A new constraint on millicharged dark matter from galaxy clusters

We propose a new constraint on millicharged dark matter from considerations on galaxy clusters. The charged dark matter moves under the influence of the randomly oriented magnetic fields in galaxy clusters, and the corresponding dark matter density profile can significantly differ from the concordance CDM predictions which are well supported from the galaxy cluster observations. With a typical amplitude of magnetic fields $B=\mathcal O(1)\,\mu$G and dark matter velocity $v=\mathcal O(100)\,$km/sec at a cluster radius $R\simeq 1\,$Mpc, we claim that the charge $\epsilon e$ ($e$ is the elementary charge) of dark matter with mass $m$ should be bounded as $\epsilon \lesssim 10^{-14}(m/{\rm GeV})$ which is substantially tighter than the other previous constraints.