Ultra compact dwarfs in the Perseus Cluster: UCD formation via tidal stripping

We present the results of a Keck/DEIMOS survey of Ultra Compact Dwarfs (UCDs) in the Perseus Cluster core. We confirm cluster membership for 14 UCDs, with radial velocities ~5300 km s${}^{-1}$. Two of these confirmed Perseus UCDs have extremely blue colours ($B-R<0.6$ mag), reside in star forming filaments surrounding NGC 1275, and have likely formed as massive star clusters in the last ~100 Myr. We also measure a central velocity dispersion of a third, UCD13 (${\sigma }_0=38\pm 8$ km s${}^{-1}$), the most extended UCD in our sample. We determine it to have radius $R_e=85\pm 1.1$ pc, a dynamical mass of ($2.3 \pm 0.8)\times10^{8}$M${\odot}$,andametallicity[Z\mathrm{/}H]$= -0.52^{+0.33}{-0.29}$ dex. UCD13 and the cluster’s central galaxy, NGC 1275, have a projected separation of 30 kpc and a radial velocity difference of ~20 km s${}^{-1}$. Based on its size, red colour, internal velocity dispersion, dynamical mass, metallicity and proximity to NGC 1275, we argue that UCD13 is likely the remnant nucleus of a tidally stripped dE, with this progenitor dE having $M_B\approx -16$ mag and mass $\sim {10}^9$ M${}_{\odot }$.