Cluster members selection in A2390. Left: galaxy number density in the plane of the Subaru SuprimeCam B – V color vs. Euclid I_E-band magnitude, from bulge+disk light profile fits with SourceXtractor++ (see text). The red stars indicate spectroscopic cluster members of A2390 determined by Sohn et al. (2020) and MUSE observations of the core of A2390. The dashed region indicates the cluster red sequence based on the spectroscopic members and extrapolated to fainter magnitudes. Right: sky location of galaxies enclosed by the red-sequence selection in the left panel over the joint 0.21 deg² area covered by Euclid and Subaru SuprimeCam. The size of each point scales inversely with I_E magnitude to indicate the distribution of stellar mass in red galaxies at the cluster redshift.