Euclid Quick Data Release (Q1). The shape of cluster haloes revealed by intracluster light

Intracluster light (ICL) offers a luminous route for measuring the shapes and orientations of cluster haloes. We used Euclid Quick Data Release (Q1) images to quantify ICL shapes for 205 clusters at 0.1 at most z at most 0.8 selected from the eROSITA All-Sky Survey and the Dark Energy Survey. We measured the ICL properties across five bands (I E, Y E, J E, H E and the coadded YJH E), and find that the shape parameters are consistent across the filters, with the H E band probing the profiles to the largest cluster-centric distances (0.1R 200 c about100 kpc). The ICL ellipticity (e=1-b/a) distribution peaks at 0.5, consistent with halo ellipticities inferred from strong and weak lensing studies, suggesting that the ICL traces the large-scale structure of the clusters. Cluster cores, dominated by brightest cluster galaxy (BCG) light, are round (e about0.2), while the ICL becomes progressively more elongated at large radii, reaching an ellipticity of about0.5 by 0.1R 200 c. We find that the ICL is closely aligned with the BCG (87% lie within 30), but shows a weaker alignment with the satellite galaxy distribution (61% within 30). Robust position angles could be measured using the ICL for 63% of the sample, whereas position angles could be measured for only 30% of the sample using the satellite galaxies. Stacking clusters in redshift bins allows measurements of the ICL surface brightness to 0.6 - 0.8R 200 c and ellipticity beyond 0.3 - 0.4R 200 c (300-400 kpc), but we observed a plateau in the ICL ellipticity beyond 0.1R 200 c. These results demonstrate the capability of Euclid to characterise the ICL morphology with high statistical precision, allowing new constraints on cluster halo structure and its evolution with redshift.