Ultracompact dwarfs in the Perseus Cluster: UCD formation via tidal stripping

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^-1. 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^-1), the most extended UCD in our sample. We determine it to have radius Re = 85 ± 1.1 pc, a dynamical mass of (2.3 ± 0.8) × 10⁸M_⊙, 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^-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 dwarf elliptical (dE), with this progenitor dE having M_B ≈ -16 mag and mass ~10⁹M_⊙.