The Quantity of Intracluster Light: Comparing Theoretical and Observational Measurement Techniques Using Simulated Clusters

TL;DR

This study compares various methods for measuring intracluster light in simulated galaxy clusters, revealing systematic differences and the impact of measurement parameters on ICL estimates, with implications for understanding cluster evolution.

Contribution

It provides a comprehensive comparison of observational and simulation-based ICL measurement techniques, highlighting their systematic differences and parameter sensitivities.

Findings

ICL fractions range from 9-36% at z=0 across methods.

Different techniques can vary ICL measurements by up to a factor of two.

ICL quantity generally increases over time, but not monotonically.

Abstract

Using a suite of N-body simulations of galaxy clusters specifically tailored to study the intracluster light (ICL) component, we measure the quantity of ICL using a number of different methods previously employed in the literature for both observational and simulation data sets. By measuring the ICL of the clusters using multiple techniques, we identify systematic differences in how each detection method identifies the ICL. We find that techniques which define the ICL solely based on the current position of the cluster luminosity, such as a surface brightness or local density threshold, tend to find less ICL than methods utilizing time or velocity information, including stellar particles' density history or binding energy. The range of ICL fractions (the fraction of the clusters' total luminosity found in the ICL component) we measure at z=0 across all our clusters using any definition span the range from 9-36%, and even within a single cluster different methods can change the measured ICL fraction by up to a factor of two. Separating the cluster's central galaxy from the surrounding ICL component is a challenge for all ICL techniques, and because the ICL is centrally concentrated within the cluster, the differences in the measured ICL quantity between techniques are largely a consequence of this central galaxy/ICL separation. We thoroughly explore the free parameters involved with each measurement method, and find that adjusting these parameters can change the measured ICL fraction by up to a factor of two. While for all definitions the quantity of ICL tends to increase with time, the ICL fraction does not grow at a uniform rate, nor even monotonically under some definitions. Thus, the ICL can be used as a rough indicator of dynamical age, where more dynamically advanced clusters will on average have higher ICL fractions.