tokens: Red velocity vectors are km s −1 and white ones are km s −1 . The filament is forming just left of center where the two bubbles are merging. Figure 12. Same as Figure 11 , for model M5_27T1. Velocity vectors are color-coded, ranging from 20 to 1500 km s −1 . The strength of the GW determines how filaments evolve. We note two interesting cases outlined below. 5.2. Mass Anchors Model M5_34T (Figure 11 ) has sufficient energy to form a GW, but the wind does not entirely disrupt all filaments. As shown in Figure 11 two distinct bubbles emerge from the central starburst. Their boundaries merge to form a dense filament that stretches pc back to anchor on the starburst reservoir. The 540 km s −1 wind ablates mass off the reservoir, pushes it into the filament, and by 1.5 Myr has extended the filament pc above the disk plane where it is drifting along at only 50−100 km s −1 . Due to continual mass loading at the base of the filament it stays anchored, allowing it to persist and grow. At some point the filament should disrupt entirely due to either Kelvin–Helmholtz instabilities or heating and evaporation. But our resolution is insufficient to maximize filament survival time (see Cooper et al. 2009 ). 5.3. Filament Lift Off In model M5_27T1 (Figure 12 ) the filament again forms along the bubble contact. But now, after 1 Myr it detaches from the disk reservoir and lofts into the free-flowing wind of the now merged bubbles. This filament differs from its slow counterpart model M5_34T1; it has a larger cross section to the impinging wind, so it fragments more due to Kelvin–Helmholtz instabilities. The surrounding wind flows at 1420 km s −1 while the filament moves at 0−50 km s −1 before lift off but attains 200−500 km s −1 thereafter. This filament would be analogous to the disrupted clouds studied by Cooper et al. ( 2009 ). 6. SYNTHETIC ABSORPTION LINES We showed in Section 4 that cold gas emission entrained in the GW is 4–5 dex fainter than the galactic disk. A more sensitive probe of this gas may be its absorption of background starburst continuum. We synthesize absorption lines for three temperature regimes, denoted “molecular,” “warm,” and “soft X-ray,” with temperature ranges given in Table 1 . A trivial line source function suffices for kinematical signatures of the three temperature regimes. Absorption spectra are derived by integrating optical depth in N cells along the column viewed perpendicular to the diskThe velocity channels have a resolution of 10 km s −1 and range from −1800 to 200 km s −1 . Absorption profiles are shown in Figure 13 for models M5_27T1 (top panel) and M5_34T1 (bottom). The “soft X-ray” line shows the structure of the hot free-wind inside the expanding bubble. The velocity at maximum absorption is the average speed of the free wind. The long tail of the profile back toward Galaxy systemic velocity, especially prominent in model M5_27T1, reveals gas flowing radially at the average speed of the free wind but not entirely
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