The Large-Scale Bipolar Wind in the Galactic Center

During a 9 month campaign (1996-1997), the Midcourse Space Experiment (MSX) satellite mapped the Galactic plane at mid-infrared wavelengths (4.3-21.3 (mu)m). The much greater spatial resolution and sensitivity of the MSX 8.3 (mu)m band, compared to IRAS at 12 (mu)m, has revealed diffuse emission not seen before. Here we report evidence of a spectacular, limbbrightened, bipolar structure at the Galactic center extending more than a degree (170 pc at 8.0 kpc) on either side of the plane. The 8.3 (mu)m filamentary emission appears confined to a dome-shaped shell, which extends perpendicular to the plane, and shows a tight correlation with the 3, 6, and 11 cm continuum emission over the same scales. The most likely scenario is that the extraplanar 8.3 (mu)m emission arises from dust entrained in a large-scale, bipolar wind powered by a central starburst. The inferred energy injection at the source is ~10^54/k ergs, for which k is the covering fraction of the dusty shell (k <~ 0.1). In accord with the stellar record in the Galactic center, we infer that a powerful nuclear starburst has taken place within the last several million years. The inferred energies require >~ 10^4/(epsilon) supernovae, where (epsilon) is the thermalization efficiency of the supernova ejecta. There is observational evidence of a Galactic wind on much larger scales, presumably from the same central source that produced the bipolar shell seen by MSX. Sofue has long argued that the North Polar Spur-a thermal X-ray/radio loop that extends from the Galactic plane to b = +80 - was powered by a nuclear explosion [(1-30) × 10^55 ergs] roughly 15 Myr ago, although his published models did not explain the projected loop structure. We demonstrate that an open-ended bipolar model, when viewed in projection in the near field, provides the most natural explanation for the observed loop structure. The ROSAT 1.5 keV diffuse X-ray map over the inner 45° provides compelling evidence for this interpretation. Since the faint bipolar emission would be very difficult to detect beyond the Galaxy, the phenomenon of large-scale galactic winds may be far more common than has been observed to date. The derived energetics for the bipolar wind are of the order of 10^55 ergs, in line with the energy requirement of the MSX observations. We infer that the Galactic center is driving large-scale winds into the halo every ~ 10-15 Myr or so.