Supernova-Driven Outflows in NGC 7552: A Comparison of H-alpha and UV
  Tracers
  We investigate the supernova-driven galactic wind of the barred spiral galaxy
NGC 7552, using both ground-based optical nebular emission lines and
far-ultraviolet absorption lines measured with the Hubble Space Telescope
Cosmic Origins Spectrograph. We detect broad (~300 km/s) blueshifted (-40 km/s)
optical emission lines associated with the galaxy's kpc-scale star-forming
ring. The broad line kinematics and diagnostic line ratios suggest that the
H-alpha emission comes from clouds of high density gas entrained in a turbulent
outflow. We compare the H-alpha emission line profile to the UV absorption line
profile measured along a coincident sight line and find significant
differences. The maximum blueshift of the H-alpha-emitting gas is ~290 km/s,
whereas the UV line profile extends to blueshifts upwards of 1000 km/s. The
mass outflow rate estimated from the UV is roughly nine times greater than that
estimated from H-alpha. We argue that the H-alpha emission traces a
cluster-scale outflow of dense, low velocity gas at the base of the large-scale
wind. We suggest that UV absorption line measurements are therefore more
reliable tracers of warm gas in starburst-driven outflows.
