Comparison of Kinematic with Hydrodynamic Models

Striking support for the two-zone model is obtained from a comparison with recent hydrodynamic outflow models. Initial two-dimensional simulations (e.g., [Tomisaka & Ikeuchi 1988]) proposed an evolution of the outflow which included collimation of the flow at early stages by disk material, subsequent breakout of the confined flow along the minor axis, and eventual extension of the hot wind material up to a few kpc from the nucleus.

More recent simulations ([Tomisaka & Bregman 1993]; [Suchkov et al. 1994]; [Suchkov et al. 1996]) include a separate corotating halo component and are able to reproduce much of the observed morphology through the interactions of the wind with the disk and halo. These simulations show that the outflow entrains disk gas around itself, dragging the cooler, denser material up to a couple of kiloparsecs above the plane of the galaxy (e.g., see Fig. 6 in [Suchkov et al. 1994]). The regions of densest entrained disk material, near the base of the outflow, serve to collimate the outflow beyond the height of the disk itself. The scale height of this collimation in the simulations is similar to that seen in our observations, $\sim$500 pc. The ``fingers'' of disk material entrained to heights above the collimated zone can be identified with the optical emission line filaments we observe on the outer walls of the outflow cone. This entrained gas has also been observed at molecular wavelengths (e.g., [Stark & Carlson 1984]; [Nakai et al. 1987]; [Sofue et al. 1990]). Within the collimated region, the simulations show that both the wind and the confining walls are at their densest, consistent with the observed increased levels of optical emission. Finally, it should be noted that a recent analysis of the minor-axis X-ray distribution implies a partially confined outflow of this gas as well, within 1.6 kpc of the disk ([Bregman, Schulman, & Tomisaka 1995]).