A rare sextuple-merging brightest cluster galaxy system in a disturbed galaxy cluster observed with the Einstein Probe Follow-up X-ray Telescope

Abstract: The evolutionary processes of galaxy clusters influence the properties of their member galaxies. We present a joint X-ray--optical analysis of the galaxy cluster WHY J050106.2+013714 at $z_{\rm c}=0.151$. X-ray observations with the Einstein Probe Follow-up X-ray Telescope indicate that the cluster is dynamically young. The cluster displays an average X-ray temperature of $2.8^{+0.4}_{-0.3}$ keV and a total luminosity of 9.4$\pm0.3\times10^{43}$ erg s$^{-1}$, consistent with the scaling relation of typical disturbed clusters. Remarkably, the cluster hosts a multi-merging brightest cluster galaxy (BCG) system composed of six massive galaxies, with a total stellar mass of $1.16\times10^{12}M_{\odot}$. We detected a well-defined intracluster light component extending to a size of 310 kpc. A systematic search for merging BCGs in the DESI Legacy Surveys reveals that this sextuple-merging BCG is extremely rare in the local Universe. Additionally, other merging BCGs are also likely to form in moderately disturbed clusters, which provides valuable insights into the formation of BCGs.