper Suprime Camera and Vera C. Rubin data are vital for recovering z>10 sources. Based on empirical double power-law LF models, we expect more than 100, 000 LBGs at z=6--12 and more than 100 sources as far back as z>12 in the final Euclid data release. In contrast, the steeper Schechter LF models predict no detections of z>12 LBGs. In this work, we also present two ultra-luminous (M_ ̊m UV

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