|Annu. Rev. Astron. Astrophys. 2005. 43:
Copyright © 2005 by Annual Reviews. All rights reserved
The Galaxy and the Large Magellanic Cloud are excellent laboratories for detailed studies of wind-blown events. The processes involved in these local events are scaled down versions of superbubbles and GWs in starburst galaxies. We summarize key features of these local laboratories in this section.
3.1. Large Magellanic Cloud
The largest H II region in the LMC (and indeed Local Group), 30 Doradus, is a microcosm of starburst processes. Cluster R136 powers the nested shells and superbubbles. This mini-starburst contains ~ 50 very massive stars and has an estimated initial mass of a few 104 M (Malamuth & Heap 1994; Brandl et al. 1996; Brandl 2005). For comparison, M82 is powered by the equivalent of ~ 100 R136's within a region only 2-3 times larger than 30 Dor (Rieke et al. 1980; Muxlow et al. 1994; O'Connell et al. 1995). Shells and compact knots moving at ~ 200 km s-1 are detected in 30 Dor (Chu & Kennicutt 1994; Redman et al. 2003), perhaps forming the base of a large-scale wind able to escape the LMC (vesc 150 km s-1). Many shells in 30 Dor seem to be momentum-conserving, not pressure-driven.
3.2. Nuclear Wind in the Galaxy
Only 8.0 ± 0.5 kpc (Reid 1993) distant, the Galactic Center shows remarkable energetic activity at infrared (IR), radio, X-ray and -ray wavelengths (Morris & Serabyn 1996; Yusef-Zadeh et al. 2000, 2005; Cheng et al. 1997). While this activity has proved difficult to disentangle, there is now solid evidence on scales of arcminutes to tens of degrees for powerful mass ejections from the Galactic Center. The idea of a central explosion dates back to the early discovery of peculiar H I kinematics there (see Section 1.1), but we know today that most, but not all, of the H I kinematical signature is due to streaming motions arising from a central bar (Morris & Serabyn 1996).
A particular problem with GW studies has been deriving reliable energies from multi-wavelength observations at comparable resolution. Current estimates of the energetics of our Galactic Center span a huge range. Sofue & Handa (1984) discovered the 200 pc diameter Galactic Center radio lobe (GCL; Fig. 2b), with an implied thermal energy of ~ 3 × 1051 erg. Chevalier (1992) argued for a higher value (~ 2 × 1052 erg) due to winds from hot young stars over the past 30 Myr. Others have argued from the high temperatures implied by the ASCA detection of 6.7 keV K emission from He-like Fe XXV (Koyama et al. 1989, 1996; Yamauchi et al. 1990; but see Wang, Gotthelf, & Lang 2002) that an explosive event (4-8× 1053 erg) must have occurred. Bland-Hawthorn & Cohen (2003) detected the GCL at mid-IR wavelengths (Fig. 2b). The association of the lobe with denser material raises the energetics to 1054 / erg, where is the covering fraction of the dense shell; less energy is needed if there is substantial polycyclic aromatic hydrocarbon (PAH) emission with the mid-IR continuum. These energetics assume a shell velocity of ~ 150 km s-1, a value based on the kinematics of the neighboring molecular gas after correction for bar streaming (Bally et al. 1988); this value is uncertain because of our location in the plane. The ROSAT 1.5 keV diffuse X-ray map over the inner 45° provides compelling evidence for this GW interpretation (Fig. 2a) (Bland-Hawthorn & Cohen 2003). Evidence for outflows on smaller scale may be present in Chandra X-ray Observatory (CXO) X-ray maps (Baganoff et al. 2003).
Figure 2. Aspects of the Milky Way's wind. (top) ROSAT 1.5 keV diffuse X-ray map that shows a biconical pattern emerging from the Galactic Center on scales of tens of degrees. (bottom) The inner ~ 2.5 × 2.5° around the Galactic Center. Above the plane in red is the Galactic Center Lobe (GCL), here imaged by Law & Yusef-Zadeh with rasters from the Green Bank Telescope (GBT). Elsewhere, the color image comes from 8.3 (B), ~ 13 (G), and 21.3 (R) µm scans obtained with the SPIRIT III radiometer on the MSX spacecraft. These show embedded dust at various temperatures. Note the warm dust filaments along the edges of the GCL.
Potential energy sources are young star clusters or the 3-4× 106 M central BH (Oort 1977; Frogel 1988; Genzel et al. 1996; Schödel et al. 2003; Ghez et al. 1998, 2005). Individual star clusters have ages ranging from 5 Myr (Krabbe et al. 1995) to 20 Myr (Eckart, Ott, & Genzel 1999; Figer et al. 2000). While the star formation history is undoubtedly complicated, there is now abundant evidence that the Galactic Center has experienced several starburst episodes (e.g., Tamblyn & Rieke 1993; Sjouwerman et al. 1998; Simpson et al. 1999). Detailed models of PAH and fine structure features (Lutz 1998) suggest a starburst ~ 7 Myr ago, supported by a census of stars (Genzel et al. 1994; Krabbe et al. 1995; Najarro et al. 1997).
Activity seems to be fueled from the central molecular zone (CMZ), a "ring" at 180 pc radius with Mcmz ~ 8 × 106 M. Inflow rates of ~ 1 M yr-1 to the Galactic Center (Morris & Serabyn 1996) suffice to trigger starbursts and nuclear activity in Seyfert galaxies (~ 1043 erg s-1). Hydro simulations (Section 2.4) show that a central explosion of ~ 1055-56 erg would provide mass Mcmz with sufficient radial impulse to make the observed ring (Sanders 1989; Saito 1990).