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

Abstract

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 R_{e} = 85 ± 1.1 pc, a dynamical mass of (2.3 ± 0.8) × 10⁸ M_⊙, 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 ∼10⁹ M_⊙.