SWIFT Observations of the Arp 147 Ring galaxy system
  We present observations of Arp 147, a galaxy system comprising a
collisionally-created ring galaxy and an early-type galaxy, using the Oxford
SWIFT integral field spectrograph (IFS) at the 200-inch Hale telescope. We
derive spatially resolved kinematics from the IFS data and use these to study
the interaction between the two galaxies. We find the edge-to-edge expansion
velocity of the ring is 225 +/- 8 km/s, implying an upper limit on the
timescale for the collision of 50 Myrs. We also calculate that the angle of
impact for the collision is between 33 degrees-54 degrees, where 0 degrees
would imply a perpendicular collision. The ring galaxy is strongly star-forming
with the star formation likely to have been triggered by the collision between
the two galaxies. We measure some key physical parameters in an integrated and
spatially resolved manner for the ring galaxy. Using observed B-I colours and
the H-alpha equivalent widths, we conclude that two stellar components (a young
and an old population) are required to simultaneously match both observed
quantities. We constrain the age range, light and mass fractions of the young
star formation in the ring, finding a modest age range, a light fraction of
less than a third, and a negligible (<1%) mass fraction. We postulate that the
redder colours observed in the SE corner of the ring galaxy could correspond to
the nuclear bulge of the original disk galaxy from which the ring was created,
consistent with the stellar mass in the SE quadrant being 30-50% of the total.
The ring appears to have been a typical disk galaxy prior to the encounter. The
ring shows electron densities consistent with typical values for star-forming
HII regions. The eastern half of the ring exhibits a metallicity a factor of ~2
higher than the western half. The ionisation parameter, measured across the
ring, roughly follows the previously observed trend with metallicity.
