Hubble Space Telescope survey of the Perseus cluster - IV. Compact stellar systems in the Perseus cluster core and ultracompact dwarf formation in star-forming filaments

We present the results of the first search for ultracompact dwarfs (UCDs) in the Perseus cluster core, including the region of the cluster around the unusual brightest cluster galaxy NGC 1275. Utilizing Hubble Space Telescope Advanced Camera for Surveys imaging, we identify a sample of 84 UCD candidates with half-light radii 10 < r_e < 57 pc out to a distance of 250 kpc from the cluster centre, covering a total survey area of ∼70 arcmin². All UCDs in Perseus lie in the same size-luminosity locus seen for confirmed UCDs in other regions of the local Universe. The majority of UCDs are brighter than M_R=-10.5, and lie on an extrapolation of the red sequence followed by the Perseus cluster dwarf elliptical population to fainter magnitudes. However, three UCD candidates in the vicinity of NGC 1275 are very blue, with colours (B-R)₀ < 0.6 implying a cessation of star formation within the past 100 Myr. Furthermore, large blue star clusters embedded in the star-forming filaments are highly indicative that both proto-globular clusters (GCs) and proto-UCDs are actively forming at the present day in Perseus. We therefore suggest star-forming filaments as a formation site for some UCDs, with searches necessary in other low-redshift analogues of NGC 1275 necessary to test this hypothesis. We also suggest that tidal disruption of dwarf galaxies is another formation channel for UCD formation in the core of Perseus as tidal disruption is ongoing in this region as evidenced by shells around NGC 1275. Finally, UCDs may simply be massive GCs based on strong similarities in the colour trends of the two populations.