Supernova-driven outflows in NGC 7552: a comparison of Hα 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 (UV) absorption lines measured with the Hubble Space Telescope Cosmic Origins Spectrograph. We detect broad (similar to 300 km s(-1)) blueshifted (-40 km s(-1)) 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 a emission comes from clouds of high-density gas entrained in a turbulent outflow. We compare the H a 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 similar to 290 km s(-1), whereas the UV line profile extends to blueshifts upwards of 1000 km s(-1). The mass outflow rate estimated from the UV is roughly nine times greater than that estimated from H a. We argue that the H a 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.