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The consequences of feedback for the global ISM can be evaluated quantitatively in terms of the interstellar porosity parameter Q, which is the ratio: (total area or volume occupied by superbubbles) / (total area or volume of the galaxy). Thus it is essentially the filling factor of hot gas, assuming hot gas is contained within all of the superbubbles. Values of Q near unity indicate the HIM dominates the multiphase ISM, and values ≫ 1 imply an outflow, with the galaxy generating more hot gas than it can contain.

It is straightforward to use the analytic expression for N(R) (equation 3) to derive Q in terms of a galaxy's star-formation rate, Psi (Oey et al. 2001; see also Clarke & Oey 2002):

Equation 6 (6)

for beta = 2, a Salpeter (1955) IMF for stellar masses 0.1 leq m leq 100 Modot, and P0 / k = 9500. Rg and h are the radius of the gaseous star-forming disk and gas scale height, respectively. We caution that Q depends on ambient interstellar parameters, for example, P0-1 as indicated.

Oey et al. (2001) estimated Q for all the galaxies in the Local Group. The Milky Way yields Q ~ 1 for some methods and Q < 1 for others, consistent with the ambiguous results found in the past (e.g., McKee & Ostriker 1977; Slavin & Cox 1993). The LMC yields Q ~ 1, implying that hot gas dominates the ISM volume. The remainder of the Local Group galaxies all show Q ≪ 1, with the sole exception of IC 10, a starburst galaxy for which Q ~ 20, thereby unambiguously predicting an outflow. Oey et al. (2001) crudely estimate the mass-loss rate in this outflow dot{M}out, assuming that the material is largely evaporated from shell walls by thermal conduction. We find that dot{M}out ~ Psi; indeed, absorption-line studies of local starburst galaxies by Heckman et al. (2000) also show that empirically, the outflow and star-formation rates have the same order of maagnitude for that sample.

Since Q ~ 1 represents a rough threshold for the escape of superwinds from the galactic disk, this also implies the simultaneous escape of newly-synthesized metals, which are contained in the hot gas. Likewise, the shredding of the ISM into filaments facilitates the escape of ionizing radiation, thus Q ~ 1 also represents an escape threshold for ionizing photons (Clarke & Oey 2002). We finally note the extensive body of numerical work on superbubbles and blowout conditions. Mac Low (1999) and Strickland & Stevens (2000) provide overviews of this field. The details of the numerical predictions are presently difficult to confirm empirically, but observations with XMM-Newton and Chandra are beginning to constrain the dominant processes.

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