2014MNRAS.439.3808P

We present the results of a Keck/DEIMOS survey of Ultracompact dwarfs (UCDs) in the Perseus Cluster core. We confirm cluster membership for 14 UCDs, with radial velocities ∼ 5300 km/s. Two of these confirmed that 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 (σ0 = 38±8 km/s), the most extended UCD in our sample. We determine it to have radius R_e = 85 ±1.1 pc, a dynamical mass of (2.3±0.8)x10⁸M_☉, and a metallicity [Z/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. 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 dwarf elliptical (dE), with this progenitor dE having MB ~ -16 mag and mass ∼ 10⁹M_☉.