Supernova-Driven Outflows in NGC 7552: A Comparison of H-alpha and UV Tracers

TL;DR

This study compares H-alpha and UV tracers of supernova-driven galactic winds in NGC 7552, revealing that UV absorption lines better trace the high-velocity, warm gas component of the outflow.

Contribution

It demonstrates that UV absorption lines provide a more comprehensive measure of outflow velocities and mass loss rates than H-alpha emission in starburst-driven winds.

Findings

UV lines extend to higher blueshifts (~1000 km/s) than H-alpha (~290 km/s).

Mass outflow rate from UV is about nine times greater than from H-alpha.

H-alpha traces dense, low-velocity gas near the star-forming region.

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 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.