Kinematic Decomposition of the BCG and ICL in Abell S1063 and MACS J1206

Abstract

The brightest cluster galaxy (BCG) and the intracluster light (ICL) overlap in space and merge smoothly in surface brightness. Beyond the nearest clusters, measurements of the ICL have therefore relied on photometric definitions. We present a spectral decomposition of the light in cluster cores into BCG and ICL, using deep VLT/MUSE integral-field spectroscopy of Abell S1063 (z=0.35) and MACS J1206.2$-$0847 (z=0.44). Spectral templates of the two components are derived iteratively from the BCG center and the ICL-dominated outskirts, and their weighted sum is fitted to the spectrum of every spatial bin. This enables us to measure the kinematics and the light of the BCG and ICL separately without assuming a boundary or a functional form for their light profiles. The velocity dispersions of the BCG and ICL are $316\pm5$ and $753\pm25\ \mathrm{km\ s^{-1}}$, respectively, in AS1063, whereas we obtain $313\pm5$ and $695^{+47}_{-45}\ \mathrm{km\ s^{-1}}$ in MACS1206. The ICL is blueshifted relative to the BCG by $75\pm22$ and $148\pm38\ \mathrm{km\ s^{-1}}$, respectively, and is ~0.7 dex more metal-poor, at a combined statistical significance of ~$2σ$. Defining the ICL as the light fainter than $μ_{B}=25\ \mathrm{mag\ arcsec^{-2}}$ assigns two-thirds to three-quarters of the kinematically defined ICL to the BCG, while a double Sersic model fits the total profiles well but leaves the division between the two components poorly constrained given the depth and resolution of the current MUSE data. Our kinematic decomposition provides a direct, physically motivated reference for photometric definitions of the ICL.