Next Contents Previous


Based on the above picture, we can broadly classify the gas in a superwind into two categories. The first is the ambient interstellar medium, and the second is the the volume-filling energetic fluid created by the thermalization of the starburst's stellar eject. The thermal and kinetic energy of this fluid is the "piston" that drives the outflow and dominates its energy budget. The observed manifestations of superwinds arise when the primary wind fluid interacts hydrodynamically with relatively dense ambient interstellar gas.

This has long been known to apply to the optical emission-line gas. In the case of superbubbles, the limb-brightened morphology and the classic "Doppler ellipses" seen in long-slit spectroscopy of dwarf starburst galaxies (e.g. Meurer et al. 1992; Marlowe et al. 1995; Martin 1998) are consistent with the standard picture of emission from the shocked outer shell of a classic wind-blown bubble (e.g. Weaver et al. 1977). Typical expansion velocities are 50 to 100 km s-1. Similarly, the morphology and kinematics of the emission-line gas in the outflows in edge-on starbursts like M 82, NGC 253, NGC 3079, and NGC 4945 imply that this material is flowing outward on the surface of a hollow bi-polar structure whose apices correspond to the starburst (e.g. Heckman, Armus, & Miley 1990; Shopbell & Bland-Hawthorn 1998; Cecil et al. 2001). The deprojected outflow speeds range from a few hundred to a thousand km s-1. This material is presumably ambient gas that has been entrained into the boundary layers of the bipolar hot wind, or perhaps the side walls of a ruptured superbubble (e.g. Suchkov et al. 1994; Strickland & Stevens 2000). In both superbubbles and superwinds, the optical emission-line gas is excited by some combination of wind-driven shocks and photoionization by the starburst.

Prior to the deployment of the Chandra X-ray observatory, it was sometimes assumed that the soft X-ray emission associated with superbubbles and superwinds represented the primary wind fluid that filled the volume bounded by the emission-line gas. If so, its relatively low temperature (typically 0.5 to 1 kev) and high luminosity ( $ \sim$ 10-4 to 10-3Lbol) required that substantial mass-loading had occurred inside the starburst ( $ \cal {L}$ $ \sim$ 10). The situation is actually more complex. The superb imaging capabilities of Chandra demonstrate that the X-ray-emitting material bears a very strong morphological relationship to the optical emission-line gas (Strickland et al. 2000, 2001; Martin, Kobulnicky, & Heckman 2001). The X-ray gas also shows a limb-brightened filamentary structure, and is either coincident with, or lies just to the "inside" of the emission-line filaments (Fig. 1). Thus, the soft X-rays could arise in regions in which hydrodynamical processes at the interface between the wind and interstellar medium have mixed a substantial amount of dense ambient gas into the wind fluid, greatly increasing the local X-ray emissivity. Alternatively, the filaments may represent the side walls of a ruptured superbubble left behind as the wind blows-out of a "thick-disk" component in the interstellar medium. In this case the H$ \alpha$ emission might trace the forward shock driven into the halo gas and the X-rays the reverse shock in the wind fluid (see Lehnert, Heckman, & Weaver 1999).

Figure 1

Figure 1. Soft X-ray and H$ \alpha$ emission in several edge-on starburst galaxies, showing the spatial similarities between the two phases. NGC 253 & NGC 4945 have kpc-scale limb-brightened nuclear outflow cones (the opposite outflow cone is obscured in both cases) with a close match between X-ray & H$ \alpha$ emission. In NGC 3628 a 5 kpc-long H$ \alpha$ arc on the eastern limb of the wind is matched by an offset X-ray filament.

Ambient interstellar material accelerated by the wind can also give rise to blueshifted interstellar absorption-lines in starbursts. Our (Heckman et al. 2000) survey of the NaI$ \lambda$5893 feature in a sample of several dozen starbursts showed that the absorption-line profiles in the outflowing interstellar gas spanned the range from near the galaxy systemic velocity to a typical maximum blueshift of 400 to 600 km s-1. We argued this represented the terminal velocity reached by interstellar clouds accelerated by the wind's ram pressure. Very similar kinematics are observed in vacuum-UV absorption-lines in local starbursts (Heckman & Leitherer 1997; Kunth et al. 1998; Gonzalez-Delgado et al. 1998). This material (Figure 2) ranges from neutral gas probed by species like OI and CII to coronal-phase gas probed by OVI (Heckman et al. 2001a; Martin et al. 2001). Heckman et al. (2000) showed that there are substantial amounts of outflowing dust associated with the neutral phase of the superwind. Radiation pressure may play an important role in accelerating this material (e.g. Aguirre 1999).

Figure 2

Figure 2. FUSE spectra of the OVI$ \lambda$1031.9 interstellar absorption-line, tracing outflowing coronal-phase gas. The absorption covers the range from vsys to a maximum blueshift of $ \sim$700 km s-1 in the powerful starburst NGC 3310 (left panel) and $ \sim$140 km s-1 in the starbursting irregular galaxy NGC 4214 (right panel).

Extended radio-synchrotron halos around starbursts imply that there is a magnetized relativistic component of the outflow. In the well-studied case of M 82, this relativistic plasma has evidently been advected out of the starburst by the primary energy-carrying wind fluid (Seaquist & Odegard 1991). The situation in NGC 253 is less clear (Beck et al. 1994; Strickland et al. 2001)

Next Contents Previous