Euclid Early Release Observations Overview

• Euclid Early Release Observations are pioneering imaging campaigns that validate instrument performance with high-resolution optical and NIR data across diverse astrophysical environments.
• The ERO datasets achieve ultra-deep surface-brightness limits (e.g., μ_VIS >30 mag/arcsec²) enabling robust detection of intracluster light, tidal streams, and faint galaxies.
• Advanced processing pipelines combine precise calibration, tailored background subtraction, and dual-channel imaging to support investigations into galaxy evolution and dark matter.

The Euclid Early Release Observations (ERO) comprise a set of pioneering imaging campaigns executed by the ESA Euclid space mission during its commissioning and performance-verification phase, prior to the full commencement of the cosmology survey. Targeting a diverse selection of astrophysical environments—from galaxy clusters and nearby galaxies to star-forming regions and globular clusters—the ERO deliver high-resolution, wide-field, deep optical and near-infrared data optimized for both compact and diffuse emission science. These datasets are intended not only to validate Euclid’s end-to-end system performance but also to showcase its unique potential for low-surface-brightness (LSB) studies, early science, and community engagement across the landscape of galaxy evolution, structure, and dark matter research.

1. Instrumentation, Data Acquisition, and Pipeline

The Euclid satellite is equipped with two main imagers: the visible-band VIS channel (IE; ~550–900 nm, pixel scale 0.1″, FWHM ~0.16″) and the Near-Infrared Spectrometer and Photometer (NISP) channel (YE, JE, HE; ~1.0–1.7 μm, pixel scale 0.3″, FWHM ~0.49″). ERO imaging campaigns employed the standard Reference Observation Sequence (ROS): four dithered exposures per band (totaling 9056 s in VIS, 1395 s in each NISP band for deep cluster fields), yielding contiguous fields-of-view up to ~0.8 deg².

Data processing utilizes a pipeline whose five pillars are detrending, astrometric calibration, photometric calibration, image stacking, and catalogue production for compact and extended sources (Cuillandre et al., 2024). The pipeline achieves a PSF FWHM of 0.16″ in VIS, 0.49″ for NISP, absolute VIS flux calibration precision <1%, and NISP calibration to ~10%. Typical 5σ point-source depths are IE ∼27.1 mag (VIS), NISP ∼24.5 mag, with surface-brightness limits reaching μ_VIS ≳ 30.0 mag arcsec⁻² in optimal fields.

ERO pipeline optimizations ensure minimal instrumental signatures, robust astrometry tied to Gaia DR3 (VIS internal RMS 6 mas, external 8 mas), and precise stacking that preserves both compact-source photometry and diffuse, extended emission.

2. Science Demonstrations: Surface-Brightness Limits and LSB Phenomena

The ERO images reveal unprecedented sensitivity to diffuse emission across scales. Extended PSF wings closely approximate a diffraction-limited halo out to hundreds of arcseconds, ensuring negligible scattered-light contamination (<1% at 200″). For a typical LSB feature spanning 10″×10″, the 1σ background limit is measured at μ_VIS = 29.9 AB mag arcsec⁻² and μ_NISP ≈ 28.3–28.4 AB mag arcsec⁻² (Cuillandre et al., 2024). These depths enable robust detection and morphological study of LSB structures including intracluster light (ICL), tidal debris, stellar halos, faint galaxies, and extended features in nearby galaxies and clusters.

Optimization for both compact source and LSB science is achieved by parallel processing of “compact-source” (background-subtracted) and “extended-emission” (background-preserved) image stacks, each with dedicated catalogue production. The pipeline simultaneously delivers accurate photometry for millions of sources and preserves large-scale diffuse emission with systematic background control.

3. Key Findings from Cluster Scale ERO Campaigns

Cluster fields receive the deepest ERO integrations, with Perseus and Abell 2390 as key exemplars.

Perseus Cluster: ERO achieves μ(IE) ≈ 30.5 mag arcsec⁻² and traces ICL, dwarf galaxies, globular cluster (GC) systems, and shell/stream features to projected radii ≳600 kpc. The ICL and associated intracluster GC (ICGC) distributions present coherent, large-scale, flattened morphologies, with centroids significantly offset (~60 kpc) from the BCG and hot gas, likely reflecting ongoing relaxation post-merger (Kluge et al., 2024). The ICL+ICGCs account for 38±6% of the cluster’s total stellar luminosity within 500 kpc, with the ICL (outer Sérsic component) contributing 67±7% of the BCG+ICL light.

Abell 2390: Multi-method analyses (2D CICLE fitting, wavelet decomposition, 1D masking) converge on ICL fractions of 18–36% (mean 24%) and BCG+ICL fractions of 21–41% (mean 29%) within 600 kpc, revealing ICL to larger radii at fainter μ than previous works (Ellien et al., 10 Mar 2025). The ICL exhibits a strong negative Y–H color gradient, mapping to metallicity range [Fe/H] ∼ 0 (r≲10 kpc) to [Fe/H] ∼ −0.7 at r∼200–300 kpc. Substructure analyses show that ~30% of core ICL arises from pre-processed intragroup light, reinforcing theoretical expectations from hierarchical assembly models.

