Mapping Large-Scale Gaseous Outflows in Ultraluminous Infrared Galaxies with Keck II ESI Spectra: Spatial Extent of the Outflow

The kinematics of neutral gas and warm ionized gas have been mapped across ultraluminous starburst galaxies using the Na I λλ5890, 5896 absorption-line and Hα emission-line profiles, respectively, in Keck II ESI spectra. Blueshifted, interstellar absorption is found over extended regions, exceeding 15 kpc in several systems. An outflow diverging from the nuclear starburst would have to reach large heights to cover this area in projection. The scale height of the absorbing material could be lower, however, if the outflow emanates from a larger region of the galaxy. The large velocity gradient discovered across several outflows is inconsistent with a flow diverging from the nuclear starburst. Widespread star formation, triggered by the merger, probably drives these extended outflows via mechanical feedback from supernovae, although shocks generated by the galaxy-galaxy merger may also contribute to the formation of a hot wind. In a typical ULIG, the mass carried by the cool phase of the outflow is ~108 M☉; i.e., a few percent of the dynamical mass in the starburst region. Assuming the starburst activity has persisted for 10 Myr, the kinetic energy of the cool outflows is a few percent of the supernova energy. The cool wind is expected to be accelerated by momentum deposition, possibly from radiation pressure as well as the ram pressure of the hot, supernova-induced wind. The turnaround radii of the cool outflows are at least ~30-90 kpc, which presents a significant Na I absorption cross section. If most L > 0.1L* galaxies pass through a luminous starburst phase, then relics of cool outflows will create a significant redshift-path density. Galaxy formation models should include this cool phase of the outflow in addition to a hot wind in feedback models.