3.2. Nearby Starburst Galaxies

Two classic examples of starburst-driven outflows are described in this section to illustrate the wide variety of processes taking place in these objects.

M 82. This archetype starburst galaxy hosts arguably the best studied galactic wind. Some of the strongest evidence for the wind is found at optical wavelengths, where long-slit and Fabry-Perot spectroscopy of the warm ionized filaments above and below the disk shows line splittings of up to ~ 250 km s^-1, corresponding to deprojected velocities of order 525 - 655 km s^-1 (e.g., McKeith et al. 1995; Shopbell & Bland-Hawthorn 1998). Combining these velocities with estimates for the ionized masses of the outflowing filamentary complex, the kinetic energy involved in the warm ionized outflow is of order ~ 2 × 10⁵⁵ ergs or ~ 1% of the total mechanical energy input from the starburst. The ionized filaments are found to lie on the surface of cones with relatively narrow opening angles (~ 5 - 25°) slightly tilted (~ 5 - 15°) with respect to the spin axis of the galaxy. Deep narrow-band images of M82 have shown that the outflow extends out to at least 12 kpc on one side (e.g., Devine & Bally 1999), coincident with X-ray emitting material seen by ROSAT (Lehnert, Heckman, & Weaver 1999) and XMM-Newton (Stevens, Read, & Bravo-Guerrero 2003). The wind fluid in this object has apparently been detected by both CXO (Griffiths et al. 2000) and XMM-Newton (Stevens et al. 2003). The well-known H I complex around this system (e.g., Yun et al. 1994) may be taking part, and perhaps even focussing, the outflow on scales of a few kpc (Stevens et al. 2003). Recently published high-quality CO maps of this object now indicate that some of the molecular material in this system is also involved in the large-scale outflow (Walter, Weiss, & Scoville 2002; see also Garcia-Burillo et al. 2001). The outflow velocities derived from the CO data (~ 100 km s^-1 on average) are considerably lower than the velocities of the warm ionized gas, but the mass involved in the molecular outflow is substantially larger (~ 3 × 10⁸ M), implying a kinetic energy (~ 3 × 10⁵⁵ ergs) that is comparable if not larger than that involved in the warm ionized filaments. The molecular gas is clearly a very important dynamical component of this outflow.

NGC 3079. An outstanding example of starburst-driven superbubble is present in the edge-on disk galaxy, NGC 3079. High-resolution HST H maps of this object show that the bubble is made of four separate bundles of ionized filaments (Cecil et al. 2001). The two-dimensional velocity field of the ionized bubble material derived from Fabry-Perot data (Veilleux et al. 1994) indicates that the ionized bubble material is entrained in a mushroom vortex above the disk with velocities of up to ~ 1500 km s^-1 (Cecil et al. 2001). A recently published X-ray map obtained with the CXO (Cecil, Bland-Hawthorn, & Veilleux 2002) reveals excellent spatial correlation between the hot X-ray emitting gas and the warm optical line-emitting material of the bubble, suggesting that the X-rays are being emitted either as upstream, standoff bow shocks or by cooling at cloud/wind conductive interfaces. This good spatial correlation between the hot and warm gas phases appears to be common in galactic winds (Strickland et al. 2000, 2002; Veilleux et al. 2003, and references therein). The total energy involved in the outflow of NGC 3079 appears to be slightly smaller than that in M 82, although it is a lower limit since the total extent of the X-ray emitting material beyond the nuclear bubble of NGC 3079 is not well constrained (Cecil et al. 2002). Contrary to M 82, the hot wind fluid that drives the outflow in NGC 3079 has not yet been detected, and evidence for entrained molecular gas is sparse and controversial (e.g., Irwin & Sofue 1992; Baan & Irwin 1995; Israel et al. 1998; but see Koda et al. 2002).