LSB Methodologies: All results emphasize the requirement for careful PSF subtraction, rigorous flat-fielding, star/galaxy masking, and aggressive, multi-scale background modeling. Empirical PSFs (VIS, NISP) are modeled from bright stars and subtracted to suppress scattered light. For NIR data, wavelet-based cirrus subtraction (DAWIS) ensures minimal Galactic foreground residuals, pushing the SB limits deeper.

4. Stellar Populations and ICL/GC Co-Evolution

ICL and ICGCs share spatial density, ellipticity, and centroid evolution at large radii (>60 kpc), indicating a common origin or potential (Kluge et al., 2024). The double-Sérsic surface-brightness profile of the ICL aligns with a scenario where the inner component represents the BCG and the outer the classical ICL envelope (outer n≈1.2, r_e≈266 kpc). The ICL color gradient and the GC luminosity function (broad and narrow Gaussian components) suggest the ICL and a substantial fraction (40–50%) of the ICGCs originate from hierarchical accretion and stripping, with dominant progenitors being stars and clusters from massive satellites (~a few × 10¹⁰ M_⊙), and an increasing relative contribution from disrupted dwarfs at large clustercentric radii.

The specific frequency S_N of ICGCs peaks at 60-80 GC per 10⁹ M_⊙ in the outskirts (r∼300–600 kpc), reflecting characteristic values of dwarfs or halo regions rather than in situ BCG populations, further corroborated by the shape of the GC luminosity function at large radii.

5. Morphology, Substructure, and Dynamical State

The Euclid EROs provide high-fidelity maps of ICL and ICGC isophotes and density contours out to ≳600 kpc. Isophotal modeling in Perseus reveals that the ICL becomes increasingly elliptical (ε ∼ 0.4) and offset from the BCG center at >100 kpc, while position angles stabilize (PA ∼ 0–20°). Spatial offsets between ICL/ICGC and hot-gas centroids (∼60 kpc west for ICL/ICGC, ∼70 kpc east for X-ray gas) are a clear signature of recent or ongoing cluster-scale dynamical relaxation. The kinematics of the BCG match the cluster mean, but dwarf galaxies and GCs are offset in velocity space, suggesting oscillatory motion of the dark-matter halo rather than the BCG itself.

In Abell 2390, advanced lensing and member galaxy analyses connect ICL substructure to galaxy subgroups and demonstrate that a substantial ICL contribution in the cluster core originates from the pre-processing in infalling galaxy groups. The elongation of the ICL at large radii traces ongoing dynamical evolution, whereas X-ray gas appears more round and relaxed, reflecting differential relaxation timescales (∼100 Myr for gas, ≳Gyr for stars and dark matter).

6. Limitations and Prospects for the Euclid Wide Survey

ERO limitations include restricted sky coverage (∼0.7 deg² per field, limiting accurate SB background estimation at the largest radii) and the impact of early mission instrumental systematics such as stray light and cirrus, particularly in VIS (Cuillandre et al., 2024, Urbano et al., 2024). In NISP, undersampling of the PSF currently challenges faint point-source detection, but refinements in modeling and the design of the Wide Survey will mitigate these factors.

The upcoming Euclid Wide Survey will provide near-continuous sky coverage, enabling robust background modeling, mapping of ICL/ICGC out to splashback radii (1–2 Mpc), and extension of LSB and star cluster studies across hundreds of clusters and thousands of galaxies (Kluge et al., 2024). The survey’s scientific yield is expected to include mapping of mass assembly, halo ellipticity fields, and GC/ICL globular cluster scaling relations across cosmic environments.

7. Legacy and Impact on Extragalactic Astronomy

Euclid ERO confirm the instrument’s capability for transformative low-surface-brightness science at optical and NIR wavelengths. The combination of sub-arcsecond resolution, field uniformity, and surface-brightness sensitivity is enabling detection and quantitative analysis of ICL, GCs, tidal streams, stellar halos, and faint galaxies at a depth and scale previously unattainable for wide-field surveys.

The coherent spatial structures and metallicity gradients recovered in the ICL, joint mapping of GC populations, and connection to cluster substructure are establishing new empirical baselines for the assembly histories of massive haloes and their constituent populations (Kluge et al., 2024, Ellien et al., 10 Mar 2025). Future work leveraging the Euclid pipeline will enable ensemble studies of ICL, ICGC, and stellar halo formation, providing independent constraints on dark matter via both stellar and GC tracers, and advancing the understanding of hierarchical structure formation in the Universe.