November 9, 2025Open Access

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

Key Points

Findings suggest over 100,000 Lyman break galaxies may be discovered at z = 6-12, supporting early star formation study.
Forecast indicates over 76% recovery of z > 6 LBGs with a contamination rate below 10% using synthetic photometry.
Analysis features near-infrared imaging and spectral energy distribution models for optimal high-z galaxy selection.
Study emphasizes the role of auxiliary data in distinguishing genuine Lyman breaks and enhancing bright end constraining.

Abstract

The evolution of the rest-frame ultraviolet luminosity function (UV LF) is a powerful probe of early star formation and stellar mass build-up. At z > 6, its bright end (MUV 6 Lyman break galaxies (LBGs) and constrain the UV LF's bright end. With NIR coverage extending to 2um, Euclid can detect galaxies out to z = 13. We present forecasts for the number densities of z > 6 galaxies expected 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 MLT dwarfs, we explore optimal selection methods for high-z LBGs. A combination of S/N cuts with SED fitting (from optical to MIR) yields the highest-fidelity sample, recovering >76% of input z > 6 LBGs while keeping low-z contamination 10 sources. Based on empirical double power-law LF models, we expect >100,000 LBGs at z = 6-12 and >100 at z > 12 in the final Euclid release. In contrast, steeper Schechter models predict no z > 12 detections. We also present two ultra-luminous (MUV 9, highlighting Euclid's power to constrain the UV LF's bright end and identify the most luminous early galaxies for follow-up.

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