Star Formation in the Outer Filaments of NGC 1275
  We present photometry of the outer star clusters in NGC 1275, the brightest
galaxy in the Perseus cluster. The observations were taken using the Hubble
Space Telescope Advanced Camera for Surveys. We focus on two stellar regions in
the south and south-east, far from the nucleus of the low velocity system (~22
kpc). These regions of extended star formation trace the H alpha filaments,
drawn out by rising radio bubbles. In both regions bimodal distributions of
colour (B-R)_0 against magnitude are apparent, suggesting two populations of
star clusters with different ages; most of the H alpha filaments show no
detectable star formation. The younger, bluer population is found to be
concentrated along the filaments while the older population is dispersed evenly
about the galaxy. We construct colour-magnitude diagrams and derive ages of at
most 10^8 years for the younger population, a factor of 10 younger than the
young population of star clusters in the inner regions of NGC 1275. We conclude
that a formation mechanism or event different to that for the young inner
population is needed to explain the outer star clusters and suggest that
formation from the filaments, triggered by a buoyant radio bubble either rising
above or below these filaments, is the most likely mechanism.
