Collapse of Axionic Domain Wall and Axion Emission
  We examine the collapse of an axion domain wall bounded by an axionic string. It is found that the collapse proceeds quickly and axion domain walls disappear. However axions are emitted in the collapse and its energy density increases during radiation dominated era and contributes significantly to the present mass density of the universe. In particular the axion emitted from the wall can account for the dark matter in the universe for $F_a\gsim 10^{10}\GeV$. 
Michiyasu Nagasawa and Masahiro Kawasaki