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 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 MB ≈ -16 mag and mass ~109M⊙. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.