Improving the accuracy of observable distributions for galaxies classified in the projected phase space diagram

Abstract

Context. Studies of galaxy populations classified according to their kinematic behaviours and dynamical state using the ProjectedPhase Space Diagram (PPSD) are affected by misclassification and contamination, leading to systematic errors in determining thecharacteristics of the different galaxy classes.Aims. We propose a method to statistically correct the determination of galaxy properties’ distributions accounting for the contami-nation caused by misclassified galaxies from other classes.Methods. Using a sample of massive clusters and galaxies in their surroundings taken from the MultiDark Planck 2 simulationcombined with the semi-analytic model of galaxy formation sag, we compute the confusion matrix associated to a classificationscheme in the PPSD. Based on positions in the PPSD, galaxies are classified as cluster members, backsplash galaxies, recent infallers,infalling galaxies, and interlopers. This classification is determined using probabilities calculated by the code roger along with athreshold criterion. By inverting the confusion matrix, we are able to get better determinations of distributions of galaxy propertiessuch as colour.Results. Compared to a direct estimation based solely on the predicted galaxy classes, our method provides better estimates of themass-dependent colour distribution for the galaxy classes most affected by misclassification: cluster members, backsplash galaxies,and recent infallers. We apply the method to a sample of observed X-ray clusters and galaxies.Conclusions. Our method can be applied to any classification of galaxies in the PPSD, and to any other galaxy property besidescolour, provided an estimation of the confusion matrix. Blue, low-mass galaxies in clusters are almost exclusively recent infallergalaxies that have not yet been quenched by the environmental action of the cluster. Backsplash galaxies are on average redder thanexpected.