Supernova-driven outflows in NGC 7552: a comparison of H α and UV tracers

Abstract

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 (∼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 α emission comes from clouds of high-density gas entrained in a turbulent outflow. We compare the H α 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 α-emitting gas is ∼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 α. We argue that the H α 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.