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After a burst of star formation has taken place, the first supernovae appear after about 10 million years. These Type II supernovae arise from collapsing massive stars which have exhausted most of their nuclear fuel. Type II supernovae are distributed like the young stars in a disk galaxy, i.e. concentrated along spiral arms. A billion years after the initial starburst, the Type I supernovae appear. Type Ia supernovae are thought to be due to accretion of matter by a white dwarf in a binary pair. These are to be found among the older stellar population and therefore have an exponential distribution with radius like the underlying disk.

The increase in Type Ia supernovae towards the Galactic centre implies that here the ISM likely becomes supernova-dominated, resulting in a two phase ISM with only an HIM and a CNM. The warm phases are disrupted as diffuse clouds are shocked and heated to high temperatures. The molecular clouds survive the assault of supernovae and remain essntially intact, having only their outer warm layers stripped away. The scale height of the oldest stellar populations is higher (~ 300 pc) than for the atomic hydrogen (~ 100 pc), such that half of all Type Ia supernovae explode above the gas and therefore deposit much of their energy directly to the halo. This picture produces a volume filling fraction of about 25% for the HIM, and presumably explains the HI holes seen in external galaxies.

The correlated distribution of Type II supernovae has an important consequence. The time interval between successive supernovae is less than the bubble lifetime. This can result in a large-scale wind of energy into the Galactic halo. Heiles (1987) derives a two-dimensional porosity parameter

Equation 2 (2)

where sigma is the supernova rate in kpc-2 Myr-1, s (~ 2) is a correlation factor, and N (~ 40) is the number of Type II supernovae that occur in a single association. But this leads to a volume filling factor of more than 95% along the spiral arms, and about 80% outside of the arms, contrary to observation. Furthermore, the Type II energy flow is expected to break out and produce a mass flow rate of ~ 20 Msun per year, which would deplete the total gas content of the disk in 109 yr. How are we to reconcile this?

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