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 |$R_{\text{e}} = 85 \pm 1.1$| 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 MB ≈ −16 mag and mass ∼109 M⊙.