Hubble Space Telescope survey of the Perseus Cluster -IV: Compact
  stellar systems in the Perseus Cluster core and Ultra Compact Dwarf formation
  in star forming filaments
  We present the results of the first search for Ultra Compact Dwarfs (UCDs) in
the Perseus Cluster core, including the region of the cluster around the
unusual Brightest Cluster Galaxy (BCG) NGC 1275. Utilising Hubble Space
Telescope Advanced Camera for Surveys imaging, we identify a sample of 84 UCD
candidates with half-light radii 10 pc < r_e < 57 pc out to a distance of 250
kpc from the cluster centre, covering a total survey area of ~70 armin^2. 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.
