Euclid: Early Release Observations. A combined strong and weak lensing solution for Abell 2390 beyond its virial radius

Euclid is presently mapping the distribution of matter in the Universe in detail via the weak lensing (WL) signature of billions of distant galaxies. The WL signal is most prominent around galaxy clusters, and can extend up to distances well beyond their virial radius, thus constraining their total mass. Near the centre of clusters, where contamination by member galaxies is an issue, the WL data can be complemented with strong lensing (SL) data which can diminish the uncertainty due to the mass-sheet degeneracy and provide high-resolution information about the distribution of matter in the centre of clusters. Here we present a joint SL and WL analysis of the Euclid Early Release Observations of the cluster Abell 2390 at z=0.228. Thanks to Euclid’s wide field of view of 0.5 deg$^$2, combined with its angular resolution in the visible band of 0.“13 and sampling of 0.“1 per pixel, we constrain the density profile in a wide range of radii, 30 kpc < r < 2000 kpc, from the inner region near the brightest cluster galaxy to beyond the virial radius of the cluster. We find consistency with earlier X-ray results based on assumptions of hydrostatic equilibrium, thus indirectly confirming the nearly relaxed state of this cluster. We also find consistency with previous results based on weak lensing data and ground-based observations of this cluster. From the combined SL+WL profile, we derive the values of the viral mass $M_{200} = (1.48 \pm 0.29)\times10^{15}, \Msun$, and virial radius $r_{200} =(2.05\pm0.13 , {\rm Mpc}$), with error bars representing one standard deviation. The profile is well described by an NFW model with concentration c=6.5 and a small-scale radius of 230 kpc in the 30,kpc $< r <$ 2000,kpc range that is best constrained by SL and WL data. Abell 2390 is the first of many examples where Euclid data will play a crucial role in providing masses for clusters.

💡 Research Summary

This paper presents the first joint strong‑ and weak‑lensing (SL + WL) analysis of the galaxy cluster Abell 2390 (z = 0.228) using data from Euclid’s Early Release Observations (ERO). The authors exploit Euclid’s unique combination of a wide 0.5 deg² field of view, high angular resolution in the visible band (0.13″ PSF, 0.1″ pixel sampling), and deep imaging (VIS 5σ limit ≈ 27 mag) to map the mass distribution from the innermost 30 kpc out to ≈ 2 Mpc, well beyond the virial radius.

The weak‑lensing (WL) catalogue is built from Euclid VIS shapes after a rigorous cleaning process. Three independent shape‑measurement pipelines (SE++, KSB+, LensMC) are cross‑matched, and foreground or cluster‑member galaxies are vetoed using photometric redshifts derived from a combination of Euclid photometry and extensive ground‑based data (Subaru Suprime‑Cam in six optical bands and CFHT u‑band). The resulting “clean catalogue” contains 10–50 background galaxies per square arcminute, matching Euclid’s expected source density.

Strong‑lensing (SL) constraints are taken from previous HST imaging and recent MUSE spectroscopy (Richard et al. 2008, 2021). Seven multiply‑imaged systems are used, spanning a redshift range of 0.53 ≤ z ≤ 4.88. The most prominent giant arc (system 1, z = 1.036) provides a clear visual check of the model’s critical curve.

Mass reconstruction is performed with the WSLAP+ algorithm, originally a free‑form, grid‑based method that has been extended to incorporate WL shear constraints and a hybrid treatment of cluster member galaxies. In the hybrid mode, the light distribution of luminous members is used to define a parametric component whose total mass is a free parameter, while the diffuse dark‑matter component remains non‑parametric. This approach simultaneously exploits the high‑resolution SL information in the core and the large‑scale WL shear at larger radii, while mitigating the mass‑sheet degeneracy that plagues WL‑only analyses.

The resulting radial mass profile is well described by a Navarro‑Frenk‑White (NFW) model over the full 30 kpc – 2 Mpc range. The best‑fit parameters are: scale radius rₛ ≈ 230 kpc, concentration c ≈ 6.5, virial mass M₂₀₀ = (1.48 ± 0.29) × 10¹⁵ M⊙, and virial radius r₂₀₀ = (2.05 ± 0.13) Mpc. These values are consistent with earlier X‑ray hydrostatic mass estimates (which assumed a cooler core and yielded a lower concentration) and with previous WL studies based on ground‑based data, confirming that Abell 2390 is a relatively relaxed, massive system.

The paper highlights several methodological advances: (i) Euclid’s space‑based imaging provides a WL source density and shape fidelity unattainable from the ground, especially in the low‑latitude field of Abell 2390 where cirrus contamination is significant; (ii) the construction of a clean background catalogue dramatically reduces contamination bias; (iii) the WSLAP+ hybrid framework efficiently combines SL and WL constraints, delivering a high‑resolution mass map in the core while preserving accurate large‑scale shear information; (iv) the joint analysis breaks the mass‑sheet degeneracy, leading to tighter constraints on the concentration parameter.

Finally, the authors argue that Abell 2390 serves as a proof‑of‑concept for Euclid’s capability to deliver precise cluster masses for hundreds of systems. By applying the same SL + WL pipeline to the full Euclid cluster sample (≈ 10⁶ clusters above 10¹⁴ M⊙), future work will refine the cluster mass function, improve constraints on cosmological parameters such as Ωₘ and σ₈, and enable detailed studies of cluster assembly, baryonic physics, and dark‑matter substructure across cosmic time.