The refill of superbubble cavities
  In this paper we study the late evolution of model galaxies after a single episode of star formation of different durations. The aim of the paper is to discover the timescale needed to refill with cold gas the center of the galaxy. This timescale strongly depends on the amount of gas initially present inside the galaxy and ranges between 125 and 600 Myr. A HI hole can therefore survive several hundred Myrs after the last SNII has exploded. If, as a consequence of the refill of the center of the galaxy, a second episode of star formation occurs, it pollutes the surrounding medium in a very short timescale (of the order of 10--15 Myr), at variance with what happens if the center of the galaxy is still occupied by hot and tenuous gas. 
Simone Recchi and Gerhard Hensler (Institute of Astronomy, Vienna   University)